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2006 Buick Lucerne CXS P0121 Check Engine Light: Is It a Real Issue or Misdiagnosis?

Model: 2006 Buick Lucerne CXS Fault Code: P0121 Posted: 2014-08-02 09:45

I own a 2006 Buick Lucerne CXS with 120,000 miles and a Northstar V8 4.6L engine. About two weeks ago, the check engine light came on. I took it to the dealership, where they diagnosed a faulty wiring harness along the firewall and replaced it. After the repair, the light turned off—but returned the very next day. To investigate further, I purchased a $100 Innova OBD2 code reader from Walmart and scanned the vehicle myself. The result showed DTC P0121 — Throttle Position Sensor (TPS) issue. However, this device does not provide live data readings or real-time TPS values, so I don’t know what actual sensor input is being detected. My car runs smoothly with no noticeable issues during acceleration, shifting, or idle—only the check engine light remains on. When I called the dealership to share the code, they explained that P0121 is a generic OBD-II code and can indicate multiple underlying problems. I’ve already spent $700 on repairs (the harness replacement), yet the issue persists. Given that my vehicle performs normally, I’m now questioning whether this diagnosis is accurate or if there’s another root cause. Can anyone confirm if P0121 is actually applicable to the 2006 Buick Lucerne CXS with its electronic throttle system? Thanks for any insights or guidance on how to properly diagnose and resolve this issue.

Related fault codes

P0121

Comments (6)

Anonymous 2014-08-02 12:33

A basic OBD2 reader like the one you used typically lacks live data feeds and cannot show real-time TPS readings. To accurately interpret P0121, you'll need a professional scan tool that provides live sensor data and full diagnostic history. For detailed explanations of DTC P0121, refer to: https://www.obd-codes.com/p0121

Anonymous 2014-08-02 18:26

There is no official P0121 code for the 2006 Buick Lucerne CXS. This vehicle uses a drive-by-wire system with dual throttle position sensors inside an electronic throttle body. The correct diagnostic codes would be either P0120, P0122, P0123, P0220, P0222, or P0223. Additionally, the vehicle features a Throttle Actuator Control (TAC) system—so any issue should be evaluated in that context.

Anonymous 2014-08-02 18:34

Did you experience reduced engine power when the check engine light was on? This is an important symptom to note, especially for drive-by-wire systems where throttle response may be affected.

Anonymous 2014-08-02 18:38

Yes, I did notice a slight reduction in power before taking it to the shop. The dealership replaced the harness, and I believe that was the original cause—though I’m not certain if P0121 or P0120 was actually present. Given the intermittent nature of the issue, it’s possible the problem stems from a wiring fault or sensor miscommunication rather than a direct TPS failure.

Anonymous 2014-08-02 18:47

Reference: #07-06-04-019D — Intermittent MIL Illumination with DTC P2138 and Reduced Engine Power (Repair Instrument Panel to Body Harness Connector). This TSB applies to GM vehicles from 2005–2011, including the 2006 Buick Lucerne. Note: Many scan tools do not read manufacturer-specific codes, making it difficult to determine stored faults. The issue may be related to a wiring connection between the instrument panel and body harness—especially if symptoms appear during idle or when no air conditioning is running.

Anonymous 2014-08-02 18:54

Thanks for the helpful info! I believe the dealership replaced the firewall harness, which was likely the original source of the intermittent issue. The light has come back twice—both times while I was sitting in my car at lunch with no airflow. I didn’t have a scan tool on the second occasion, but if it comes on tonight, I’ll be ready to check. My car still runs smoothly and feels slightly more responsive than usual. After years of ownership, I can sense when something is wrong—but this one has me stumped because performance is otherwise excellent. Appreciate the input!