As an update:
The fault code was a P2404, 'Evaporative emission system leak detection pump sense circuit range/performance.'

This indicates a lead in the emission system/exhaust. It's likely a stuck valve as there are no outward indications of any problems. I'll have it looked at when I'm back in Scottsdale at AM. According to them, it's not a big issue.

 

Hi @SeanMTX

Familiar issue. Please see my replies in this thread

https://www.6speedonline.com/forums/...04-2405-a.html

 

As usual, a great and comprehensive walkthrough. Thanks @BWings !

 

No problem! Hope it helps.

If you have this diagnosed by the dealer, the TSB they have pretty much follows a similar "process of elimination" like I did on my own. Only they might be putting new parts in and testing each one until the system becomes good again. Once the parts are in a non-warranty car they have to bill you for those parts and labor. They're very expensive. Just FYI for you...

For an evap leak it can be any one of or a combination of:
Fuel filler Capless tube
Carbon canister
leak detection pump
the two purge valves
And (or) any of the connecting lines going to/from these devices.

If this happens and they do try to bill you for this, you might want to look into whether the "Emissions Systems Components" have any extended warranty outside the typical powertrain warranty. I am pretty sure, for example, that catalytic converters are warrantied, by law for more than most factory warranties.

Finally, BTW, this particular fault does not impact engine performance. It is strictly an emissions system designed to keep escaping fuel vapors out of the atmosphere.

Good luck and all the best!

 

Aaannnnd, as I took it out for a spin today, the check engine light is off.

Goofy AM electricals.

 

What's happening is this:
The evap system is not constantly being monitored. The CPU only does a system check every xx miles (50 to 100 based on how the firmware was written). If you have a flakey leak detection pump, or, let's say, one of the purge valves intermittently going bad, then good - after the CEL event, if there were a number of ignition cycles, then another evap test that passed, it could extinguish the CEL. But, with the bad part, eventually a future test will fail again.

So my expectation is - on another future ignition cycle, the "bad component" will again fail, and reset the light.

You're not doing any harm (except to those who would say - the environment).

Through my own experience with this leak detection pump. This is an odd device in itself. When it is asked to perform a "test" it takes a fresh air sample on one side of the unit, then a separate pump takes a sample (sniffs) directly out of the carbon canister and compares the sample. Then it reports a failure if the sniff comparison does not fall between expected limits. There are so many conditions where the fresh air sample can be "less than fresh air" and cause the test to fail and report a P2405. Then the next cycle it tests fine and the CEL clears. The leak detection pump is only consistent when the carbon canister is saturated. It becomes quickly saturated when owners continue to fuel after the pump clicks. On these cars we should always stop at the first click and not continue to top off the tank. The extra fuel just runs directly into the canister and kills it.

This leak detection pump is used on a few cars, not all cars' systems use this method. Aston, of course, Audi. I didn't check that far but likely Porsche, Bentley, Lamborghini but I see no evidence GM uses it, or Ford.

Sorry for the length.