I had a P0455 fault code, I replaced the gas cap, canister close valve and checked the hoses and cannisters. Everything looked good. Now I got a restricted performance message along with a P0171 and P0174 fault code. No more P0455 code. Any suggestions?
I have a 2001 XK8.

Could be your O2 Sensors are reading a bad air/fuel mix; or the O2 Sensors it self is bad.

A code P0171 may mean that one or more of the following has happened: The MAF (Mass Air Flow) Sensor is dirty or faulty Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry. There could be a vacuum leak downstream of the MAF sensor Possible cracked vacuum or PCV line/connection Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1) Sticking/plugged or failed fuel injector Low fuel pressure (possible plugged/dirty fuel filter!) Exhaust leak between engine and first oxygen sensor

Read more at: http://www.obd-codes.com/p0171
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Clear them and see if they come back.

Thanks Gordo, I replaced the MAF today, I will let you know what happens. Thanks

MAF replacement did not help. I pulled the air cleaner box thinking a mouse might have built a nest in it to cause a lean P0171 and P0174 code. No mouse nest, but I did find my air cleaner element soaked in oil and oil in the bottom of the air cleaner box. Not sure how the oil got in the box. I am now looking at the breather hose, but that's downwind of the filter box.

Check the How to section for oil in the airbox.

I had these codes today on my 98 xj8. It turned out to be a hole in the part load breather. Put a new one on and codes went away. Also with the vacuum leak the AC didn't blow cold air.

Went to the oil in the aircleaner from the how to section. It directed me to the breather on the left front of the engine. Found that hole restricted. Cleaned out with a 3/32 drill bit. That fixed it. Thanks everyone.