Code scanner

[crsgmc]

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