The P2123 code on your 2023 GMC Sierra indicates a problem with the throttle position sensor B circuit. This code affects your Sierra's 5.3L V8, 6.2L V8, 2.7L turbo, or 3.0L diesel engine's dual throttle position sensor system, signaling high voltage or out-of-range readings from the secondary TPS.
Understanding P2123 on Sierra
Modern throttle bodies use two position sensors (A and B) for redundancy and verification. Sensor B typically operates inversely to sensor A - as one increases, the other decreases. Code P2123 sets when sensor B circuit voltage is higher than expected for the current throttle position, indicating sensor drift, circuit fault, or internal sensor failure.
Common Symptoms
- Check engine light on
- Reduced engine power mode
- Erratic throttle response
- Hesitation during acceleration
- Unstable idle
- Cruise control may not function
- Possible limp mode
Primary Causes
- Faulty throttle position sensor B
- Short to voltage in TPS B circuit
- Corroded throttle body connector
- Damaged wiring between throttle body and PCM
- 5-volt reference circuit issue affecting sensor B
- Throttle body internal failure
- PCM input circuit problem (rare)
Diagnostic Procedure for 2023 Sierra
Monitor TPS A and TPS B voltages with a scan tool. As throttle opens, TPS A should increase while TPS B decreases (or vice versa depending on design). If TPS B is stuck high or doesn't correlate properly with TPS A, sensor or circuit fault is indicated. Check for wiring shorts to battery positive. Test TPS B circuit at the PCM connector. Inspect throttle body connector for corrosion or damage.
Repair Solutions
Since TPS sensors are integrated into the throttle body, replacement of the entire throttle body is typically required for TPS B failure. Repair any wiring shorts to voltage sources found during inspection. Clean corroded connectors. After throttle body replacement, perform relearn procedure. Clear codes and verify both TPS circuits read correctly through full throttle range.