I had erroneous codes p0171 p0174 on my 2005 XJL, even while the car ran perfectly.
I replaced the MAF....no change, except that the car seemed to need a slight delay after turning on the ignition and starting.
I was sure the codes were not being correctly thrown.
I did a ECU Hard reset, and it is no longer a problem. You can reset the ECU by detatching the two battery cables (Pos and Neg) and shorting them together for about 20 secs. Then reconnect them to the battery. It also reset my saved radio settings but that's ok. No cost and all done in 15 minutes.

 

You've forced a relearn. As the S-Types are known NOT to throw these codes wrongly, it would be likely the codes would reflag in time. Your XJ is likely the same...

Have a look at fuel trims. What are they?

 

Listen to JagV8. Get the car hooked up to a scanner tool and look at the fuel trims in real time. Then spray cleaner/propane what ever you want at the possible leak points while watching the fuel trims on the computer screen.


It's just a big guess where your vacuum leak is without watching the fuel trims.
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