• Troy Wierenga

11 shortcuts to an OBDII diagnostic rabbit hole


“Beware of false knowledge; it is more dangerous than ignorance.”

- George Bernard Shaw



Assumption taken as fact is the root cause of diagnostic mishaps. The following is a list of the most common automotive diagnostic pitfalls for beginners.


1. You replaced a suspected electrical part and assume it’s good.

You diagnosed it correctly, replaced the faulty part but to no avail. Self doubt and confusion sets in. Remember only those parts from a known good running vehicle are proven good. Acronym for NEW - Never Ever Worked.


2. You install a junkyard part, see number 1


3. Your wire harness wiggle test should be classified as a yank and twist test. You now have more than 1 problem to sort through.

Mimic engine movement and vibration, no more. It's easy to create another problem without realizing it. When you finally find it and fix it, it may take a little bit to realize you're back at square one.


4. You find something else isn’t working and assume it's all connected.

Sometimes it is, but never eliminate the possibility of separate faults. Someone once replaced a brake light switch for no brake lights. Then they replaced the multi-function switch. Still no brake lights. It ended up being 2 burnt out bulbs. “I didn’t even think to check the bulbs since they were both out.”


5. Your scan tool will not communicate, or it keeps losing communication. Bad module theories lead you down a time consuming and possible expensive path.

Try a different scanner first. Some vehicle and scanner models don’t get along too well. I spent too long one time checking PCM powers and grounds due to my scan tool acting up. I was pissed.


6. Believing coincidences are not a coincidence, and assume you found the fault.

There was a guy who had replaced his starter for an intermittent free wheeling of the starter. After replacement, the symptoms remained. He showed me where he had taken the panel loose by the ignition switch and said "If I reach back in here and wiggle these wires I can get it to start." Those wires had nothing to do with it. It was a defective rebuilt starter. See number 1



7. You clear the code. A few days later the light is back on with the same code stored. You assume it’s an intermittent fault without thinking about monitor completion.

Depending on the code, such as p0420 or p0430, some monitors need to see a list of operational criteria, or drive cycles before the PCM will even look for that problem. If you start thinking " it's intermittent", it is easy to become biased toward an electrical fault, when it might not be.


8. You see live data values that 'seem' off. You can’t verify what known good is and assume it’s a clue towards the root cause.

Many sources tell you that when in doubt, check a known good similar vehicle. Most of us do not have these sitting around. Pidfusion.net is a database for checking your live data values to similar known good vehicles.


9. You have checked powers and grounds with only a test light. You assume they’re good and no longer suspect this.

When testing circuits, make sure to load test them. Speaker wire will have continuity, but you can’t jump start your car with it. I use a headlight to load test suspected wiring.


10. While working on someone else’s vehicle, they only give you part of the story. You assume it’s the whole story.

I’ve spent hours testing systems, components just to find out that one little bit of information would have cut the possibilities down significantly. “Oh, I’m sorry, I completely forgot to tell you that it only does it when it's 20 degrees or below outside…”

Unfortunately some people see a scan tool as a handheld kiosk with little human intervention needed. "Don't you have one of those thingies (scantool) that you plug in to see what's wrong?"


11. Your scan tool shows a PID value for something not supported by the vehicle, and you assume it is.

Many of the OBDII apps allow you to select PIDs you want logged without being connected to a vehicle. IF you select a PID that's not supported, you may get some value that doesn't make sense. Also many vehicles will report that short term fuel trim sensor 2 is supported and gives a value of %99.2 all of the time. If you see this, just know this means that O2 sensor 2 does NOT affect your fuel trim. Ref - (if B==$FF, sensor is not used in trim calculation)



Whether it's a check engine light with vague codes, many codes, rough idle or low power and no codes, remember the saying, "Assumption is the mother of all !&%# ups.."




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