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P0120 Generic (SAE)

Throttle / Pedal Position Sensor / Switch "A" Circuit

Updated 2026-10-02 · Fuel & air metering

Severity
High
Typical repair cost
40 – 650 EUR
System
Powertrain

What it means

Throttle position sensor "A" is the primary track of the potentiometer or Hall sensor on the throttle body (on cable throttles) or in the throttle body of an electronic throttle. The ECU expects its output to stay within roughly 0.3-4.7 V and to track sensor "B" inversely. P0120 sets when the signal is out of range, frozen, or jumps erratically compared with the second track and with engine load. On drive-by-wire cars the ECU usually limits throttle opening or forces limp-home idle because it can no longer verify the throttle plate position.

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Symptoms

  • Check engine light and often an EPC or throttle warning lamp
  • Limp mode: engine held at 1200-1500 rpm with no response to the pedal
  • Hesitation, jerking or sudden surges while driving
  • Rough or hunting idle, stalling when coming to a stop
  • Automatic transmission shifting harshly or at the wrong time because load signal is wrong

Common causes

  1. Worn potentiometer track in the throttle position sensor, causing dropouts at particular openings
  2. Carbon build-up in the throttle body causing the plate to stick and the signal to disagree with the commanded position
  3. Damaged or chafed wiring and corroded connector at the throttle body
  4. Loss of 5 V reference or ground to the sensor
  5. Faulty throttle body motor or gear wear on electronic throttles
  6. ECU input fault or a throttle adaptation that was never performed after a battery or part change

How to diagnose

  1. Read TPS A and TPS B in live data with the key on: typically A reads 0.5-0.9 V closed rising to about 4.0-4.5 V open while B does the opposite, and their sum stays near 5 V throughout the sweep
  2. Slowly open the throttle (pedal or by hand on cable systems) while watching the voltage on a scope or fast scan tool: any dropout, spike or flat spot indicates a worn track
  3. Check the supply at the connector: 4.9-5.1 V reference and under 0.1 V on the ground pin with the engine running
  4. Unplug the sensor and check continuity (under 1 ohm) of each wire to the ECU and insulation to ground (infinite)
  5. Inspect the throttle plate for carbon; clean with throttle cleaner only if the manufacturer permits, then perform the throttle adaptation (basic setting) with a scan tool
  6. If the sensor is integrated and all electrical checks pass, replace the throttle body and carry out adaptation

Frequently asked questions

Can I drive in limp mode with P0120?

Only carefully and briefly. The car will barely exceed walking pace on some models and may surge unpredictably; have it checked promptly.

Why does the fault come back after cleaning the throttle body?

Cleaning changes the closed-plate position. Most electronic throttles must be re-adapted with a scan tool afterwards or the ECU sees an implausible idle position.

Terms to know

Throttle position sensor (TPS)

Related codes

P0121P0122P0123P0124P0220P2135

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