Got a truck for my grandson about two weeks ago. 2007 Gmc Sierra classic (not sure what that means) v6, automatic, king cab with 174,000 miles on it. Basic, simple truck. I got it because these trucks seem sturdy and reliable. Didn't expect perfection and figured it would need some repairs - just not this soon. The check engine light was not on when I test drove it but it was making a bunch of noise. Dealer said the exhaust manifold needed repaired - I was told one side of it was faulty. Dealer had that fixed and new spark plugs and wires put in. Drove the car off the lot and 3 hours later the check engine light came on. It blinked a couple of times then stopped. Grandson said it was when he was going up a hill and the truck wasn't getting gas like it should. Took it to my mechanic and he checked it all out. Unable to find anything. Got it home and it did it again. Took it back. Mechanic read code history and said it misfired but unable to determine cause of misfire. Picked it up and it did it again. I got it from an independent lot with no guarantees. Looked like a good first time starter vehicle, one owner. I have been looking on line and discovered this can be a tricky thing to identify since it is intermittent flashing. Any suggestions?
Chances are the dealer erased codes before you bought it, though that should have been noticed during pre-purchase inspection.
Many things can cause a misfire. You might want to do a compression test to verify internal engine condition.
Thanks for the input. I had it a little wrong. Apparently, the light is on all the time but blinks now and then. It blinks 3 times and when it does doesn't have as much power. Back to the mechanic Monday. I will ask if he did compression test.