My father in-laws truck is a 2007 GMC sierria and on thier way to our house his check engine light came on. We took it to Autozone and they read it. It showed a P3400 error, but they couldnt tell us what that error was in a GMC. They reset the code and it hasnt come back on yet, but we have only driven it a few miles so far.

Can anyone tell me what that code is?

Thanks,
Eric

 

I think thats for cylinder deactivation. Get on the highway and cruise until the motor shuts down 4 cylinders. If it comes back on then it needs to be serviced by the dealer.

 

Sounds about right. They were crusing down the highway when it came on the first time. Thanks for the info!