Code P0120: throttle position sensor A: circuit fault

Original definition (SAE J2012): Throttle Pedal Position Sensor/Switch A Circuit Malfunction^[1][2]

• P0120Throttle/pedal position sensor/switch "A" circuit: malfunction.
• P0121Throttle/pedal position sensor/switch "A" circuit: operating or limit-value problem (range or performance). The signal exists, but does not match what is expected.^{[4][9][10][11]}
• P0122Throttle/pedal position sensor/switch "A" circuit: low input signal, that is, the sensor voltage is below the expected range.^{[5][12][13][14]}
• P0123Throttle/pedal position sensor/switch "A" circuit: high input signal, that is, the sensor voltage is above the expected range.^{[6][15][16][17]}

1. 1Faulty throttle position sensor (TPS) or accelerator pedal sensor: it is the first cause cited by Autodoc and CarParts, and it also appears in Ross-Tech.^{[1][3][7][18][19]}
2. 2Sensor wiring and connectors in poor condition: damaged or corroded wires, loose or dirty connectors, a short circuit or a faulty earth. HELLA adds that contact faults in the connector and internal short circuits caused by moisture or oil are common causes of throttle potentiometer failure.^{[1][3][18][19]}
3. 3Dirty, worn or faulty throttle body: Autodoc cites wear on the throttle shaft and CarParts a dirty or faulty throttle body, and Ross-Tech points to a dirty or stuck actuator.^[1][3][7]
4. 4Faulty mass airflow (MAF) sensor: it appears in Ross-Tech for VW group cars, and CarParts says that, with P0121, the throttle signal may stop agreeing with that of the pressure (MAP) or air flow (MAF) sensors.^[7][10]
5. 5Sensor poorly fixed to the throttle, according to Autodoc and CarParts for P0121 and P0122.^{[9][12][13][16]}
6. 6Faulty ECU (ECU/PCM) or one with out-of-date software: it is the least likely cause.^[1][3]

• Engine warning light on in the instrument cluster.^[3][7][9]Engine warning light: faults that cause it
• Delayed or abrupt response when accelerating, with jerking or shuddering.^[1][15][18]Jerking: faults that cause it
• Rough idle: the engine runs unsteadily when stationary.^[1][9][15]Vibrations: faults that cause it
• The engine stalls, especially at low revs.^[1][10]Engine stalls: faults that cause it
• Noticeable loss of power when driving, to the point of barely being able to accelerate in some cases.^[1][13]Less power: faults that cause it
• Difficulty starting the engine.^[1][3][18]Hard to start: faults that cause it
• Increased fuel consumption.^[1][18]Uses more fuel: faults that cause it
• On some cars limp-home mode kicks in, with reduced power.^[10][15]Limp mode: faults that cause it

1. 1Start with the simplest thing: inspect the sensor, its connector and the wiring to the ECU for damage, dirt, moisture, corroded terminals or loose connections.^[1][9][18]
2. 2With a diagnostic tool, watch the sensor signal while you slowly open the throttle or press the pedal: it should rise smoothly and continuously, with no jumps or drops. HELLA suggests measuring the potentiometer resistance while slowly opening the throttle, to detect breaks in the sliding contact.^[11][18]
3. 3With a multimeter, check that about 5 V of reference reaches the sensor connector and that the earth circuit is fine.^[3][9]
4. 4Check the throttle body: that it is not dirty or stuck and that the throttle moves without binding. After cleaning it, Carista recommends doing an adaptation (relearn) with the diagnostic tool. On VW group cars, also check the mass airflow sensor.^[3][7][11]
5. 5Only when the wiring, voltage and signal are fine, replace the sensor or the throttle body. The ECU is the last thing to suspect; in some cases updating its software is enough.^[1][3]