CEL was on for a day, then disappeared

Is this supposed to happen? I thought the check engine light was supposed to stay on and the code stored until you hook up a scan tool and erase it. A week later I plugged in my Scanguage II and got no code, so it's not that the bulb is blown.

:confused1

It fixed itself.

I think some codes, such as loose gas cap, are not stored at all.

Downstream O2 sensor codes tend to come and go. My 99 goes on and off, often depending on what kind of gas I've last filled up with. My son's 96 does the same. It may be that by the time the light goes off, the code has also been cleared from memory.

Some codes clear after 3 runs. I've had an intermittent CPS throw a code that clears itself after 3 runs. Of coarse that's until the CPS dies :gee:

OK so I just have to wait til it comes on again.


I did notice that it felt like the engine was running rough but it might have been in my head... I spent a week driving a brand new rental GMC Colorado while the XJ was on the boat to Maui so of course the old 4.0 would feel rough. Since you can't ship a vehicle with ANY leaks I had it checked out completely before shipping it, and had the rear main seal and timing belt replaced. So I would think if something else major was wrong they would have caught it.

BTW, right before I shipped it from Houston I tried running it with a tank of half 93 octane and half 87, because someone on here was saying that his manual recommends higher octane for high temperatures, towing, etc... And my gas mileage was the highest I ever saw, 18.5. I could swear it felt smoother too but again it's probably just psychological, so I went back to 87.

Could it be that I have carbon buildup causing pinging with lower octane? What exactly does ping sound/feel like? Would that trigger a CEL?

You could try some Mopar combustion chamber cleaner get it at the dealership.