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Hi I have a 2001 XK 8 the engine check light came on whilst driving, no change in performance
I have turned off the engine, Now when I turn the key at start up the check engine light stays on.
Good battery . car starts first time.
Any ideas could it be a poor connection on a fuse if so which one, :
Can anyone help me on this.

Could be a number of things - I had this recently on mine and it turned out to be the oxygen sensor pre cat - i had the same symptons as you no effect on performance at all but engine management light on. You need to have your codes checked as this will confirm where your problem my lie.
If it is your sensor then let me know if you are going to tackle it your self as I can advise the best way to get at it
Thanks
Ian

Hi Ian
I have just checked the codes PO 171 and PO 174 System too lean on both banks

I can't see any visible signs of a leak and I don't have a smoke test tool any ideas where to look first

I have looked at the MAS sensor it looks clean enough as does the air filter I have a K&N filter in my housing and conecting pipes look good

My guess would be you may have a small air leak somewhere there are some plastic pipes that connect around the cam covers /throttle body they become very brittle with age and can split / crack I would check these as a start point -strange you don't have an effect on performance does it run lumpy on start up but smooth out as the ecu adjusts if you can't find an air leak you may have to get a jaguar dealer to give you more definition on the codes / problem

Air leaks are favourite but there is a long list of other possibilities including plugs, MAF sensor, oxygen sensors and wiring. You really need a session on a Jaguar specific diagnostic machine, either the full Jaguar dealer system or a specialist with the Jaguar modules of the Autologic system. An hour's diagnostics could save a lot of time and money compared with testing by substitution all the possible problem items. Both David Marks and XJK Garages (contact details in the magazine) have the kit and are very good at diagnostic issues. Mike.

an easy way to check for air leaks on the induction side, or Vacuum pipes is.....WD40 ,

spray a small amount on each joint and pipe while the engine is running

any WD40 that gets into the induction system will cause the revs to rise instantly for a few seconds,

dont spray it all over at once, as you will never know where the leak is

the main cause will be the part load breather pipe from the top of the left cam cover, down the valley to the TB , these break in the valley somewhere , good news is they are only £15 from jaguar

the full load breather is on the right , short pipe on top of induction pipe after the air box

BB