This is a 2019 Subaru Impreza, and the episode is about a problem with its anti-lock braking system. The big issue is that the car’s computers aren’t communicating correctly after an ABS-related repair.
Concept
option model code
An option model code refers to the vehicle’s configuration identifiers that determine which parts and calibrations are compatible. In ABS diagnostics, the correct option/model coding can matter because the “same-looking” module may require the right variant to match the car’s equipment.
Network codes mean the car’s computer network isn’t working the way it should. Since many modules share information, one communication problem can cause several different warning codes.
ABS is the system that helps prevent the wheels from locking up when you brake hard. The ABS module is the computer that runs that system, and here it’s reporting communication problems with other parts of the car.
Modern cars have computers that talk to each other. If the communication network has a problem, different systems may all throw warning codes because they can’t get messages from the other computers.
The steering angle sensor tells the car how turned the steering wheel is. If the car can’t get that information, systems that help with stability and driving assistance may log communication errors.
Hard fault codes are the “current” warning codes—problems the car is detecting right now. The host is saying these communication issues are not just old stored errors.
Eyesight is Subaru’s camera-based safety system that helps with things like detecting vehicles and lane-related driving support. In this case, it’s throwing codes because it can’t communicate with the rest of the car properly.
Electric power steering uses a motor to help you turn the wheel. Here, the steering system is also reporting that it can’t communicate correctly with other computers.
U0100 is a specific code that usually means one computer can’t talk to another computer. In this diagnosis, it’s part of the ABS’s “lost communication” set of faults.
U1235 is a code that points to communication trouble between vehicle computers. The host is using it to show the ABS is reporting it can’t talk to other modules.
VDC is Subaru’s system that helps keep the car stable when traction is low. If the car starts to slide or behave unexpectedly, it can brake individual wheels to help bring it back under control.
EPS means electric power steering. Instead of relying only on a hydraulic pump, an electric motor assists the steering, and it can report faults to the car’s diagnostic system.
Term
data stream codes
Data stream codes are related to what the car’s computers are reporting in real time. If the numbers coming from a module don’t make sense, the scan tool can flag it and other systems may complain too.
The key idea is to fix the most direct problem first. If the scan tool points at the eyesight system, you still check other connected systems (like steering and braking) for their own fault codes, because they can be the real source of the warning.
A U code usually means the car’s computers aren’t talking to each other properly. The problem may be that one module is sending wrong information, not that a single sensor is broken.
ABS is the system that prevents your wheels from locking up during hard braking. The “ABS unit” is the computer that controls that behavior, and if it’s throwing hard faults, it can affect other systems too.
The ECM is the main computer that controls the engine. If another module can’t communicate with it, the scan tool may show communication-related faults.
The steering angle sensor tells the car how much you’re turning the steering wheel. Driver-assistance systems use that information, and in this case the scan tool can’t talk to the sensor directly even though the data is still shared.
The CAN bus is the set of wires and rules that lets the car’s computers talk to each other. If modules can’t communicate on the “high speed” network, it can trigger multiple warning codes.
The DLC is the diagnostic plug under the dash where you connect a scan tool. In this case, the tool is connected in a way that lets it read data from the car’s high-speed network.
Modules need proper electrical power and a solid ground connection to work correctly. If those connections are bad, the car may show communication errors even if the module itself was replaced.
It’s how a shop updates or sets up a car’s computer module so it works correctly in that specific car. Without the right programming, the module may not communicate or may throw faults.
Power feeds are the wires that supply electricity to a car computer. Some wires are always on, and some only turn on when the ignition is on—if the wrong one is missing, the module may not work.
A headlight bulb can be used like a basic tester to see if a wire can actually deliver power when something is drawing current. It’s a quick way to catch weak or faulty power connections.
These are error codes that mean one of the car’s computers isn’t talking properly to the others. It’s not necessarily a broken sensor—sometimes it’s a module or network issue.
A key cycle is when you turn the ignition off and then back on again. Some problems only show up after the car “restarts,” so technicians use key cycles to reproduce the fault.
BCM stands for Body Control Module, the car’s electronics hub that controls many convenience and electrical functions. Here, the BCM is involved because it provides the ignition relay output that powers the ABS-related fuse circuit.
An “open” means there’s a break in the wire or a connection that isn’t making contact. That prevents the ABS module from getting the power it needs through that circuit.
A parasitic draw is when the car keeps using a little bit of battery power even after you turn it off. If too much current is left on, the battery can go dead.
The ignition feed is the power that turns on when you turn the key. The host says they repaired a wire so the ABS gets that correct “key-on” power and can shut off properly when the key is off.
A body network is a part of the car’s computer wiring that runs comfort and convenience features. The host is comparing how those networks behave versus the ABS/high-speed network in this case.
TCM means Transmission Control Module. It’s the computer that manages how the automatic transmission shifts, and the host says it turns off normally in this case.
Voltage fluctuation means the car’s electrical system voltage briefly changes. The host is saying some networks may wake up or keep talking if they sense those voltage changes, even when the key is off.
To “scope” the network means to measure the car’s data signals with a special tool. It helps them see whether the communication looks normal while driving and whether it keeps happening after the key is turned off.
The CAN network is the car’s “message system” that connects different computers. They’re saying you can watch that network after the key is off, and if it’s still active, that’s a big clue something isn’t behaving normally.
They’re using the moment when the key is off as a test point. Normally the car should quiet down; if the network is still active, it hints that something is staying awake or misbehaving.
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Welcome to the Automotive Diagnostic Podcast.
We're going to explore ways to sharpen our diagnostic skills, find learning resources,
and hear from experts in the automotive field.
This show is brought to you by Auto Rescue Tools and Isaac Rodel. If you've been looking for a
programming laptop, you're not sure which one to buy or how to set it up, especially if you
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questions. Isaac's your guy. He can custom set up programming laptops that are ready to tackle any
make or model. I've got one of these laptops myself, and I can say that it is outstanding,
and it really streamlines the process by having everything you need in one device. So if you're
looking for something like that, I highly recommend checking out AutoRescueTools.com. You'll also
find scan tools, diagnostic equipment, key cutting equipment, and much more. Check out the link in
the show notes. I highly recommend it. Hey, what's going on? Automotive World, welcome to another
episode of the Automotive Diagnostic Podcast. My name is Sean Tipping. I will be your host once again
for this week's episode. Thank you so much for joining me. Today on the show, I've got another
case study for you. This was one we dealt with recently at our shop. It was an interesting ride,
but it was a 2019 Subaru Impreza. This started at one of our customer shops, and they were having
issues with the ABS unit on this one. We've done some ABS module stuff on these. I just had an
episode recently on a Subaru with an ABS issue where it was the wrong one for the vehicle. We've
talked about the option model code and stuff like that. So that was actually pretty fresh on my mind,
but this one was a little different. We were actually dealing with some communication codes on
this one, some network codes, both in the ABS module and a few other modules, but not able to
easily get to a solution on this one. And the ABS module had been replaced. Now, of course,
the first thing I'm suspicious on these particular Subaru's is whether or not the right ABS module
was put in there, especially if it's a used one. But turns out, after doing some digging on this
one, the ABS module that was in there, not only matched the one that was coming out, but also was
correct for the vehicle. So they actually did get the correct one. It already had the correct one
in there, but both of them had the exact same issue. So that's what we'll be chasing here today,
is the codes and the problem that this one had. So this was a network communication issue with
codes in the ABS or the VDC, they call it. There was four primary codes that would set in the ABS,
lost com with the ECM, lost com with the TCM, lost com with the steering angle sensor, and lost com
with eyesight. And those were current, hard fault codes. There were some other ones in history
as well. But we did have some battery voltage issues, which I'll touch on a little bit later.
And we had some other history codes and other modules because of that. But we also had code
setting in the electric power steering and the eyesight. So the ones in the eyesight said,
eyesight communication, parentheses, VDC, which is the ABS in this one, vehicle dynamics control,
and eyesight communication, EPS, which is electric power steering. And then we had in electric power
steering, eyesight system abnormal. Okay, so those were our hard fault codes. And between the codes
and the ABS, the electric power steering and the eyesight, the entire dash, you know, lit up like
a Christmas tree. And this is pretty consistent, can't really clear it out. But we've got those
codes. So what I first am digging into here, and actually, my guys helped me out quite a bit on
this one. So I definitely want to give them credit. But I thought the what was actually going on here
was interesting, how we got there, and then why it was causing the codes that we had. And
having an understanding of some of these codes and what they mean, I think that's all important and
why it's worthwhile talking about on the podcast here. So in that ABS module, and we've got those
lost communication codes. And if you look up the definitions of those codes, so that's like a U0100,
a 101, a 126, and a U1235, those are all actual, hey, I am the ABS, and I lost communication with
these other modules. And that is the definition. And there's a flow chart you can go through and
everything. But it is the ABS saying, I cannot talk to those modules plain and simple. Now,
if we go over to the eyesight and the power steering, it's a little different. So the eyesight has,
it's a B28A2 and a B28A4, which says eyesight communication, and then in parentheses says VDC
and EPS. Now, those are codes that you definitely need to actually look at the definition, because
it is not just a straight up, hey, I lost communication with that module. If you look up
these codes in service information, what it's saying first is there's a land check that it has
you do. And if you go through really all that's having you do is check communication with both
the module that set the code, which you got the code out, it probably communicates,
but also the module it's setting the code for, right? So where it says eyesight communication,
VDC, I would check, can I communicate with the ABS module? And I can. And then same thing for the
power steering, can I communicate with the power steering? I can. Now it says eyesight system
abnormal, as far as a code in the power steering, but I can talk to it, data stream codes,
special functions, all of these modules, it's all there. All right. But here's the thing,
if you continue to follow the code, right, it says, check to make sure that you can talk to
these modules. Okay. If you can, it says, are there codes set in these modules? And if yes,
it says to diagnose those codes first. Okay. And that's a lot of network codes that we see that
are like implausible or invalid data, which may even set as a U code. They didn't hear for the
eyesight that's Subaru's designation is it's a body code. But it says right there, eyesight
communication. So if you're taking it for face value on the scan tool, you might think exactly
like, Hey, it's communication. But really, what this is, there is codes or data that I'm getting
from a module that I don't like, that module has a problem of some sort, diagnose the code that
is in that module. Okay. So if you're following this, you look and say, can I talk to the VDC
in the EPS? Yes. Do they have codes? Yes. Diagnose those codes first, and then come back to the
eyesight. That's the order that you would go with this and the one that actually worked for this.
So I have codes in the power steering, and I have codes in the ABS. Now, the power steering says
eyesight system abnormal, which if you follow that code, it basically says, Oh, there's a code
in the eyesight module, figure that one out. Okay, now, if I just had the EPS
code, and then the eyesight code in the EPS, it's a little bit of the chicken and the egg.
But I also have the VDC code in the eyesight, which tells me to go to the VDC or the ABS.
So right now, I am right back to the ABS unit. And I could kind of forget about the other codes,
even though they say communication, I am focused on why do I have communication codes
that are hard faults in my ABS module. Okay, modules already been replaced. Now,
something to consider here, the modules it says it can't talk to the ECM, the TCM,
the steering angle in the eyesight, I can talk to all of them minus the steering angle sensor,
because it is not a module that actually talks directly to the scan tool. It shares data over
the CAN bus, but you're not directly talking to it on this car. It does, however, live on the same
bus. In fact, all of these modules live on the same high speed CAN bus for this vehicle.
They all share the same bus. And I can talk to all of these modules on the high speed bus
using my scan tool, which is also hardwired onto the high speed bus at pins 6 and 14 on the DLC.
I can talk to all of them. And like the ECM and the TCM, they don't actually have codes saying
that anything's wrong with communication. They're happy. Yes, we do have something in the eyesight.
And I can't talk to the steering angle, but I can talk to everything else. And no one else is
upset about the steering angle. That's the other thing. Like, I'm sure there's other modules,
like the power steering, for instance, they use the steering angle data that comes onto the CAN bus.
He's not sending any codes for that. So again, everything's pointing back to this ABS unit.
And you can see why maybe it was replaced, because that doesn't quite make sense. So of course,
the next natural place that you would go is to check powers and grounds at the module. So those
were checked. Hey, have you ever been faced with a challenge of sourcing, installing and programming
a used control module in a vehicle? I know a lot of us have. It seems to be happening more and more
often today with the volume of control modules on vehicles, the cost of some new ones, or even the
availability of new control modules in some cases used may be the only option. So what do you do
here? I strongly recommend checking out SJ auto solutions and Tommy Oliva. Tommy offers a cloning
service for use control modules to make these things plug and play for the vehicle that you're
working on. In a lot of cases, he is also able to source the control modules if you're unable to
locate one for the vehicle that you're working on. But once you get connected with Tommy, he's
going to offer fantastic support from start to finish to make sure that that control module is
going to work in your application. He's also got tech support that he offers through his website,
along with some free resources there as well on information about used control module programming.
So make sure to check out SJ auto solutions. I can't recommend that enough.
And they were checked by a couple of my guys and everything was there that was supposed to be there.
Okay, so we actually have three power feeds into this module. Two of them are constant and one of
them is switched. And then it's got a ground going to it. So it's fairly straightforward. And we have
the two can lines going in. Again, I can talk to this thing, but we have powers on all three power
inputs. We have a ground on the ground, you know, load test them with a headlight bulb. Looks good.
Everything is there. Can scope the CAN bus. But again, it looks great. And if here's the other
thing, if you actually disconnect the ABS unit from the CAN bus, then a whole bunch of other people
then set code saying, I can't talk to the ABS now, you get those communication codes. So like,
even in the ECM, so clearly, the thing is talking on the bus and other modules are observing it.
But for some reason, it is not happy. Oh, what the clue ended up being here. And again, credit to
one of my guys, because he really picked this out as he was looking at it was that
when the codes would set in the ABS module actually mattered a lot for getting to the
bottom of this one. So the codes for communication in that ABS module would only set on a key cycle,
meaning that you would clear the codes, you would turn the key the off position, and then you would
turn it back on, and then the codes would reset at that point. If you just cleared the codes,
and you didn't cycle the key left it running, or you did like a really fast key cycle,
the codes would not set. And the ABS was happy with everybody talking on the bus and everybody
was happy with the ABS and you could clear all the codes and that was it. And so what this indicated
was something is happening during the key cycle. And what this clued my technician into is really
paying attention to the power feeds, not just any power feed, but the switched power feeds.
Is there a switched power feed that is staying on that has power when the key is in the off
position? And that's actually exactly what he ended up finding was that there is a yellow
hour feed, which comes from an ignition relay in the BCM, which was receiving power when the key
was in the off position. Okay, so this ignition to relay that's in the BCM normally feeds fuse
number 10, which then feeds the ABS unit among a couple of other components as well.
But it feeds the ABS unit on this ignition feed. And it is supposed to turn off within about five
seconds of turning the key off. That's normal operation. And the fuse that it feeds, you could
go to and it does what it's supposed to do. Approximately five seconds after turning the key
off, there's no more power there. That's what it's supposed to do. But I still have power on
the yellow wire at the ABS that didn't make sense. Double check the diagrams, some things, you know,
not adding up here. Well, what it was, someone did a repair to the circuit at some point or another,
and it probably wasn't that long ago, where they tied in constant power to that yellow wire in the
harness so that it was always on. And once we removed that, we found that there was an open
somewhere between the ABS module and that yellow wire where it comes out of the BCM.
And that was our that was probably the original issue is there was no power on this wire at all.
They jumped constant power to it. And not only did it cause a battery drain, which would have been
the other clue. And it's something to think about too, like, maybe you don't get to the solution.
Picking up the clues that you would want to pick up. But sometimes, if you notice there is a problem,
and at least has something to do, or maybe maybe it's not even that clear that it does,
you could follow that problem, and you might get to the solution for everything, right? And it might
even be a little bit of a stretch to say, like, I got a communication issue on this ABS module,
there's a parasitic draw, should I really follow the parasitic draw? If you have nothing else,
it's something we're like, well, I mean, it's an issue, it's killing the battery,
maybe it's related, maybe it's not. In this case, it would have led you to the same place,
because the ABS module is the thing that was staying awake. So the fix was just repairing
this wire so that the switch ignition feed got to the ABS module where it was supposed to.
And then it would actually power down the way it was supposed to. What was happening here
was that the ABS module is staying awake. And in the off position for the key,
the module was still reaching out to all of these other modules on the bus saying, hey,
where'd you guys go? And then setting communication codes. And that's why we would get them on every
key cycle. Because the ABS module never fully went to sleep. And it's looking for modules when
everybody else is shut down, because they're getting the power feed in the way that they're
supposed to. Now, something else that's interesting about this is there are some
networks on vehicles, and it seems to be mostly body can or body networks, although it's not
100% exclusive to this by any means. But there's a lot of networks that if you have traffic on
that bus, you have somebody still talking and reaching out, nobody will go to sleep. Everybody
keep talking until that module actually goes to sleep. That was not the case on this Subaru.
Now, was there anybody else talking on the high speed bus there might have been, but
a series of modules at the very least shut down, even though the ABS was clearly still
talking on the bus, the engine control, the TCM, the eyesight module, and then the steering angle
all shut down correctly at when the key was off and stopped talking on the bus and stopped
observing what was happening on the bus. And we don't see that all the time. There are body
networks and can networks that will continue to talk or be able to talk as soon as there's any
voltage fluctuation on the can line. And those are generally networks that need to operate when
the key is off. In this particular case with this high speed can on this Subaru, that's not the way
that they engineer it, at least for the modules that were actually shutting down. Basically,
the logic there is, well, the keys off, we don't need to talk, which actually makes a lot of sense
to me. But I get why certain modules and certain systems would need the ability to talk even with
the key being in the off position. All depends on the network, the modules, the construction of the
vehicle. But that's the way it ended up on this one. So we had communication codes that weren't
really a network fault. Yes, was there a lack of communication between modules? For sure. But
it's because of when the ABS was reaching out to the rest of the network. If you were to scope
that network, you wouldn't see anything's wrong outside of the fact that there would have been
activity long after the key was off. And maybe that's your clue. You know, maybe that's the way
that you find it is you're observing the can network after you shut the key off. There was
different paths that someone could have taken to get to the solution on this one. But I find it
interesting, the various types of communication and network codes that can set and having a full
understanding of how a particular module is reaching out onto the network to communicate with other
modules and when and being able to look at the systems dynamically like that. And honestly,
the biggest clue on this was the when the codes would set versus when they would not. The key
cycle was the big piece of information. And so you can play around with that a lot too with the
vehicles that you work on is trying to figure out, okay, when did these codes set? And does that tell
me anything about what's going on? Will it, you know, lead me to a different test because of
the when of the code? So something to look at for sure. And I thought this one was pretty
interesting and actually pretty happy with my guys that they got this figured out. I mean,
for the most part, without a ton of input from me, I'd nudged and pushed a little bit here. But
I was I was happy that they got this thing resolved because this is definitely a strange one man made
problem. Those sometimes are the worst, but you know, we got to figure out those out too. So
anyways, I thought you would enjoy that case study. That's where I'm going to wrap this one up. Thank
you everybody for listening. I always appreciate it. With that out of the way, let's get out there,
start fixing the world one car at a time.
About this episode
A 2019 Subaru Impreza arrived with ABS/VDC network communication hard codes and a dash full of warnings, even after the ABS module had been replaced. The team confirmed the replacement module was correct and that most modules could communicate on the high-speed CAN bus. The breakthrough came from code behavior: the ABS “lost communication” codes only set on a normal key-off/key-on cycle, not after clearing while leaving the ignition on. That pointed to an intermittent issue tied to the ABS switched power feed during startup, rather than a true CAN failure.
This week on the show, I share a diagnostic on a 2019 Subaru Impreza that has communication codes setting in the ABS module for several other modules on the car. The scan tool can talk with those modules, as well as the ABS. The ABS has been replaced with the same issue. Listen to find out the cause.
