service engine soon light

[02_denali_guy]

my service engine soon light on my 02 denali is on and dont know y? Stopped at the parts store and had them hook their tester up to it, it said it was the gas cap(which i checked and it was tight) or a vaccum leak. Installed a 2nd battery in the truck and bout a week later the light came on, checked under the engine cover i pulled off when i did the big 3 upgrade and didnt see or hear anything leaking. Any ideas?

 

[performance1985]

The problem with a check engine light on these things, is that it can be a conundrum of problems triggering it.

I had the same problem and put it on an OBD which triggered that an O2 sensor was bad - bank 2 sensor 1. After I looked further into the problem, I realized I had three bolts on the stock exhaust manifold that were broken off - but no markings on the valve cover.

From what I know about these motors, I would look at my exhaust manifolds first and make sure all the bolts are good - before diving into new o2 sensors

That being said, I don't know what engine you have and the mileage. High mileage can also be upper end issues, but my bet is on the common exhaust manifold bolts being broken or sheared off - causing a leak you don't even know you have.