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Usually when something goes wrong with my vehicle the check engine light comes on. I had an issue recently where the check engine light did not turn on, but the vehicle ceased functioning. I read the check engine light comes on frequently for any emissions related problems, and thinking back that lines up with my personal experience, but my experience is limited.

What types/categories of issues will the check engine light turn on for?

  • You might consider adding what issue you had where the CEL light did not turn on. Someone may be able to tell you exactly why it didn't in your situation. – Nick G Oct 6 '15 at 14:22
  • I'll add it as a separate question – Josh Oct 6 '15 at 16:17
  • Why kids love cinnamon toast crunch... – Nick May 5 '16 at 14:18
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+50

A Set of Standards for Error Codes

I'm sure that someone is going to come up with a much more exhaustive answer, but for a good start, you need to understand that there are MANY different standards for reporting vehicle diagnostics.

For example, take a look at the OBD II Pids Article on Wikipedia. Think of your car's computer as only as powerful as the inputs (or sensors) that are wired up to it. Now, combine that array of sensors, constantly providing a gigantic stream of information (LOTS of raw data) with a limited list of events... individual error codes that can be reported. "I just saw a cylinder misfire, because the data told me!" Or, "There's WAY too little oxygen in the exhaust.. the engine is running rich!"

What can't the light tell you?

Imagine a vehicle equipped with ODB2 is collecting data. It knows of a limited set of condition codes that can be matched up to a particular data event. If the ODB2 standard doesn't support a particular vehicle condition, the computer can't report it.

If there aren't relevant sensors hooked up, again.. the computer can't report it.

But it's not JUST about the computer...

There's also a huge difference between being able to report a condition, and being able to interpret exactly what the error code means. It could be a code that is a symptom of another code, and hence should be ignored. The codes, by themselves, simply help narrow down the issue to possible reasons. They're a tool in the mechanic's arsenal, but physical inspection is often required for a complete analysis.

Why did the Secondary Air Injection fail (P04111)? Because the rivets that hold the casing of the fan together broke. The code lead to the correct system, but failed to tell me about the rivets.

Why was the car running rich? Maybe there is a vacuum leak.. a brittle 10-year-old rubber hose finally split. The computer can't tell you that, because it can't examine the hoses, only the data being collected.

Find out which diagnostic system your car uses, and you can check out the corresponding standards chart to see a full list of the reasons. There's a good chance that your vehicle is ODB2.

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    @Paulster2 Thank you sir.. coming from you, that means a lot. You were the "someone" that I was referring to in the opening sentence, btw. – Lynn Crumbling Oct 6 '15 at 3:28
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The check engine light comes on for issues that the ECU can detect from its many sensors. The ECU controls fuel and spark. This means the ECU will not come on for mechanical issues that cannot be detected by a sensor. However, some of these items can cause a misfire code.

Some Examples:
Air filter clogged
Exhaust leak
plugged cat
clogged fuel injector
malfunctioning fuel pressure regulator
clogged fuel filter
malfunctioning fuel pump
spark plugs, wires, cap, rotor, coil
Vacuum leak
dirty throttle body
malfunctioning fast idle thermo valve (similar to vacuum leak)
low oil (unless there is an oil level sensor)
belts
gaskets
seals
rings
low compression
bearing damage (rod knock)
Broken cam
rocker fell/broke off
broken valve spring
stuck valve
critical engine damage (windowed the block) wiring, fuses, relays
starting system - ignition switch, starter, relay
Cooling system - clogs, leaks, flow (may throw a code if you overheat)
transmission clutch(es)
driveshaft, differential, axles
brakes
suspension
tires
hvac
lights
gauges

  • I like to think of the CEL or error code as the result of a symptom that the computer feels. It isn't guaranteed to report the cause of the problem. – Zaid May 5 '16 at 11:57
  • @Zaid Yes, good way to put it. – rpmerf May 5 '16 at 12:20

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