i have a xj8 3.2 yr 2000. when i got it, it had the engine light on the dash, i took it to the local garage and the fault codes listed were p1638 p0171 and p0174 he cleared the faults and all has been fine 60 miles later the light has come back on and the codes showing are p0171 and p0174. what is my next step?? i dont think the garage ive took it to wants the job

 

Those codes are saying 'Lean Fuel Mixture Left and Right'. That is probably caused by an air leak between the MASF and the throttle body. Take the air duct off between the filter box and TB and look at the underside of the accordion section of the duct. Common place for breaks as the plastic is directly over the exhaust manifold. A new unit is less than $100, an used one half that. I repaired mine with black electrician's tape. Clean well, wrap tape tightly from one end to the other. A heat gun helps make a tighter, more invisible repair.

If that is not the issue look for another source of a leak in that area.

While you have the duct off I would clean the TB and MAFS with a spray cleaner designed for that. Available at any auto parts store. Also, clean the part load breather on the left valve cover.

Let us know what happens. Good luck!

Edit: Just reread you post and see you have a 3.2L. Don't know if you have the accordion section but everything else applies.

 

I also have the P0171 & P0174 codes coming up. I have cleaned the MAF and Throttle body using the instructions from fellow poster Gus. (His website is awesome and his info and insight is great) My engine compartment is pristine and had no cracks or anything within the air intake. I am not sure what to do next. The items I cleaned were not that dirty and the car actually is running perfectly fine. You would not know there is a problem if it was not for the light. If anyone has any other advice I would greatly appreciate it. I respect the unbelievable knowledge on this forum and I have read for years and this is my first problem.

 

Sorry I have a 2001 XJ8 4.0 with 89k miles.