Yestrerday as I was driving home in my 99 XJ8 and the check engine light came on and also the diaplay read somethhing to the effect of that the car was running in what I understand it to be low performance get you home mode. A short while later that display went off.

This morning the check engine light would not come on either indicating that the fault was temperory. After borrowing scanner I read the code P0171 (Mixture too lean Bank 1).

Any hints as to what would have caused this issue, or do I need to wait until it gets bad and permenant?

 

I had this code once (or P0170 ?). I had cleaned the part load breather (on LH cam cover) and the hose cracked and I didn't know it until I got the code and looked for air leaks. This also will set restricted running mode. Check all your vacuum hoses, especailly after the MAF going to the throttle body. You are probably getting unmetered air into the intake, this will cause a lean code to show up.

 

Thanks, I just ordered a Scanner when that gets here I will start investigating. The car is 10 years old and I need to replace all perishable items slowly.

 

Not only check the part load breather pipeing, but the bellows on the air intake where they run over the exhaust from the mafs to the throttle body. They disintegrate over time. If there are no prblems with vacuum leaks, replace the mafs