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1996 GMC 1500

On my way home last night the check engine light came on. I made it home, truck running fine. I put the scanner on the truck to read the code. The scanner has been used on this truck before with no problems. The scanner powers up and begins to check modules and all of them fail. It then gives message that it cannot connect to the ECU. I checked the fuses including the cig lighter and the ECU under the hood. All the fuses are fine. Disconnected the battery for 15 minutes and tried again. Still no connection. Before I started messing with anything, I started the truck and it was running with a high idle at 1400 rpm. It did not drop. After connecting the battery, I started it again and the high idle was gone as well as the check engine light. I shut the truck off and sat there for a few minutes and started it again.......high idle was back. Two questions why can't I read codes, and what is causing the high idle. I am thinking the egr may be causing the high idle?

Thanks for any help

Re: Code scanner

You're not getting power at the diagnostic plug, and probably other stuff too.

Re: Code scanner

Have you tried the scanner on another vehicle to ensure operability?

If the scanner is good, then it’s the truck.

Since you said the scanner powers up, it has power, just can’t communicate with the PCM.

Try plugging the scanner in and wiggling the wires behind the obd port. Maybe a short.

Also try putting the key in the on position first and then hookup the scanner.

What kind of scanner is it?

Re: Code scanner

I just tried it on my 2000 Xterra. The scanner worked just fine. I also tried plugging it in with the key in the on position in my K1500 and it would not work. I have a Creader VII+ scanner. I will play with the wires to see if there is a short. Any other ideas?

Re: Code scanner

The obd port is where I would focus my energy. I’ve had similar experiences when I plug in my Tech 2. It would say can’t communicate and after several attempts of gently plugging it in the port and some gentle wiggling, it would work fine.

Re: Code scanner

I had an issue on my 02 and it was the fuse providing power to the OBD@ port.