There have been a few cases I was sure my CEL would have come on, and it hasn't.

It does come on when I start the truck of course, then turns off.

When I had a leak from my headers and the idle was supper high until I fixed it, no CEL.

If there are error codes does the CEL light?

 

For the CEL to be turned on, there are certain parameters that need to be met. Some are almost instantaneous and others take more time. The O2 sensor would have been reading lean, but it would take a bit for the fuel trim to make it's way over i think it's around +20 for the CEL to set. But if you had a constant misfire like a plug wire had grounded out, it would have lit up almost immediately. There are different rules stored in the PCM for every code and depend on number of drive cycles with failed tests, certain voltage readings, etc.

 

There are also certain codes that won't actually turn the CEL on, but will register a code that can be pulled.

 

Thanks. This is essentially what I think I was wanting to know.