I JUST BOUGHT A 99 GRAND PRIX GTP 3.8 SC WITH 133,000. THE CAR STARTED TO HESITATE AT 40 TO 50 MPH THE SERVICE ENGINE LIGHT CAME ON AND I TOOK IT TO AUTO ZONE AN COMPUTER SHOWND PO742 (TORQUE CONVERTER CIRCUIT STUCK ON). HOW MUCH OF A PROBLEM IS THIS AND HOW CAN I FIX IT.
The hesitating feel is probably the torque converter unlocking when you accelerate and slipping a bit.
Typically when a GM has a TCC problem, it is that the TCC torque converter clutch. Will not disengage when coming to a stop.
The car will buck badly and stall just as you come to a stop. It is just like stopping in a manual transmission car and forgetting to push in the clutch. This is because the same thing is happening.
When your engine temp is over 140 degrees and you get up to speed, especially when it is in overdrive (4th ) gear about 38-42 MPH. The TCC locks the converter solidly so engine and trans are directly coupled together. This gives more MPG and theoretically more life to the converter and trans fluid.
If yours is not bucking and stalling, I would assume it is FAILING to lockup at all. Check all fuses and look for any marked TCC or CLUTCH.A blown fuse can cause this. Probably the most likely cause is a TCC solenoid problem which on the 99 I believe is in the side pan of the transmission on the driver's side.
Don't get suckered into a new transmission or a rebuild. This is a simple fix for a knowledgeable trans guy who is honest.