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1997 Buick Lesabre P0121 Code Not Clearing After TPS Replacement - Fix & Wiring Issues

Model: 1997 Buick Lesabre Fault Code: P0121 Posted: 2015-11-03 16:29

I own a 1997 Buick LeSabre with the 3.8L V6 engine, second-hand vehicle with only 44,000 miles and excellent condition. This morning, while pulling into the garage, I noticed the car was idling roughly and exhibiting slight surging. My wife is disabled, so I helped her get settled and then started the car again—still rough idle and the check engine light remained on. I connected an OBDII scanner and found a P0121 code: Throttle Position Sensor (TPS) - Insufficient Activity. I attempted to clear the code but it did not disappear. I cleaned the TPS connection, but the issue persisted. I replaced the TPS with a new one. After installation, the rough idle improved significantly and the car ran smoothly. However, the transmission would not shift from second to third gear, and the P0120/P0122 code (Throttle Position Sensor - Low Input) remained active and refused to clear. I disconnected the battery for 10 minutes and reconnected it. After restarting, the car started with a slight bump but idled smoothly when stationary and performed well under load with brakes applied—still no shift from second to third gear, and the code persists. I consulted a neighbor who works on cars—he suspects wiring or harness issues. Could there be a problem with the TPS wiring or signal connection? Any insight into what might have gone wrong? Thanks for any help or troubleshooting tips!

Related fault codes
P0121
Comments (2)
Anonymous 2015-11-03 16:38

The P0121 and P0122 codes indicate insufficient activity and low voltage in the throttle position sensor signal. Do you have a digital volt-ohm meter (DVOM)? Start by checking the wiring at the TPS: reference voltage should be around 5V, signal voltage must vary properly with throttle position, and ground connection must be solid. Your neighbor is correct—wiring or harness issues are likely the root cause of this problem.

Anonymous 2015-11-04 13:01

Update on the issue: When I installed the new TPS, I missed attaching the small tab that extends from the sensor and catches the rotating end of the throttle shaft. Without proper mechanical engagement, the TPS could not detect throttle position changes, so it failed to send accurate variable resistance signals to the ECM. This explains why the code remained and why shifting was affected. Thanks to the troubleshooting guidance from Jim @ rollingdead.com.