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1999 Pontiac Grand Am 2.4L: Sluggish Performance, Poor Shifting, and Engine Revs While Driving

Model: 1999 Pontiac Grand Am Fault Code: P0171 Posted: 2016-12-27 13:48

My 1999 Pontiac Grand Am 2.4L is exhibiting poor driving performance. The vehicle feels sluggish while in motion, shifts improperly between gears, and unexpectedly increases engine RPM on its own during driving. I've been retrieving diagnostic trouble codes (DTCs) including P0171 (Catalyst System Efficiency Below Threshold), P0136 (O2 Sensor Bank 1 Sensor 2), P0141 (O2 Sensor Bank 2 Sensor 1), and P0420 (Catalyst Efficiency). These codes suggest issues with the exhaust system or sensor performance. Despite recent maintenance, including a replacement of the catalytic converter at a muffler shop (which confirmed a 2 lb pressure difference), the shifting problem persists. The car appears to momentarily enter neutral while driving and then revs up unexpectedly. I suspect this may be related to transmission control rather than engine performance.

Related fault codes
P0171P0731P0758
Comments (5)
Anonymous 2016-12-27 14:40

Please provide a complete list of all diagnostic trouble codes (DTCs) and details about any recent maintenance or repairs performed on the vehicle.

Anonymous 2016-12-27 15:09

I have an InnovA OBD-II scanner, but it only shows red light warnings on the screen — no actual error codes. My brother-in-law recently replaced the catalytic converter, and the car still shifts poorly. It seems to go into neutral unexpectedly and then revs up.

Anonymous 2016-12-27 15:34

try a free scan at most auto parts stores - a second opinion and its FREE Scan even if engine light is off. I do not use or know innova code readers. If its as poor performing as you say there should be codes Why did you change cat?

Anonymous 2016-12-27 19:30

The catalytic converter was full — the muffler shop performed a pressure test and found a 2-pound pressure difference between the inlet and outlet.

Anonymous 2016-12-29 11:44

This issue appears to be more related to transmission performance than engine operation. The symptoms of unexpected neutral shifts and high RPMs during driving strongly suggest a transmission control or shift solenoid problem.