When I started my truck this afternoon (70 degrees at the time, sat outside for 10 hours), the check engine light went on. At first, I thought it might be a loose gas cap, so I shut the truck off, tightened the cap (didn't need it, but what they hey) and started back up. Nope, still have the light. I hadn't put gas in the truck for about 10 days (sitting at 1/2 tank) and the DIC didn't show the usual "check gas cap" warning message, so I assumed a bit more. Truck only has 5K miles on it, so this is definitely not something I would expect already.

Contacted Onstar and they told me there was a code and to see if it "goes away" in a few days. Nuts to that. I was only 4 miles from the dealership so I stopped in. They threw it on the computer and it came back with a "P0442" code which the service writer told me was a small evaporator leak. They checked everything by hand (values were opening and closing fine, gas cap was fine, etc) but couldn't pin it down. They reset the code and told me to keep and eye open for it.

I only run name-brand gas (mostly Mobil, sometimes BP) in the vehicle so it shouldn't be that. I wonder if I'm getting a crack in a vacuum line somewhere ....

Anyone else have this happen to them?

 

It could very well be a slight leak in an under body evaporative emissions line. The diagnostic may need several more park cycles like you described to set again.

If it does it will be real and the dealer will need look harder.

 

Ding Ding Ding!! You are right on the money!

Light came back on last night, happened to be driving by the dealer so I stopped in. They checked and yup, the valve was leaking (it failed the smoke test). They replaced it (twice actually since the replacement was also bad) and I was on my way!

The Service Manager (I've known him for over 20 years!) and I was chatting about this and he said the Tahoe's don't usually have this problem.

Other than that, I'm good to go!