P0121 - Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance (P0121). The reported signal is electrically valid but disagrees with other operating parameters (RPM, load, or other sensors).
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance – Signal Voltage | Voltage (V) | 0.50 - 4.50 V (tracks operating range) | Ignition ON, connector engaged |
| 5 V Reference Voltage | Voltage (V) | 4.95 - 5.05 V | Ignition ON at sensor connector |
| Sensor Ground Voltage Drop | Voltage (V) | < 0.05 V (< 50 mV) | Under load to chassis ground |
| Signal Wire Continuity | Resistance (Ω) | < 0.5 Ω (Open = OL) | Disconnected harness between sensor and ECM |
Ignition ON, connector engaged
Ignition ON at sensor connector
Under load to chassis ground
Disconnected harness between sensor and ECM
Oscilloscope Waveform Analysis
100 ms/div, 1 V/divPossible Causes
Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance is electrically operational but performing outside calibrated tolerance
Channel A:Verify dedicated wiring pins and signal pair for circuit A against the schematic.
Chafed or pinched wiring harness along the signal path to Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance
Defective internal output driver or reference circuit in the control module for Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance
Symptoms
Recommended Solution
Compare signal voltage and resistance between circuit A and companion circuit B at module connector.
Log component live data across varying load points and verify plausibility against engine target model.
Verify loaded supply voltage and harness terminal pins at Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Throttle/Pedal Position Sensor/Switch 'A'' Range/Performance.
Unplug the sensor:the signal must swing to a known rail. Replace the sensor or repair the wiring based on the result.
Mechanic's Tip
Always run a loaded voltage-drop test on the sensor ground with ignition ON. A drop above 0.1 V shifts the whole signal and tricks the module into setting sensor codes on a good sensor.