I have a 2008 C2S Cab with about 32,000 miles on it. My check engine light (CEL) came on at start-up about three months ago. Even though the car continues to run fine, I took it to my independent service center and they ran diagnostics with the following results: P0456 Evaporative System Leak, Very Small Leak, Not Present (it must have been reset by them, 2 occurrences.

The shop ran a smoke pressure test on the EVAP system and were unable to see anything. But based on their experience with other similar cars, they thought it might be the tank vent valve, so they changed that out and reset the code.

I drove the car for the next several days with all systems working normally. Then less than a week later, the CEL came back on, again at start-up. The shop told me to continue to drive the car for a while to see if the light would go out on its own. In fact, after about 2-3 days the light *did* go out (i.e., did not come on at start-up) and stayed out for about 3-4 days. Then the CEL came back on again at start-up. On the advice of the shop, I have continued to drive the car since, and the CEL has stayed on most of the time, occasionally going out for 1-2 days (roughly 5-6 start-ups) here and there.

Given this information, my shop has recommended changing out the carbon canister, and even they are not highly confident this is the problem. It seems to me if we are going to guess, starting with the least expensive possibilities would be a better approach. Also, I have never heard of a relatively static carbon canister failing. I would expect a moving part to be more likely, say a solenoid that is worn and does not close quite properly every time. Also, based on the inconclusive smoke pressure test, I would expect the failure to be somewhat internal, hence hidden.

I recognize that a "very small leak" could be very hard to find, and that it would be easier to locate over time if it gets worse. But I would like to see what, if anything, can be done to locate it sooner so it can be repaired more quickly. I am concerned that driving with the CEL on all the time might mask an even more serious fault. I know my shop has the same concerns.

What's my next move?

 

If you want to start cheap, I'd try a new gas cap.

 

I should have put that in my write-up, but the gas cap was the first thing I did. No change.

 

New clue: The CEL was on most of the time until the temperature dropped below 40 degrees F, but has been off since (several weeks so far). It must be something significantly affected by thermal expansion/contraction.