359: 2019 Subaru Impreza No Communication Diagnostic
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.
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2019 Subaru Impreza
"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."
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.
The 2019 Subaru Impreza is a compact car known for its AWD-focused platform and electronics-heavy systems, including ABS and multiple control modules. In this case study, the focus is on diagnosing ABS-related communication problems across the car’s modules.
option model code
"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."
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
"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."
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.
Network codes are ABS- and module-related diagnostic codes tied to the vehicle’s data network (how modules exchange messages). If the network isn’t functioning correctly, multiple modules can report related faults even if the underlying hardware is otherwise okay.
ABS module
"one, the ABS module that was in there, not only matched the one that was coming out, but also was correct for the vehicle."
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.
The ABS module is the car’s anti-lock braking controller. It monitors wheel speed and manages brake pressure during hard braking to prevent wheel lockup, and in this case it’s also reporting that it can’t communicate with other modules.
network communication issue
"So this was a network communication issue with codes in the ABS or the VDC, they call it."
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.
A network communication issue means the car’s electronic control units (ECUs) can’t exchange messages over the vehicle network. When that happens, multiple systems can log “lost communication” faults even if their hardware is otherwise intact.
steering angle sensor
"lost com with the steering angle sensor, and lost com with eyesight."
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.
The steering angle sensor measures how much and how quickly the steering wheel is turned. Stability and driver-assist systems use it to estimate vehicle direction, so losing communication with it can trigger multiple network fault codes.
hard fault codes
"And those were current, hard fault codes. There were some other ones in history as well."
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.
Hard fault codes are diagnostic trouble codes that represent active, current failures rather than past or intermittent issues. The host contrasts these with history codes, emphasizing that the communication faults are happening now.
Eyesight system
"lost com with the TCM, lost com with the steering angle sensor, and lost com with eyesight."
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.
The Eyesight system is Subaru’s driver-assistance suite that uses cameras/sensors to monitor the road and support functions like collision avoidance and adaptive cruise. In this diagnostic, Eyesight is logging communication faults with other vehicle modules.
electric power steering
"So the ones in the eyesight said, ... eyesight communication, parentheses, VDC ... and eyesight communication, EPS, which is electric power steering."
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.
Electric power steering (EPS) uses an electric motor to assist the steering effort instead of a hydraulic pump. In this segment, EPS is one of the modules that logs communication-related faults alongside ABS and Eyesight.
U0100
"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."
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.
U0100 is a standardized OBD-II diagnostic trouble code that indicates a communication problem between modules. Here, the host lists U0100 (along with other U-codes) as the ABS reporting it lost communication with other systems.
U1235
"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."
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.
U1235 is another standardized communication-related OBD-II code. In this segment, it’s grouped with other U-codes as evidence that the ABS module is unable to communicate with other control modules.
VDC
"it's a B28A2 and a B28A4, which says eyesight communication, and then in parentheses says VDC [352.8s] and EPS."
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.
VDC is Subaru’s stability-control system. It monitors vehicle behavior and helps correct loss of traction by applying brakes and/or adjusting engine output.
EPS
"it's a B28A2 and a B28A4, which says eyesight communication, and then in parentheses says VDC [352.8s] and EPS."
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.
EPS stands for electric power steering. It’s the steering system controlled by an electric motor and sensors, and it communicates with other modules over the car’s network.
data stream codes
"And then same thing for the [396.5s] power steering, can I communicate with the power steering? I can. Now it says eyesight system [401.3s] abnormal, as far as a code in the power steering, but I can talk to it, 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.
Data stream codes refer to diagnostic codes that are tied to what a module reports in its live data (the “data stream”) during scanning. If the live data is implausible or inconsistent, it can trigger communication or system-abnormality codes elsewhere.
diagnose those codes first
"Okay. If you continue to follow the code, right, it says, check to make sure that you can talk to [415.8s] these modules. Okay. If you can, it says, are there codes set in these modules? And if yes, [422.9s] it says to diagnose those codes first."
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.
This is an important diagnostic workflow: when a scan tool shows an “eyesight communication” symptom, you don’t assume the eyesight module is the root cause. Instead, you verify network communication with the related modules (VDC/EPS/ABS), check whether they have their own stored codes, and diagnose those first before returning to the eyesight-related code.
U code
"Okay. And that's a lot of network codes that we see that [427.9s] are like implausible or invalid data, which may even set as a U code."
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.
A U code is a diagnostic trouble code related to communication between modules on the vehicle network. Instead of a sensor being “bad,” the issue is often that one module is sending invalid data or not communicating correctly.
ABS unit
"So right now, I am right back to the ABS unit. And I could kind of forget about the other codes... Okay, modules already been replaced. Now, something to consider here..."
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 ABS (anti-lock braking system) unit is the controller that manages wheel-speed sensing and brake pressure modulation to prevent wheel lockup. When communication or hard faults point to the ABS module, it often indicates the ABS controller isn’t responding correctly to the rest of the car.
ECM
"the modules it says it can't talk to the ECM, the TCM, the steering angle in the eyesight... And like the ECM and the TCM, they don't actually have codes saying that anything's wrong with communication."
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.
ECM (engine control module) is the car’s main computer for engine management. In diagnostics, “can’t talk to the ECM” points to a network or module communication issue between the ABS and the engine computer.
steering angle in the eyesight
"the steering angle in the eyesight, I can talk to all of them minus the steering angle sensor... It shares data over the CAN bus, but you're not directly talking to it on this car."
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 steering angle sensor provides the car’s steering input angle to driver-assistance systems (here, “EyeSight”) and other controllers. The host notes they can’t communicate with the steering angle sensor directly with their scan tool, even though the data is shared on the vehicle network.
high speed CAN bus
"In fact, all of these modules live on the same high speed CAN bus for this vehicle. They all share the same bus."
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 CAN bus (Controller Area Network) is the vehicle’s internal data network that lets modules exchange information. A “high speed CAN bus” is the faster backbone used by many control modules, so communication faults often trace back to network, power/ground, or module behavior on that bus.
DLC
"using my scan tool, which is also hardwired onto the high speed bus at pins 6 and 14 on the DLC."
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.
DLC (data link connector) is the standardized diagnostic port where a scan tool plugs in. The host mentions the scan tool is hardwired to the high speed CAN bus at specific DLC pins, which is why they can access multiple modules.
powers and grounds
"of course, the next natural place that you would go is to check powers and grounds at the module. So those were checked."
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.
Power and ground are the electrical supply and reference connections that control modules use to operate and communicate. When diagnosing module communication issues, checking power and ground at the affected module is a common step because weak connections can mimic “network” faults.
control module programming
"along with some free resources there as well on information about used control module programming."
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.
Control module programming is the process of configuring a vehicle’s electronic control unit (ECU) so it matches the car’s hardware and software needs. In practice, it often includes loading the correct calibration and enabling the module to communicate properly with the rest of the vehicle.
power feeds
"we actually have three power feeds into this module. Two of them are constant and one of them is switched."
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.
Power feeds are the separate electrical supply lines that provide voltage to a module. The transcript distinguishes constant power (always hot) versus switched power (only hot with ignition), and checking them is a common step to rule out electrical causes of module communication failures.
headlight bulb
"you know, load test them with a headlight bulb. Looks good."
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.
Using a headlight bulb as a load test is a simple way to verify that a circuit can supply current under load, not just show voltage. In diagnostics, it helps confirm that power feeds are truly capable of powering the module.
communication codes
"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."
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.
Communication codes are diagnostic trouble codes (DTCs) that indicate a module isn’t communicating correctly with the rest of the vehicle over the network. They often appear when a module drops off the bus, responds incorrectly, or only fails under certain conditions.
key cycle
"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."
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.
A key cycle is the sequence of turning the ignition off and then back on, which causes modules to reboot and re-initialize. Some communication faults only appear after a specific key cycle pattern, which can be a key clue during diagnostics.
BCM
"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."
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.
open somewhere between
"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."
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.
An open circuit (often described as “open somewhere between”) means the wire path is broken or not electrically connected, so current can’t flow. Here, the technician found an open between the ABS module and the yellow wire coming out of the BCM.
parasitic draw
"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?"
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.
A parasitic draw is electrical current that continues to pull power from the battery when the car is off. In this case, the circuit was modified so constant power stayed on, which can drain the battery and also complicate module behavior.
ignition feed
"So the fix was just repairing this wire so that the switch ignition feed got to the ABS module where it was supposed to."
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.
The ignition feed is the switched power supply that should be present only when the ignition is in the appropriate position. The fix described is repairing a wire so the ignition feed reaches the ABS module correctly, allowing it to power down when the key is off.
body network
"there are some networks on vehicles, and it seems to be mostly body can or body networks... But there's a lot of networks that if you have traffic on that bus..."
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.
Body networks are CAN sub-networks that handle convenience and body functions (like lighting, locks, and other comfort-related modules). The host contrasts these with the high speed CAN behavior, noting that some body networks may keep talking longer or wake on voltage changes.
TCM
"the engine control, the TCM, the eyesight module, and then the steering angle all shut down correctly at when the key was off..."
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.
TCM stands for Transmission Control Module. It controls automatic transmission shifting and related functions, and in this diagnosis it’s mentioned as one of the modules that shuts down correctly when the key is off.
voltage fluctuation
"there are body can 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."
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.
Voltage fluctuation is an unintended change in electrical voltage on a circuit or network line. The host says some CAN networks can wake up or keep communicating when they detect voltage changes on the CAN line, even with the key off.
scope that 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."
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.
“Scope that network” refers to using test equipment (typically an oscilloscope) to observe the electrical signals on the data bus. In diagnostics, scoping helps confirm whether the bus traffic looks normal during driving and whether it continues after key-off.
CAN network
"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."
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.
The CAN network (Controller Area Network) is the car’s internal data bus that lets modules—like ABS, engine, and body electronics—share messages. The diagnostic clue here is observing CAN activity after the key is off, which can reveal a module that isn’t shutting down correctly or is misbehaving on the bus.
key was off
"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."
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.
“Key was off” is used as a diagnostic condition to check whether modules and the CAN network properly go to sleep. If there’s still bus activity after shutdown, it suggests a module is not entering its expected low-power state or is continuing to transmit.
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