I have a 2004 XJ8 w 60K mi. Recently the engine light came on and I took it to Auto Zone to read the codes. I was seeing code P0300, 301, 303 306......random misfire codes and it wasn't running with restricted perf mode. It would sometimes hesitate and sputter a little on acceleration my wife said. I thought it was time to replace the spark plugs(original) and air filter and clean the MAF. Afterwards the car ran much better but I cant get the engine light to go out. I tried removing the battery cables for ~30min and shorting the cables together for a few minutes and while they were removed I entered the car and put in the key and it was dead. I have done this in the past and I knew it reset the computer because it would also lock out the stereo and loose it's programming. This time it all comes back on like nothing ever happened.

What am I doing wrong?
Mike

 

A hard reset doesn't clear CEL. You need an OBD reader.
You say I entered the car and put in the key and it was dead.
Do you mean no lights etc. ?

 

Yes, I mean there was no lights, no buzzers, no anything.... which was good. Normally this process would cause the radio to loose it's programming.

Auto Zone won't clear codes here, just read them. I guess it's a liability issue.

 

You could take the battery out, and put a new one a year later and the codes will still be there.

Two things will clear the CEL.

A) An OBD Code reader tool

B) Resolving the problem and then going through 3 complete drive cycles without throwing any more codes. So if you truly resolved the issue, drive the car a bit, and the light will go out.

The code will not stay on permanently if the underlying source of the code is resolved. BUT it's not an on off switch either, without clearing the code, the proper drive cycles must be completed before the code clears.

Take care,

George