P0121

What it means

P0121 is the generic SAE J2012 code for a range or performance fault in the throttle or pedal position sensor/switch 'A' circuit. The engine control unit has compared the signal from this sensor with other data, such as the second throttle sensor, manifold pressure, mass air flow or engine load, and found that it does not make sense for the conditions. The signal is still within electrical limits, so it is a plausibility fault rather than a simple open or short circuit.

Symptoms

• Engine management light on
• Hesitation or flat spots when accelerating
• Erratic, high or hunting idle
• Car enters limp-home mode with reduced power
• Poor or delayed throttle response
• Stalling or rough running at idle or when pulling away

Possible causes

• Dirty, carbon-coated or sticking throttle body causing a mismatch between commanded and actual throttle anglelikely
• Worn or faulty throttle position sensor or throttle body assembly, or accelerator pedal sensor, with a dead spot or noisy signallikely
• Damaged wiring, poor connections or corroded terminals in the sensor signal, reference or earth circuitslikely
• Unmetered air from a vacuum leak or split intake hose skewing the load comparisonpossible
• Faulty mass air flow or manifold pressure sensor giving data that disagrees with throttle positionpossible
• Low or unstable sensor reference voltage or poor earth at the control unitpossible
• Fault inside the engine control unit or an outdated calibration, where a manufacturer bulletin identifies itrare

How to diagnose

1. Read all stored and pending codes and record the freeze frame data, noting engine load, throttle angle, speed and temperature when the fault set
2. Check for related codes for the second throttle sensor, pedal sensor, air flow or manifold pressure, as these point the diagnosis
3. Inspect the throttle body, sensor connector and wiring for damage, corrosion, chafing, water ingress and loose pins
4. Check the intake system for split hoses, loose clamps and vacuum leaks
5. View live data and compare throttle and pedal angle readings with the throttle closed, slowly opened and fully open, looking for jumps, dropouts or disagreement between the two sensor channels
6. Check the throttle plate moves freely and is not sticking, and inspect it for heavy carbon build-up (procedure and any relearn requirement depend on model and engine)
7. Measure the sensor reference voltage, signal and earth at the connector against the manufacturer's specification
8. Compare air flow and manifold pressure readings with expected values at idle and under load to rule out another sensor

How to fix

• Clean the throttle body using the method recommended for that model, then carry out any required throttle adaptation or relearn
• Replace the throttle body or throttle position sensor, or the accelerator pedal assembly, if readings are erratic or out of specification
• Repair or replace damaged wiring, connectors or terminals, and restore good earth connections
• Repair intake leaks by replacing split hoses, seals or loose fittings
• Replace a faulty air flow or manifold pressure sensor if it is shown to be giving incorrect readings
• Apply a manufacturer-issued control unit software update only if a known calibration fault applies to the vehicle

Can I keep driving?

Nearby codes

• P0120GenericThrottle/Pedal Position Sensor/Switch A Circuit
• P0122GenericThrottle/Pedal Position Sensor/Switch A Circuit Low