I have read up over the years on the P0456 code and it has come and gone for me over the years. In the past two months, it has come, gone and returned. Not driving it as much these days so it lingers. The gas cap seems to fit securely but maybe after nine years it has imperceptible leakage.

My question: How does the system check work at startup? How often do sensors check for leaks and what does it take for the CEL to go away? If I'm not asking the right questions, maybe you can enlighten me on the process.

Thank you.

 

I would definitely replace your gas cap, assuming it is the original it is due for replacement and may well be the cause of your P0456 code.

If that doesn't fix the problem, check for leaks at the evaporative hoses on top of the gas tank (items 16 and 20 in the diagram below), and hopefully it isn't a leak from item #9 in the same diagram as that means removing the gas tank to replace it!

I had the same P0456 code and it was a leak in the short rubber hose (item 16) and it was frustrating how long it takes the system to process through its full diagnosis staging. When an error code is produced and cleared, the system produces a P1000, indicating there has been an error code and the system is going through the whole process of checking all components for any continued codes. Once this has been completed and the system has found no errors, it will produce a P1111, indicating all is well, but this can take a frustrating 150-300 miles of driving to complete.

Rather than a hit and miss diagnosis of replacing suspect parts a good repair shop can do a smoke test which will reveal any leak(s) quite cheaply.

 

+1 to CK's suggestion....

You'll want to check this also, it was my problem. It was random when I got my code....maybe a week later, sometimes 6 months later. I also replaced the purge valve (firewall) a couple hoses and the fuel cap. Another thing to check there would be the crossover hose for any cracks and such.
The listed detail is for an XK8, ...for the XJ8 the Valve is on the RRear by the jack post. It literally takes all of 20 minutes to do.

 

Thanks, CK. I am now getting a P0456 and a P1111. Does that mean it cleared and I just need to drive it?

 

If you still have the P0456 code pretty sure you still have the vacuum leak.

Clear the P0456, then let it run through its test procedure, and see if the P0456 is still there.

 

You may have to get the car up to 55mph...88km/h and try to keep it there for about 10 minutes. You may have to do this multiple times.