Here are more likely causes of hard shifting when warm:
* Low or Contaminated Transmission Fluid: This is the most common cause. Over time, transmission fluid breaks down, losing its lubricating and cooling properties. Low fluid level can cause hard shifting, especially when the fluid is hot and viscosity is reduced. Contamination with debris from worn-out internal components will exacerbate the problem.
* Worn-out Transmission Components: Internal components like clutches, bands, and valve bodies can wear out, causing harsh shifts. This is more common with higher mileage vehicles.
* Solenoid Problems: Solenoids control fluid flow within the transmission. A malfunctioning solenoid can lead to improper shifting.
* Transmission Control Module (TCM) Issues: A problem with the TCM itself can cause incorrect shift commands. This is less common but possible.
* Torque Converter Issues: Problems with the torque converter can also lead to hard shifting, particularly when the transmission is warm.
Before suspecting the temperature sensor:
1. Check the transmission fluid level and condition: This is the absolute first step. Check the fluid level with the engine running and warm (according to the owner's manual) and inspect its color and smell. Dark, burnt-smelling fluid indicates contamination and needs changing.
2. Have the transmission fluid and filter changed: Even if the level seems okay, a fluid and filter change is almost always recommended for hard shifting issues, as it addresses the most likely problems (low/contaminated fluid).
If those steps don't resolve the issue, then you can consider the temperature sensor:
You could test the sensor's resistance with a multimeter following the instructions in a repair manual specific to your Grand Prix's year. However, this is more of a diagnostic step after addressing the more probable causes above. Replacing the sensor is relatively inexpensive, but it's a waste of time and money if the problem lies elsewhere in the transmission.
In short, while a faulty transmission fluid temperature sensor is *possible*, it's far less likely than other problems related to the transmission's internal components or fluid condition. Start with the simpler, more common fixes first. If those don't work, then investigating the sensor becomes a more reasonable next step. A mechanic specializing in transmissions can properly diagnose the issue.
