Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.
P0122: Throttle/pedal position sensor or switch “A” circuit low
The engine computer is receiving voltage below the accepted range from throttle or accelerator-position circuit “A”.
What P0122 means
The engine computer is receiving voltage below the accepted range from throttle or accelerator-position circuit “A”. The relevant sensor may be mounted on the throttle body, built into an electronic throttle body or, on some vehicles, associated with the accelerator pedal. The wiring diagram must decide which.
Can I still drive?
Treat this as a potentially safety-related throttle fault. Do not continue if acceleration is delayed or unpredictable, the engine stalls, reduced-power mode appears or the car cannot keep pace safely; arrange recovery. If the warning lamp is steady, the throttle behaves completely normally and there are no other warnings, a short, low-speed trip directly to a garage may be reasonable. Avoid motorways and overtaking.
Symptoms
- Engine-management light.
- Weak or delayed acceleration.
- Reduced-power or limp-home mode.
- Rough or abnormal idle.
- Stalling or hard starting.
- Unusual automatic-gearbox shift behaviour on some vehicles.
- No symptom beyond the warning lamp in some cases.
Common causes
- Failed throttle-position sensor.
- Failed accelerator-pedal position sensor where this circuit assignment applies.
- Damaged, loose or corroded connector.
- Signal wire shorted to earth.
- Missing 5-volt reference or poor sensor earth.
- Damaged loom or integrated electronic throttle-body fault.
- Control-unit fault is possible but uncommon.
Safe first checks
- Save all stored and pending codes and freeze-frame data. With the ignition off, inspect the throttle-body and pedal connectors for looseness, water, corrosion or bent pins. Look for chafing where the loom passes brackets or hot parts, and ensure a loose floor mat is not obstructing the pedal. Leave voltage, continuity and relearn tests to somebody using the correct wiring information. Do not force open a motorised throttle plate.
Typical UK fix cost
Approximately £90–£450 for diagnosis and a common connector, wiring, separate-sensor or throttle-body repair. A small wiring repair may be near the lower end; an integrated electronic throttle body generally moves the bill towards the upper end. Vehicle-specific parts and relearn requirements can take it outside the range.
Related models
This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.
Related fault codes
- P0120: Throttle/pedal position sensor or switch “A” circuit
- P0121: Throttle/pedal position sensor or switch “A” range/performance
- P0123: Throttle/pedal position sensor or switch “A” circuit high
- P0128: Coolant temperature below thermostat regulating temperature
Sources
- Honda service information — P0122/P0123 advanced diagnostics (PDF)
- GM service information mirror — P0122/P0123 circuit description
- HELLA Tech World — throttle potentiometer testing
- Fixter — UK throttle-body replacement cost guide
A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.