Fixed 2008 chevy trailblazer with a 4.2 engine will not shut off.

Hi , First time on your forum. I have been helping another mechanic on this problem. The car cranks and runs fine But will not shut down when the key is turned off. To turn it off you have to pull the fuel pump relay. My mechanic friend has been throwing parts at the problem. New relays , starter switch , starter housing ect, looking at the wiring diagram it looks as if the ECM controls the circuit that shuts the engine off. Any help in how to trouble shoot the problem and provide a fix to this. Mike in AL.

Thanks for all the input very helpful

Sounds like an ignition switch problem. Any aftermarket accessories that could be back feeding power into the Hot In Run circuits? Like a big capacitor for an amp?
Maybe a remote start or aftermarket alarm, or the remnants of such spliced into the ignition switch wires under the dash?

id start by checking the ignition feeds at the pcm that are only controlled by the ignition switch see if they agree with ignition switch position. But if you have a scanner capable of going into the bcm you can check ignition switch state while turning the key to different spots

PCM I
Fuse 28 should lose power with key off. It comes from the Pink wire, pin C, at the ignition switch.

I agree with pole71 check fuse 28 for power key on eng off first. should have power then key off and see if the power goes away.

PCM receives a 12v signal telling it to keep the engine running. Faulty components and wiring can cause 12v at the PCM ignition run input terminal. I had a box truck with the same symptom. Shorted AC relay control coil caused 12v to be present at the PCM ignition run terminal.

Want to thank you for your input. Put me in the right direction to solve this problem.

Thanks for sharing this information. This SUV had the same problem. The AC relay.
Thanks again.

That's awesome!

Wow good work guys!!