P1121: Throttle Position (TP) Sensor Circuit Intermittent High Voltage

P1121 means the ECU has detected an intermittently high voltage signal coming from the throttle position sensor circuit. The throttle position sensor tells the engine management system exactly how far the throttle plate is open, and the ECU uses that data to calculate fuelling and ignition timing. When the signal spikes above its expected range on and off rather than consistently, it gets flagged as an intermittent high voltage fault rather than a hard failure.

Intermittent faults like this are often caused by a failing sensor with worn internal resistance, a chafed or frayed wire in the sensor harness that occasionally touches a voltage source, or a connector that has developed corrosion or a loose pin. Because the fault is intermittent, it can be harder to catch in real time. Start by inspecting the wiring and connector at the sensor for any signs of damage or corrosion, then check the sensor's signal voltage live with a scan tool while moving the throttle through its full range.

At medium severity, P1121 can cause erratic throttle response, hesitation under acceleration, or the engine entering a reduced-power mode. It should be addressed promptly rather than left to worsen. If the wiring checks out clean, replacing the throttle position sensor is usually the resolution.