A powered circuit tester is a tool that doesn’t just “look” at electricity—it can actively test circuits. Because it interacts with the circuit more directly, it can be riskier if you don’t use it carefully.
Term
fused ones
“Fused” means there’s a small safety fuse built into the wire. It helps protect against big electrical damage if something goes wrong, but you still need to connect things correctly.
A logic probe is a tester that helps you understand what a signal is doing, not just whether power is present. It’s used to check electronic control signals in cars.
A three-position switch is a switch with three settings. Each setting connects the circuit differently, which can help you test wiring by sending power or ground to a specific point.
“Grounding” means connecting the test point to the car’s electrical return path. Diagnostic tools use this to see how a circuit behaves when it’s pulled to ground.
Electric and hybrid vehicles use high-voltage electrical systems that change how faults must be diagnosed compared with traditional gasoline cars. Their control modules and power electronics often require specialized testing approaches and safety procedures.
Pico Technologies sells EV diagnostic kits designed for electric and hybrid cars. The point is to give technicians EV-focused testing tools in one package.
High voltage system analysis means testing the EV’s high-voltage parts to figure out what’s wrong. It’s about making safe measurements so you know what to repair.
Installation testing checks that a repaired or replaced EV high-voltage part was installed correctly. It helps confirm the system is safe to use before you hand the car back.
ADAS stands for the car’s driver-assist safety features, like things that help you stay in the lane or brake automatically. After certain repairs, these systems may need to be checked or recalibrated.
A voltmeter measures electrical potential difference (voltage) between two points in a circuit. In diagnostics, it helps confirm whether you have the expected power feed (often around 12 volts in many vehicles) and whether the circuit has a proper ground reference.
An incandescent bulb can act like a controlled electrical “load” when it’s connected to a circuit. That lets you see if the circuit really has enough power to run something, not just show voltage on a meter.
A load is something that uses electricity on purpose while you test. It helps reveal problems that might not show up if you only measure voltage with no real draw.
Milliamps are a measure of how much current is flowing. Saying “800 milliamps” tells you how strongly the tester loads the circuit to check if the power supply can handle it.
Amps measure how much current is flowing. “1.2 amps” means the tester draws a stronger electrical load, which can help reveal power problems that a lighter test might miss.
Cars run on electricity that needs both a power side and a ground side. If you test both, you can tell whether the problem is that the part isn’t getting power or isn’t getting a proper return path.
Power windows are windows that move using an electric motor instead of a manual crank. They’re often used for electrical testing because the wiring and switches are easy to check.
A relay is like an electrically controlled switch. It lets a small signal turn on a bigger electrical load, and testing it helps you find out where the problem is.
An electric motor is the part that turns electrical energy into movement. If you can power it directly and it still doesn’t run, the motor (or its connections) is likely the problem.
Power seats are seats that adjust using electric motors. If they don’t move, testing the electrical supply and ground to the motors can help pinpoint the fault.
Jumper wires are temporary wires you use to connect two points to test a circuit. If you connect them to the wrong place, you can cause a short and damage parts of the car’s electrical system.
A power probe is a tester that lets you send electricity to a specific wire or connector. Instead of guessing, you can “command” a part to see if it works, which speeds up diagnosing electrical problems.
Cars run on a 12-volt system with two sides: the positive side and the ground (negative) side. Testing both helps you figure out whether the problem is power coming in or the return path back to ground.
“Crank, no start” means the starter spins the engine, but the engine won’t actually fire. The fix usually involves checking things like fuel and spark, not just the battery.
The fuel pump is what sends fuel to the engine under pressure. If the engine won’t start, checking whether the fuel pump can run when you command it helps you find whether the problem is electrical or the pump itself.
Most cars use a 12-volt electrical system. If you’re testing a wire or connector and you don’t see the expected 12 volts, that’s a clue the circuit isn’t getting power.
“Internally high resistance” describes a failure inside a component (like a motor) where current can’t flow normally. Even if power is present, the component may not draw enough current to operate, causing symptoms like no fuel pressure.
A resettable circuit breaker is a safety feature that shuts off power if too much current is drawn. After it trips, you can reset it and try again, which helps protect the tool and wiring.
Circuit integrity means the car’s wiring and connections are healthy. If the circuit is damaged or has a bad connection, the car may act like a part is broken even when it isn’t.
A scan tool is a gadget that talks to the car’s computer to find problems. It can be very helpful, but if you don’t interpret the results correctly, you can end up replacing the wrong parts.
DTC means the car’s computer saved a trouble code. The code helps point you in the right direction, but it doesn’t always mean the part you’re thinking of is definitely the one that’s bad.
Misinterpreting scan data means reading the car’s computer info and guessing wrong about the cause. That can lead to replacing parts that were never the real problem.
Concept
brick modules
“Brick modules” means ruining a car computer module so it won’t work anymore. The host is saying that’s possible with extreme misuse, but most of the time the bigger problem is replacing the wrong parts.
FAFO is slang that means “mess around and then you’ll find out what happens.” The host is basically saying you should be careful, because actions can have consequences.
Snap-on is a company that makes tools that mechanics use every day. The host is comparing Snap-on’s version of this kind of electrical tester to others.
Test lights are simple automotive electrical diagnostic tools used to check for voltage on a wire. They typically use a bulb/LED to indicate whether the circuit is energized, which is useful for quick “is power present?” checks.
Term
LED thing
They’re talking about a simple tester that just has an LED that turns on or off. It doesn’t show numbers, so it can be harder to diagnose issues that depend on exact voltage.
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I'm Matt Fonzell and hey, it's 10 feet.
Bob Hope could jump this with his golf cart.
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I just feel like that's the reality is what they need to be doing
is protecting the fighters and letting them fight another day.
And MMA, we don't quite see the CTE levels
that you see in boxing, not that there isn't any, just not to the same level.
A lot of that's to do with how the fights can end very suddenly.
Boxing, you get the eight counts, you get the 10 counts, standing eight counts,
you get 10 to get up, nothing of the sort in MMA.
OK, generally, they don't count to 10.
There's been some instances where they've essentially gotten that.
But that's really not how it's supposed to work.
And I say, let the fighter fight another day.
Are there times that fights get called a little too soon?
Yes, but I think that might be slightly better,
if not substantially better than letting it go too long.
Are there times where fighters were arguably the referee should have
gotten involved and didn't and then that fighter comes back and wins?
Absolutely. So it's a bit of a dichotomy.
I get it. I just tend to lean more towards protecting the fighter
and let them fight another day. Why take on what could be permanent damage?
For what? So in this case, Connor, obviously can't.
And I don't want to drag this out too long.
It's probably going too long already, honestly.
But to not need to do it and to have that drive to train and prepare
and get in there and get potentially beat up, that I mean, that's an amazing thing.
And all I guess the important part to me was and how divisive this was
is where you have the one camp of people, one group of people who are very critical of Connor
shouldn't have thrown that kick, shouldn't have been in there.
And then there's the other side are really actually to finish that thought
screwed us out of a fight, him getting hurt.
He shouldn't have been in there.
He shouldn't have thrown that kick and now we don't get a fight.
And then you have the other side of and probably one that I'm a little more on now.
Maybe I'm just a moron, but one that I tend to lean more towards, at least,
is that he put in a lot of work.
And what's to me, honestly, what's the difference?
I get the context of or what the accepted ways of fight should end or could end.
But what's the difference between Connor running to the center of the octagon,
which could really happen with Max Holloway, throw two punches
and he's laid out knocked out.
I'm sure, yes, the crowd erupts, all the things, maybe booze.
But that would be a more acceptable end than he goes, throws a kick.
Who knows what he did to his knee, ACL, MCL, both all, I don't know, fight ends.
Anyways, I just don't see the difference.
Both times you trained your butt off, worked your butt off.
And you went out there and things didn't go the way you wanted.
To me, it just doesn't matter.
It's just not that big of a difference.
Am I upset we didn't get to see the fight? Of course.
Did I pay a lot of money for a pay per view to see it?
Absolutely not, is I'm paramount.
So at least we don't have that.
And maybe we'll get a part three and see a real fight.
And he heals up from it.
I don't know.
But it is interesting how we forget about that.
And maybe the relation to a repair shop is your mechanical and or technical
specials are really any employee that's put forth effort over years or months
to be able to do something, do something well, and then they fail.
And I don't mean necessarily like fail over and over in that you cannot
probably can't do this for a living or at least or here.
You need more training or a different environment to develop these skills.
I'm just saying they mess up and fail.
And what camp are you in?
Are you in the group of very critical?
Or you're in the camp of, man, they worked their tail off to be able to do that.
And it went sideways.
I don't have an answer for you.
It's one. It's just something to think about.
Now that my idea for an episode of what to talk about, which I know I've been
on this topic now for long enough, 10 minutes, maybe not quite 78 minutes
that I guess I want to kind of talk about circuit testers, powered circuit testers.
The reason I use that word or verbiage is technically I think it's like
Kleenex or vice grips where power probes are named that from the company power probe.
And do they fit into the diagnostic arsenal or do they should they be in shops?
So should technical specialists be using these?
I say yes. Are they dangerous?
Sure. So are jumper wires.
So are jumper wires. They're dangerous, too.
We were using them, even fused ones.
So power probes or circuit testers are fused as well.
I guess we might have to separate into maybe two groups.
One is the power probe, as I think we would think of them, right,
where there is kind of a logic probe, maybe has a meter in it,
has a switch, like a three position switch, if you will,
that there's the neutral state and then one way is put 12 volt power
to the tip or go the other way and it grounds the tip.
That would be a powered circuit tester, maybe.
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The other group maybe would be like a meter
voltmeter, Tesla digital meter, Tesla digital, something of that nature.
There's two that pop into my mind that fit into that one is a little
a Lyle unit and the other one is the top down CT 100
where they have a lead and you can clip that onto power or grown.
And then when you touch whatever circuit, it gives you a meter reading.
And then there's a voltmeter reading and it might glow red or green,
depending on if it's power feed or 12 volts there near 12 volts and then ground.
The other thing is they usually have a button or both of them have a button.
In these cases, the Lyle and the top done, they have a button on there.
And inside they have some sort of bulb, if you will, an incandescent bulb, a load.
The Lyle is, I think they've rated that around 800 milliamps, something to that
degree that is the load it will apply to the circuit.
And the top down I want to say is like up to just over an amp, like 1.2
amps, something like that.
I like both of them.
I really do.
I use both of them.
I like the meter reading.
I like being able to put a load on there and knowing that that loads kind of in
that one amp range roughly.
I think that's very handy and it's telling me more than just a straight up incandescent
test light.
I have very little need or use for an LED test light.
I have one for really just in case where I don't want the load on it.
This is very, very rare, meaning I don't know if I've used it in the last year or
even two.
I just have it there, just in case I come up with an idea of a very, very light
load, no load type of a test, something I probably just go grab a meter for.
On the flip side, these things, it's kind of fast.
Honestly, in testing fuses, applying that load helps you find issues.
And I know just in circuits in general, but I guess I bring that up because that's
kind of the first thing I grab for checking fuses is one of those two meter
test lights is because it's fast, you know, clip to better negative and then start
bouncing down fuses like you would with a test light.
But now I see a meter reading and I can apply a little bit of a load.
I like that.
I like knowing the voltage and I like putting that load on there because I
don't know about you in Minnesota.
Sometimes fuses get some corrosion in there.
And if you check it with just a meter, it'll pass.
Unless it's, you know, that circuits for sure loaded, then sure.
But in certain cases, sometimes you can get burned.
And this helps you not get burned.
I like it a lot and I use them every day for sure.
Every day they're getting used one way or the other.
The back to the power probes and low powered circuit testers.
I have the power pro version.
I prefer the straight up three, you know, the original three, if you will.
I know they've got so many variations on that.
I get it like some of the features are kind of neat, but I like the speed
and simplicity of the original three or not the maestro or I think they got.
I mean, there's a bunch of them.
I think there's like an easy and I don't know.
I just think you get away from the formula that really worked.
I get what the appeal might be, but it kind of slows them down, starts
to overcomplicate them.
I just like grabbing the dang thing, touching it, you know, whatever surface
if you will, a circuit, a wire, a conductor and seeing the voltage there.
And then depending on what I'm doing, applying power or ground,
testing power windows, man, that's so great.
Testing relays, I can apply power, ground, both if I want.
There's a kind of an alligator nearby that's ground all the time.
I can flip flap directions for testing electric motors or, you know, power seats.
Is there danger? Yeah, just like I said, though, jumper wires, there's danger.
You touch the wrong circuit with a jumper wire, you're smoking something.
No different than a power probe.
Power probe makes it a little too easy, but I don't know.
If you understand what it's doing, there's such a time saver.
And the long lead to go right to battery, you know, positive and negative.
Now you're checking all over the car.
And depending on the circuit, you can load test.
I mean, it's not fictitious because I do it, but just as an example,
I have a crank, no start, no fuel pressure.
I'm at the fuel pump.
I got my power probe, power feeding ground right there.
At a flip of switch, I can apply power.
I can apply ground.
I can check like I can command the fuel pump on.
Be getting 12 volts there.
Put my power probe on there, see 12 volts.
But if something's messed up with the motor where it's not drawing,
you know, internally open, internally high resistance,
I apply ground to there.
That's right, apply ground.
I think the power probe three, I think is around eight amps,
resettable circuit breaker, it pops.
I'll do it again, reset it, do it again, reset it, do it again.
I know that circuit can carry eight amps.
Probably got good power.
Then I move it over to the ground and I apply power. Boom.
Does it pop? Yep.
I probably have a good enough circuit integrity.
Probably a bad fuel pump.
That's what I do.
That's such a time saver.
Are there other ways to do that? Sure, sure.
Absolutely.
But they kind of get a bad name because, yeah, we've seen what they can do.
In the wrong hands, we see what a lot of things can do.
In the wrong hands, a scan tool can be extraordinarily expensive
with parts being replaced that aren't bad,
be it DTC is essentially leading to a part number.
You know, not that they actually spit out a part number, but you see
as DTC and equals a part replacement.
That could be a very expensive misuse of the tool
or looking at scan data and misinterpreting it leads to parts replacement.
I suppose there's the argument one is misreplace, you know,
making a mistake, replacing a part that doesn't need to be replaced.
You're not breaking anything.
OK, but it's still misuse,
misunderstanding, something like that.
And can you break something with scan tool?
I suppose we could really work really hard and come up with some scenario
where it's possible brick modules, I suppose, with misuse.
But the reality is in the wrong hands, there's a lot of things that can do damage.
So I just don't know that that's the way to argue against them.
And if you don't know, it's a good reason to learn.
So, yeah, for me, I love that tool.
I use it all the time.
As far as I know, I haven't wrecked anything, but I try to be aware.
And I'm sure now that I've said that tomorrow, I am wrecking something.
So I find a way to wreck stuff anyways,
usually because what's the worst that could happen?
And then I find out, yeah, FAFO.
Auto makes a powered circuit tester.
I have that. I do like that.
It works quite well.
I think there's a bunch of them now.
The markets getting flooded, the snap on one.
I do not like the design.
It probably works just fine.
But the design to me is ridiculous.
It might be even built by Power Pro for all I know,
but I don't like where the displays at at all.
The regular power pros make sense to me for the newer ones.
I think it's just like a four.
Again, it just start adding more and more features.
I get why, but really the three to me is kind of where it's at.
That's the one I suggest co-workers get or what we get for co-workers
when they're at that level and they're ready for that.
And then, yeah, the digital meter
test lights, namely Lyle and top down are the ones I'm aware of.
I'm a big fan. I'm a really big fan.
And I don't necessarily have a suggestion
for which power probe to get.
I mean, like I said, the auto ones.
Priced right and does what I needed to do.
And the power probe one's been around for a long time and it works.
For me, that was leading up to that one.
I couldn't see the point of getting one.
The just the LED thing doesn't work for me.
I want to see numbers.
When they put the numbers in the display, that became that became
I don't know, must have, but I really wanted it and I still use it.
So that is my thoughts on powered circuit testers.
And I guess digital meter test lights.
So, yeah, I am very interested to hear the feedback on this one
because it seems to be a somewhat controversial topic.
Yeah, I want to hear where I went wrong on that or how many of you agree
or if you have a different favorite.
I have a hook power probe hook.
I use it sparingly, right or wrong.
I just I grab the simple ones.
I find that they do what I need them to do when I need them to do it.
That's what I need. like.
If I need more, then I'm probably busting out scopes.
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About this episode
The hosts kick off with a sponsor pitch for advanced measurement tools, then dive into the “controversial” side of automotive electrical diagnostics: powered circuit testers, power probes, and how to use them safely. They compare load-based fuse testing versus simple meter checks, explain why numeric readings matter, and walk through practical workflows like commanding a fuel pump during crank/no-start. Along the way, they warn that scan tools and DTCs can be misused into costly parts replacement, and that jumper wires can cause damage.
Powered circuit testers tend to produce strong opinions. Some specialists consider them indispensable time-savers. Others see them as dangerous tools that make it far too easy to damage a circuit.
In this episode, Matt examines where tools such as the Power Probe, Autel powered circuit tester, and metered test lights fit into an automotive diagnostic process. The ability to apply power or ground can be dangerous without an understanding of the circuit, but the same can be said of jumper wires, scan tools, oscilloscopes, and nearly every other useful diagnostic tool.
Matt also opens the episode with thoughts on the unexpectedly short Conor McGregor and Max Holloway fight. Rather than focusing only on the disappointing result, he considers the discipline and preparation required to attempt something difficult, even when the attempt ultimately fails. That leads to a broader shop question: How should a leader respond when a capable employee puts in the work, takes on a difficult task, and still comes up short?
In This Episode
The difference between a powered circuit tester and a metered test light
Why Power Probe-style tools remain controversial
The risks of applying power or ground to an unknown circuit
Why Matt prefers the speed and simplicity of the Power Probe III
Using powered testers on motors, relays, power windows, and power seats
Testing fuel-pump power and ground circuits
Using the tester’s circuit breaker to evaluate whether a circuit can carry current
Why unloaded voltage readings can be misleading
The value of placing a known load on a circuit
Why corrosion can pass a voltage test but fail under load
Matt’s experience with the Lisle and TOPDON metered test lights
Why an incandescent test light is often more useful than an LED test light
The difference between a dangerous tool and an improperly used tool
Key Takeaways
A voltage reading does not necessarily prove that a circuit can perform useful work. A connection may show battery voltage through a meter and still fail when current is required.
A metered test light can combine the speed of a traditional test light with an actual voltage reading and a known electrical load. Matt regularly uses this type of tester when checking fuses and evaluating circuit integrity.
A powered circuit tester can also shorten the process of separating a failed component from a power-feed or ground problem. Matt describes using one at a fuel pump to verify the power side, ground side, and pump operation without repeatedly changing tools or building several different jumper-wire arrangements.
The tool does make applying power or ground unusually easy. That convenience is precisely why the user needs to understand the circuit before pressing the switch.
The Larger Question
Should a tool be dismissed because it can cause damage in the wrong hands?
A powered circuit tester can damage a circuit. A jumper wire can do the same. A scan tool can lead to expensive parts replacement when its information is misunderstood. The potential for misuse does not necessarily make the tool inappropriate. It may mean the specialist needs more knowledge before using it.
Listener Question
Do powered circuit testers belong in a professional diagnostic arsenal?
Are they valuable time-savers, unnecessary risks, or a little of both? What version do you use, and where has it proven most useful?
Mentioned in This Episode
Power Probe III
Power Probe Hook
Autel powered circuit tester
Lisle metered test light
TOPDON TC001/CT100-style circuit tester
Incandescent and LED test lights
Fuel-pump circuit testing
Relay and electric-motor testing
Thanks to our Partner, Pico Technology
Are you chasing elusive automotive problems? Pico Technology empowers you to see what's really happening. Their PicoScope oscilloscopes transform your diagnostic capabilities. Visit PicoAuto.com
Thanks to our Partner, Autel
From drivability diagnostics and TPMS service to ADAS and advanced safety systems, Autel helps technicians follow OEM procedures and repair with confidence. Learn more at Autel.com
