My mechanic read a code PO456 after I got a yellow check engine light. He says that indicates a minor leak in the evaporative system. Also said I could drive the car (it runs fine) and wait to see if light comes on again. He indicated that finding the leak could be "quite expensive".

Any idea what he means by "quite expensive"? He was unwilling to give me an estimate. The guy has been in business since 1956, and really knows his stuff.

 

I like your guy's answer! To find a "small" leak, which by the way has a definite definition according to OBDII, it could be a short or protracted troubleshooting time spent. I If I were you, I would look carefully at the gas cap mating surface for cracks or other distortions, make sure the cap is on tight each time you fill the car, have the code reset, and see if it comes back. These cars apparently do not have many problems with split evap hose like some vehicles (Jeep for one).

 

The only hose I've ever seen cracked in the EVAP system on a Jaguar is the XK8 body style. There is a small hose between the passanger side canister and the canister close valve that cracks and causes leaks. Your XJ doesn't have that hose, so I would close the valve electrically, and smoke test the system. A lot of times you will see the the close valve not close all the way and smoke will go out of the vent. Then you know the canister close valve is stuck open or sticking open.

 

Thanks guys, I failed to mention that the car is a XJ8 non-SC 2001 Vanden Plas. I had the shop reset the code. The fuel cap was tight, I always check it, but the wife who is primary driver, does not always tighten. My mechanic said that the computer onboard checks the fuel cap about every three starts, and since it did not reset on its own over the course of a week, I have to guess it's a leak. It has been 4 days since the shop reset it, an so far so good. I'm keeping an eye on it for now.

 

One nitpick technical clarification. AFAIK, the computer does not "check the cap every three starts", but intead, does a evap system check during startup, and after a number of succesive failures, lights the light. It can auto reset, but apparently the timing can be affected by fuel tank level, ambient temperature and so on.