how long does it take for a CEL to return
 

I had a check engine light a few months ago for a slow O2 sensor. I cleared the code, and it hasn't yet come back. But it looks as if the gallons per hour on my SG2 changes more slowly than it should. I'm not sure if this is due to that O2 sensor, or it is just my imagination.

So, how long should it take for a CEL to return if the underlying problem isn't addressed. And should my MPGs jump up and GPH jump down when I take my foot off the gas, or is it normal for it to slowly change.

Mpgs should jump up and GPH should go down very quickly once you have your foot off the gas.

I've heard emission CELs on Toyotas take three trips to turn on. I'm sure more severe problems turn the light on right away. Did you ever get the code while it was on?

It is hard to tell you a solid time for when they "return"

Without knowing the specific logic for turning on the CEL in your car, generally certain conditions must be met for the diagnostic to pass/fail which may include the engine coolant warming up a certain amount, the car being on for a certain amount of time, etc.

Also, if the sensor isn't hard failing and is intermittent failing then it may make the CEL take even longer to set.

In general, any emissions related diagnostic trouble code will set a CEL on the first fail as emissions is the reason OBD was invented.

It has been a few months since I cleared it, so by now any criteria for the light to return must have been fulfilled. The more I think about it, I guess I will just replace the thing. It's not that expensive, and may or may not solve a problem I may or may not be having.

i have a code on my truck po420 and i can reset it at work and it i will be back in about 20 miles..

Taco,

I believe that is a catalyst code. Sounds like you need a new cat...that would be hurting your mileage as well most likely.

sorry I should also add that it could be your O2 sensor(s) that are going bad as well. Is the car driving normal?

yeah *whistle whistle* i havnt had a cat for about 3 years now......

haha so why even post that you are setting that code?