Had my truck in for service today, as the muffler was rusted through and the rest of the system didn't look too good either. Since I had it in to an exhaust expert, I had them look for the cause of the P0455 code (Evaporative Emission Control System Leak Detected (no purge flow or large leak). I've had codes for this system for years, it started with the P0422 code, then changed to P0455 6 months or more ago. I've been ignoring it as there's been no driveability issues.
Turns out Twig (the mechanic) doesn't have a smoke machine, but he did a physical inspection of the system, and didn't find anything untoward. The only thing that was done to the truck was replacement of the muffler (and the shocks, but that's not important here) - he said the rest of the system is solid, despite the rust.
Odd thing was, after resetting the code, it didn't come back! In the past, when I've "rebooted" the truck to clear the code, it came back very quickly. Now, I'm not counting chickens yet, but will see what happens tomorrow when I drive it to work.
If the code comes back, the mechanic does have a line on a smoke machine he can borrow, so I'll take it back for a smoke test.
Just wondering if it's possible that it was actually the muffler that was causing the codes, and if someone could explain why. Twig said he thought it was unlikely that the system could "sense that far down the system" to throw this code because of a muffler problem.
He didn't know the proper way of resetting the code on my Dak (they don't have them here). You should have seen the look of surprise on his face when this lady pulled a truck manual from under the front seat :)
jmc