So I have an interesting problem and need some guidance. I have an LLY with 65,000 miles and recently received a notification email from Onstar that I had an engine problem that needed immediate attention. The check engine light did not come on and the dealer said it wasn't worth the drive in to have diagnosed since there was no engine light. Well I decided to call Onstar and they did their diagnostic and told me that there is a critical problem with the glow plug system and I should have it looked at immediately.

On top of this I have noticed more engine noise and far less turbo whistle. In fact, I can hardly hear my turbo at all when it used to be pretty loud. Not sure if these 2 things are related but figured it was worth mentioning. The only mod to the truck is magnaflow exhaust.

Of course I am out of warranty so I am trying to decide what needs to happen. Does anyone have any advice for me? Thanks in advance!

Drive to an auto parts store, and have them see if you have any stored CEL's. If you do not (as I suspect) I would ignore onstar and be on your way.

you could always take it to the dealer for an oil change and ask them to just give everything a once over for good measure -- like a 125 point inspection -- and see what they have to say -- you do not have to have them fix anything ,, just see if they say anything is wrong.
you could even be honest with them and tell them onstar told you there was a problem and you dont know for what but could they check things out for your well being.

OnStar doesn't seem to be much help sometimes. I had a CEL come on in the wifes van so I rang up OnStar and they saw no problem at all. It cleared and we kept rolling.
If you have a good dealer, I would go with the above advise to have them give it a quick check.

Is your glow plug light coming on when you start the truck? How long does it stay on or does it just flash real quick and go off? The only critical problem with the glow plug system that I would think on star would see would be a P0670, glow plug control module failure. This set a code, but it will not set a CEL. I don't have onstar but my GPCM failled and thats how you can tell before other hard cold start symptoms.

update?

Here's an update... I took it to Schucks Auto Supply and their code scanner only found a P0700. Not what I wanted to see because it meant that the TCM had a stored code but at least their was nothing regarding the glow plugs. Well, I'm not so lucky... My buddy plugged in his Snapon scanner which can actually read codes (unlike the cheap one at Schucks) and I have 4 seperate codes.

P0101 - Mass Air Flow Issue. Cleaned the MAF sensor and hope the problem is resolved

P0676 - Glow Plug #6 has an issue

P0700 - TCM is reporting an issue

U0101 - ECM lost communication with TCM

Not the morning I wanted to have but from what I'm reading these could all be relatively simple fixes. Even better, it turns out my extended warranty will cover all of this so into the dealership it goes on Monday. I'll let everyone know what they find. Thanks for the pointers!

Nice... always good to do some homework before you turn it over to the dealer.

Just wanted to say thank you and let everyone know what the dealer ended up having to do. The biggest issue was the #6 glow plug that OnStar had warned me about. The dealer replaced this and the code has been cleared ($27 for the part and $200 for labor if I didn't have a warranty). I've found quite a few posts from people with similiar issues on here so I hope this helps you understand what the dealer will charge if you can't fix it on your own.

They say the transmission codes were nothing so I'll keep an eye on this and make sure nothing else happens before my extedned warranty is up. My biggest concern was that all of this happened without popping a check engine light so I guess I need to make sure and scan my truck more often to find these codes. Thanks again!