What really is involved in electrical diagnostics

When most people hear the word diagnostics in relation to cars, they picture “plugging it into a computer” and the computer magically pointing to the faulty part. I wish it were that simple. In reality, a diagnostic scan tool (the proper name for that “computer”) only reads data from the onboard control units: fault codes, live data, and sometimes freeze frame information. In about half of the cases, the component named in a fault code really is the faulty part. That’s why when a beginner first uses a scan tool and happens to get one of those “easy” cases, they walk away with a false sense of confidence — as if the tool itself found the problem. But in the other half of the cases, things are not that straightforward.

The Case: 2008 Dodge Nitro

This Nitro came in with a full light show on the dash, wipers running nonstop, and a laundry list of fault codes across multiple modules. The codes didn’t say “there’s a broken wire under the passenger seat (white with an orange stripe)”. Instead, they were vague: “lost communication,” “low voltage,” “CAN bus fault.” The instrument cluster even displayed “no bus” where the odometer reading should be.

The customer had already tried swapping the TIPM module (the big fuse/relay unit under the hood). That only made things worse — nothing worked with the replacement TIPM. He reinstalled the original unit and brought the vehicle to me.

The Data Bus

If you imagine a vehicle data bus as a fancy wiring harness, you’d be disappointed. It is nothing like you see inside or outside of a computer system:

In a car, "data bus" is just a couple of twisted wires. Since the modules only swap short messages and not big files, those wires do the trick.

Since there were several signs pointing to issues with the data bus and communication, I kicked things off by setting up a digital scope to check out the CAN INT signal waveform. Typically, when you're reading voltage from those two twisted wires, the CAN bus signal waveform should look like this (red and white graphs).

This waveform represents a single message being transmitted on the CAN bus, taking just 230 microseconds to appear on the wires. In other words, about four thousand such messages could be sent in one second over a network consisting of two simple twisted wires. This is why I need to use a scope (and a high-quality one, at that) to diagnose CAN bus issues: distinguishing between good and bad signals is impossible with a multimeter or a test bulb (connecting a test bulb to the CAN network will actually shut it down, similar to shorting the network wires to the ground). Even a cheap scope with a low sample rate won't suffice. Here is another waveform from a different CAN bus operating at a slower speed. Each of those short signal peaks represents the same messages as before, but at a different zoom level (zoomed out to 1:500 here compared to 1:1 in the previous screenshot). This waveform fragment spans about 200 milliseconds—one-fifth of a second—and shows that 21 messages were transmitted in this short period. These messages represent real-time data sent from the TIPM module to a scan tool when reading live data from the module. They are not the messages exchanged between onboard vehicle modules; those would have a much higher density and thus be less "visual" in this example.

Now, here is the waveform on the CAN INT bus on the Dodge Nitro I am working on today:

The modules linked to the network cannot identify this chaos as legitimate messages, preventing them from communicating with one another. Consequently, nothing functions correctly on that network anymore.

The Grind

The typical next step in this scenario is to disconnect each module from the network one at a time until the signal returns to normal. If the signal returns to normal after disconnecting a specific module, that module will be deemed faulty. It seems fairly straightforward.

This isn't a minor task. The units in this network are spread across the vehicle's interior: the instrument cluster, HVAC control module, radio, front door modules, audio amplifier, and the DVD screen, if that feature is included in the specific trim level.

Locating, removing, and disconnecting all the modules will require significant effort. Some modules might appear on a diagram but not actually be present in the vehicle, so it's important to carefully verify each module's presence or absence. Therefore, I've decided to begin by inspecting the most probable locations for wiring issues—specifically, the front door wiring harnesses and their connectors. These areas, where wires transition from the body to the doors, are prone to damage due to years of constant bending, which can cause the wires to break or short-circuit.

After struggling with the extremely tight placement of the wires and connectors, I discovered some corroded pins on the passenger side. However, disconnecting both door harnesses still didn't bring back the normal network signal pattern. That's frustrating... The next step was somewhat drastic: I severed the network wires at the TIPM connector and examined the network signal on both sides of the cut. The signal returned to a normal pattern on the TIPM side, indicating that the issue lies on the opposite side, not within the TIPM. Consequently, it is now unavoidable to proceed with disconnecting all the network modules.

So now here I am, taking out all the seats, the center console, the center dash panel, and the instrument cluster...

...trim panels around the door openings and the large right trim panel in the trunk, since this is where the audio amplifier (the large box circled in yellow) is situated. I need to access it to test the network with the amplifier disconnected. If there are other modules in certain trim levels, they are generally installed nearby as well.

During this inspection, I found a lot of rust, moisture, and dirt under the floor mats where the large wiring harnesses are situated. I also noticed a couple of ground connections that appeared to be in bad condition. Any of these issues could lead to problems; where should I even begin?

Unfortunately, after all the effort to access the control units, disconnecting any single one didn't change anything! The signal only returned to normal when all of them were disconnected simultaneously. However, reconnecting any combination of those units to the network caused the signal to become erratic again.

Could it be a wiring issue? The moisture and rust surrounding the wires likely caused some damage. Checking the resistance between the network wires hasn't revealed any problems or irregularities. There were no short circuits between the wires, nor any short circuits to ground or power. Consequently, it's time to begin stripping the wiring harnesses in those damp and dirty areas, in hopes of discovering a corroded splice or a decayed, broken wire.

No success here either! The wires were dirty but undamaged. I also addressed the potentially faulty grounds under the driver's seat by using a jumper cable to connect them to a good ground, but this didn't make any difference.

I also had the other TIPM that the customer attempted to install. Although that unit didn't permit the ignition to turn on, the network signal stayed intact, regardless of whether other modules were connected to the network. Could the original TIPM be defective? Is it possible that something failed within the TIPM module, preventing it from supporting network operations when additional "load" is connected to the network?

I disassembled the original TIPM as far as I could, looking for any signs of moisture, corrosion, or burnt components. Nothing — it looked clean.

A new TIPM costs about $2,500, which makes it a tough call. What if my suspicion about an internal fault was wrong? The safest advice at that point was to try another used unit from the same year, model, and trim. Maybe that would work?

Meanwhile, things took a turn for the worse. While I was digging through all this, the car quit operating the starter and threw a code for loss of communication with the Vehicle Security Module. Great. It had at least been running when it came in — now it was completely dead.

Ugh! It is one of those days when I really hate this job...

By the time another TIPM arrived, I thought of a new idea: let's check the network signal at the TIPM and simultaneously at several other points on the CAN INT bus. There might be a difference in signals after a specific location, and perhaps this could help me identify a wiring fault.

The Breakthrough

While waiting on another TIPM, I decided to compare signal waveforms at different points along the network. And finally — luck. A difference showed up between two test points.

The culprit: a single rotten wire under the passenger floor mat. Corroded from the inside out, it looked fine outside but had broken inside the insulation. I spliced in a fresh twisted pair, soldered and sealed it. The network came back to life.

That was a win — but the car still wouldn’t start. Time to dig deeper...

I notice that there is currently no communication between the TIPM and the Vehicle Security Module, Airbags Module, and Occupant Classification Module. These modules are all connected to a different network, the CAN C network, along with several other modules.

Was there any common circuit for these three modules? A shared fuse, a shared ground? Nope — nothing in common except the network connection. So I tried the same trick I used earlier on the CAN INT bus: comparing waveforms at different points. And sure enough, there was a clear difference. The signals looked healthy but didn’t match, almost as if they were coming from two separate networks. That meant part of the bus had become isolated from the rest.

Tracing the wires again took time — especially since their colors changed along the way and the wiring diagram didn’t reflect it. Frustrating. But with the harnesses already stripped out and loose, I eventually found the break. And here’s the kicker: apparently, a Dodge Nitro won’t start if the passenger seat connector is unplugged, because someone thought it was a great idea to splice the network cables inside the seat harness.

For real, dude? What was the thinking here? And why isn’t that shown on the wiring diagram? Probably the logic was about limiting branch length — the short piece of wire that runs from the main CAN bus to each individual module. In data networks like CAN, branch length is critical: if it’s too long, the signal can reflect back into the main line, causing interference and communication errors. By splicing the network inside the seat harness, the engineers kept that branch very short and technically within spec.

From an engineering standpoint, it makes sense. From a serviceability standpoint, though, it’s a nightmare. The wiring diagram gave no hint that such a splice existed inside the seat harness. Without that knowledge, a technician could waste hours — or days — chasing what looks like a random communication fault, when in reality the “broken network” is just a disconnected seat. This kind of undocumented design choice is exactly why diagnostics can turn into such a time sink.

I put the passenger seat back in, reconnected the harness, and — It’s alive! Like a fossil beast dragged out of extinction and jolted into motion, the Nitro came back to life: engine started up, codes cleared, lights gone. Finally!

Wrapping it up

Do you think it was done at that point? Not quite. All the stripped wiring harnesses had to be wrapped back in tape, secured with proper holders, and routed correctly so they wouldn’t get damaged again. I also re-routed the bad grounds to a dry, reliable location, cleaned out the rust and debris, and reassembled everything: interior trim, seats, carpets, the whole lot. More hours of work — but finally, the Nitro was ready to be billed out.

And here’s the worst part: I spent 16 hours chasing down a single broken wire. On paper, that sounds like a quick and simple task — “just find the bad wire and fix it.” In reality, it turned into a two-day marathon of stripping out interior panels, lifting carpets, disconnecting modules, probing harnesses, comparing scope waveforms, and testing one dead-end theory after another.

That’s the hidden truth of electrical diagnostics: the repair itself took maybe 10 minutes once the fault was found. But finding that invisible break, buried in a harness under the passenger floor mat, consumed two full working days.

No Estimates on Diagnostic Work

Situations like this are why I don't offer fixed estimates for labor time or costs on diagnostic tasks. Identifying the root cause of a complex electrical fault, an intermittent drivability issue, or even an interior water leak is fundamentally unpredictable in terms of required labor time.

In a way, diagnostics is a lot like scientific research. We’re not discovering a cure for cancer here — we’re working on restoring a vehicle to proper operation. But the process of discovery is essentially the same: form a hypothesis, test it, analyze the data, and keep going until the true cause is identified. And just like in research, rigid timelines can’t be set on when the answer will appear.

Past experience can point me in the right direction, but it doesn’t guarantee a quick solution. For example, with network communication problems, the “typical” pattern might be: check for moisture under the floor mats, dry everything out, repair the rotten wiring, and the issue is resolved.

However not every case follows the pattern. Sometimes the fault is hidden, takes much longer to uncover, and may even lead to unnecessary parts being tried along the way (hopefully used, not brand new). What sounds like “just finding a broken wire” can easily turn into a two-day job.

Naturally, the value of the vehicle also has to be considered. Spending more on labor than the vehicle is worth is usually impractical. But that doesn’t mean the technician’s time and expertise suddenly lose value. Even if the outcome leads to a decision not to proceed with repairs, the diagnostic work already performed represents the cost of real labor and knowledge — it cannot simply be given away for free.

The real paradox of diagnostics is that it’s like solving an equation with an inherently unknown variable: does the value of the vehicle justify the diagnostic expense when that expense cannot be predicted in advance? That uncertainty is built into every diagnostic job, and it’s what makes this kind of work both so challenging and so difficult to “estimate” in the conventional sense. In practice, the question often becomes: should we invest in diagnosing and repairing this low-value vehicle, or is it more sensible to send it to the scrapyard? The trouble is, that decision usually can’t be made until after a significant amount of diagnostic work has already been done. Sometimes the problem is straightforward and well within the vehicle’s value; other times, the hours add up quickly, and the repair cost edges beyond what the car is worth. This is why diagnostics requires a different mindset than routine maintenance or mechanical repairs. It isn’t just about turning wrenches — it’s about navigating uncertainty, balancing harsh economic reality with the excitement of technical discovery, and then clearly communicating the findings to the customer. After all, these are their vehicles, not mine, and the decision of whether to proceed ultimately rests with them. My role is to provide the best possible information so they can make that call.

Jeez… I really did get myself into a job that’s far too hard for what it pays...