Here's what you should do:
* Don't drive it further: Continuing to drive the car could cause further damage and potentially lead to a complete transmission failure, stranding you. The hissing noise is a warning sign – the transmission is likely low on fluid and operating under duress.
* Check the transmission fluid: Locate the transmission dipstick (usually near the engine) and check the fluid level. If it's low, it's critical to find the source of the leak. The color of the fluid is also important. Burnt or dark brown fluid is another sign of serious problems.
* Get a professional diagnosis: Take your Grand Prix to a reputable mechanic or transmission specialist. They can properly diagnose the problem, determine the extent of the damage, and provide you with repair or replacement options. Driving it to the mechanic might be risky if it is significantly low on fluid, so consider having it towed.
Possible causes (beyond just a failing transmission):
While a failing transmission is the most likely culprit given your description, other less likely possibilities include:
* Low transmission fluid: As mentioned above, low fluid can cause various shifting problems.
* Faulty transmission solenoid: This is a part of the transmission's hydraulic system that controls shifting. A faulty solenoid could prevent downshifts.
* Transmission pressure sensor: This sensor monitors the transmission fluid pressure and if it is faulty it may give the computer incorrect data leading to improper shifting.
* Computer problem: (Less likely) In rare instances, a problem with the car's computer could affect the transmission's operation.
However, the combination of the hissing sound and the failure to downshift strongly suggests a significant internal transmission problem, making a transmission repair or replacement the most probable outcome. Get it diagnosed quickly to minimize further damage and cost.
