I have a 99 Grand Prix GT, v6, automatic trans. 1st and 2nd ok, but the speed for trans. to shift to 3rd has gone up to 50 mph. and refuses to shift any sooner. Does anyone know what could cause this problem?
Please help!

I have a 99 Grand Prix GT, v6, automatic trans. 1st and 2nd ok, but the speed for trans. to shift to 3rd has gone up to 50 mph. and refuses to shift any sooner. Does anyone know what could cause this problem?
Please help!

It would be helpful to know the mileage on your GP. When was the last time you had the tranny serviced (ATF and screen filter).

The PCM by referencing engine and drive parameters performs the shifting in a 4T65E tranny by controlling shift solenoids. There is a 1-2 upshift and 3-4 upshift. The 3-4 solenoid may be hanging up indicating a valve body related problem. Also, the PCM references to a vehicle speed sensor (VSS) to determine car speed for shifting.