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P2120 – Throttle/Pedal Position Sensor/Switch D Circuit (OBD2)

P2120 means "Throttle/Pedal Position Sensor/Switch D Circuit". The control unit has detected a fault in the electrical circuit of the throttle / pedal position sensor: the signal or control current is missing or outside the expected window. The code does not say whether the value is too high or too low, so the wiring and the component itself both have to be checked.

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P2120 quick summary

Fault codeP2120
MeaningThrottle/Pedal Position Sensor/Switch D Circuit
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityHigh
Can I drive?Only carefully. The engine often goes into limp mode with very limited power, which can be dangerous when overtaking or joining a motorway. Have it diagnosed as soon as possible.
Repair cost€30 – €500

How electronic throttle and pedal position sensors work

Throttle and accelerator pedal position sensors tell the engine control unit where the throttle valve and the accelerator pedal are – the basis of electronic throttle control (drive-by-wire).

On modern cars there is no throttle cable. The accelerator pedal has two (sometimes three) position sensors, and the throttle body has its own motor and two position sensors. The control unit compares the driver's request from the pedal with the actual throttle opening hundreds of times per second.

For safety, each position is measured by two independent sensors (A/B, or D/E for the pedal) whose signals must always agree in a fixed relationship – for example one rising while the other falls, or one at half the voltage of the other. If they disagree (P2135, P2138) or one leaves its range, the control unit cannot trust the signal and switches to limp mode: limited throttle opening, raised idle, or no response to the pedal.

Most faults are caused by worn sensor tracks, dirt or carbon in the throttle body, corroded connectors or wiring damage. On older cars with a cable, the throttle position sensor is a simple potentiometer on the throttle body.

Symptoms of P2120

  • Limp mode: very limited power, engine does not rev above about 2,000–3,000 rpm
  • Check Engine and/or EPC light on
  • Delayed or no response to the accelerator pedal
  • Irregular idle or stalling
  • Cruise control not working

Most common causes of P2120 (most likely first)

Mechanic's Pro Tip

Never clean a throttle body and drive off without running the throttle adaptation – the control unit has learned the dirty valve's position and will idle badly or throw new codes. On most cars the adaptation takes a minute with a scan tool and is the most commonly missed step.

  1. 1.Damaged wire or loose, corroded connector at the throttle / pedal position sensor

    Most likely

    Green or white corrosion, pushed-back pins and chafed insulation near hot or moving parts are the most common findings.

  2. 2.Faulty throttle / pedal position sensor

    An internal break or short in the component itself.

  3. 3.Blown fuse or missing supply voltage

    If several codes appeared at the same time, check the shared fuse, relay or ground point first.

  4. 4.Dirty or carbon-coated throttle body

    Very common on diesels and direct-injection petrol engines – the valve sticks and the actual position no longer follows the command.

  5. 5.Worn sensor track in the throttle body or pedal

    Potentiometer tracks wear at the most-used positions and cause dropouts.

  6. 6.Wiring fault or shared 5 V reference problem

    A short in another sensor on the same reference circuit can affect both position signals.

  7. 7.Throttle motor or gearbox failure

    The motor or its plastic gears inside the throttle body wear out.

Step-by-step diagnostic guide

  1. 1

    Read codes and live data

    Read all codes and check whether the fault is on the pedal side (accelerator pedal position sensor) or the throttle side. Look at both sensor values in live data while slowly pressing the pedal.

  2. 2

    Check the circuit with a multimeter

    With the ignition off, unplug the throttle / pedal position sensor and the control unit connector (or back-probe), then check each wire between them for continuity and for shorts to ground and to battery voltage. Compare the component's resistance or supply voltage with the value in the repair manual.

    Multimeter: Wire continuity: below about 1 Ω. Wire to ground with both connectors unplugged: no continuity. Supply/reference with ignition on: usually 5 V for sensors, battery voltage for actuators.

  3. 3

    Check signal agreement

    Both signals must change smoothly without jumps and keep their fixed relationship over the whole travel. A jump or a flat spot at one position means a worn track.

    Multimeter: Typical pedal sensors: signal 1 about 0.5–4.5 V, signal 2 about half of signal 1 or mirrored – check the specification for your vehicle.

  4. 4

    Inspect and clean the throttle body

    With the ignition off, remove the intake hose and inspect the throttle valve. Clean carbon carefully with throttle body cleaner without forcing the valve.

  5. 5

    Check wiring and connectors

    Check the connectors at the pedal and throttle body for corrosion, and the wiring for chafing. Measure the 5 V reference and ground.

  6. 6

    Run the throttle adaptation

    After cleaning or replacing the throttle body or pedal, run the throttle adaptation (basic setting) with a scan tool, otherwise the idle and limp mode problems can continue.

Frequently asked questions

Can I keep driving with P2120?

Only carefully. The engine often goes into limp mode with very limited power, which can be dangerous when overtaking or joining a motorway. Have it diagnosed as soon as possible.

How much does it cost to fix P2120?

Typical repair costs are 30 € – 500 €, depending on the cause and the vehicle. The most common fixes are: damaged wire or loose, corroded connector at the throttle / pedal position sensor; faulty throttle / pedal position sensor; blown fuse or missing supply voltage. Always confirm the cause with the tests above before buying parts.

Why does my car go into limp mode with a throttle code?

Because the throttle controls engine power directly. If the control unit cannot trust the position signals, it limits the throttle opening for safety so that the car cannot accelerate unexpectedly.

Is the pedal sensor or the throttle body faulty?

The code tells you which sensor is involved – pedal codes usually mention 'pedal' or sensors D/E/F, throttle codes sensors A/B/C. Live data showing both signals over the full travel confirms which part is at fault.

What is the difference between P2120 and similar codes?

P2120 belongs to a group of codes for the same component that differ in the type or location of the fault: P0510 – Closed Throttle Position Switch Malfunction; P0638 – Throttle Actuator Control Range/Performance (Bank 1); P0639 – Throttle Actuator Control Range/Performance (Bank 2); P2121 – Throttle/Pedal Position Sensor/Switch D Range/Performance; P2122 – Throttle/Pedal Position Sensor/Switch D Circuit Low; P2123 – Throttle/Pedal Position Sensor/Switch D Circuit High. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.