Ok so kind of from my build thread. My 2000 XJ had an idle issue after running some errands last week which I found out was the heat soak issue. However the the code I got was not the P0303 like it was described in the technical write up, but on P0301 has anyone else seen this before?

Also it sat overnight from thursday to friday and the code was still there, but last night when I went to go move after sitting for a couple days, the code cleared itself?..

 

303 or 301, it's just a single cylinder misfire. It just means that one or more misfires were detected on that cylinder. They can come and go for a lot of reasons.