Just purchased my 4th Grand Prix. This is a '99 GT. And Surprise...intake and valve cover gaskets need to be replaced. Not to worry, all gaskets replaced (including those ridiculous plastic elbows. Car runs just fine...except P0123, P0118 and P1635. I would appreciate any feedback from the forum on why I would just now be receiving these trouble codes. Also, the gasket kit I got came with a thermostat gasket, but when I removed the old thermostat there was only the rubber ring inside. I replaced the ring and installed the gasket. Again. car runs fine every other start. The IAC has to be reset at ignition to get normal idle.
Thanks for the help