I am sure some on here remember my problems with Code 18 and Code 19 on my '93 Buick LeSabre. The manual listed the fault for those but the problem had nothing to do with the code that was generated. In my case, the Code 18 was a misfire, a bad coil pack. The Code 19 was a misfire caused by faulty ignition wires. The manual had completely different reasons for those codes. My model LeSabre does not set a code for a faulty coil pack but the computer saw an error and set a code for it in some way that it determined Code 18, and then later Code 19. I wasted a lot of time trying to track my problem down bases on those Codes and I was following the troubleshooting guide from the GM shop manual. I was lucky that I finally found a very experienced mechanic that was willing to help me out. By the way, the Just Answer guys are worthless.

My sister had a Ford SUV that showed a trouble code for the oxygen sensor. Her garage changed out the oxygen sensor and sent her on her way. However, the engine light came back on again. Back to the garage for the oxygen sensor code and another oxygen sensor; how many they ended up putting on the car I don't know but I don't think it stopped at two. With the new oxygen sensor, the Ford service guys should have known that it was not a bad oxygen sensor. However, way too many automotive dealers mechanics just know how to read codes and not actually troubleshoot.

I learned my lesson the hard way. Hopefully others can learn from my lesson the easy way.

As a veteran mechanic you are right. A problem code isn't the car telling what part needs replaced. It is just a way of telling you what system is having a problem and gives the mechanic a direction to go. Every service manual has a flow chart to aid the mechanic in proper diagnoses of the problem. Too many times I have seen people throw parts at cars until the problem is fixed, sometimes you get lucky and hit a bullseye the first time, other times you replace about half the car before you stumble upon the actual problem. This is why you need a full understanding of how things work if you are to properly diagnose a car or any piece of machinery for that matter.

o2 sensors are bad for diy'ers, seems they just want to replace it once they see o2 in the description but there are like 6 or 8 codes that have o2 in the description and only the low voltage returned one is meant to tell you the o2 is going bad. shop should have known better though

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And most people go with cheep aftermarket sensors that don't fit factory specs and usually add to the problem.

No way! I thought they were called "Diagnostic Trouble Codes" because the ECM is psychic and knows exactly what is wrong! Yeah, just a little sarcasm...

Diagnostic Trouble Code. A code the computer reports to ASSIST with DIAGNOSIS. Sometimes, it'* a faulty sensor; other times, it'* telling a mechanic that there is something wrong in that particular system. Too many guys out there trying to do a parts-swap without actually taking a moment to think first. They don't know the basics anymore (air, fuel, ignition). LOL