Throttle/Pedal Position Sensor/Switch "D" Circuit High
Diagnostic trouble code P2123 indicates that the engine control module (ECM) has detected an abnormally high electrical voltage signal from the Throttle/Pedal Position Sensor/Switch 'D' circuit. This typically occurs when the sensor's signal voltage exceeds the maximum calibrated threshold, often pointing to a short to voltage or a failed sensor.
This code requires immediate attention because a failure in the electronic throttle control system will typically force the vehicle into a 'limp home' mode. This severely limits acceleration and vehicle speed, creating a potential safety hazard in traffic.
Common symptoms
- Illuminated Check Engine Light
- Vehicle enters reduced power mode (limp mode)
- Lack of response when pressing the accelerator pedal
- Erratic or unusually high engine idle
- Engine stalling or severe hesitation during acceleration
Likely causes
- Defective Accelerator Pedal Position (APP) sensor or Throttle Position Sensor (TPS)
- Short to voltage in the sensor 'D' signal circuit wiring
- Open ground circuit to the sensor
- Corroded, damaged, or loose electrical connector at the sensor
- Faulty Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Inspect the wiring harness and connectors leading to the accelerator pedal position sensor and throttle body.
Look for: Chafed wires, corrosion, bent pins, or loose connections that could cause an open ground or short circuit.
Connect an OBD-II scanner and monitor the live data for APP/TPS sensor 'D' voltage while slowly pressing the pedal.
Look for: The voltage should smoothly increase and decrease; a constant reading near 5 volts indicates a short to voltage or failed sensor.
Use a digital multimeter to verify the 5-volt reference and ground circuits at the sensor connector.
Look for: Exactly 5 volts on the reference pin and less than 0.1 volts on the ground pin; an open ground will cause the signal voltage to read high.
Disconnect the sensor and PCM, then test the signal wire for a short to voltage.
Look for: Any voltage present on the signal wire while disconnected indicates a short to another power wire in the harness.
Test the internal resistance of the pedal position sensor according to the manufacturer's service manual.
Look for: Resistance values that are out of specification or show an open circuit internally, confirming the sensor itself has failed.
This page explains the generic P2123 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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