P0123 — TPS/APP Sensor 'A' Circuit High Input

Quick Answer

• What it means: The Powertrain Control Module (PCM) detects a higher than expected voltage signal from the Throttle Position Sensor (TPS) or Accelerator Pedal Position (APP) Sensor 'A' circuit.
• Severity: Get towed or repair immediately; this can cause unpredictable throttle response or no throttle response at all, making the vehicle unsafe to drive.
• Most common cause: A short to voltage in the TPS/APP sensor 'A' circuit or a faulty TPS/APP sensor.

What the P0123 code means

The P0123 diagnostic trouble code stands for 'Throttle Position Sensor/Accelerator Pedal Position (TPS/APP) Sensor 'A' Circuit High Input'. This means that the vehicle's Powertrain Control Module (PCM) or Engine Control Unit (ECU) has detected a voltage signal from the TPS or APP sensor 'A' circuit that is above the manufacturer's specified limit. The TPS and APP sensors are crucial components in modern electronic throttle control systems. The TPS monitors the position of the throttle plate, while the APP sensor monitors the position of the accelerator pedal. Both send voltage signals to the PCM, which then uses this information to determine how much air to let into the engine and, consequently, how much fuel to inject. A "high input" typically indicates a voltage reading that is continuously at or near the maximum allowable voltage (e.g., 5 volts on a 5-volt reference circuit) when the throttle or pedal is not in that position, or a sudden, erratic spike in voltage.

Common symptoms

• Check Engine Light illuminated
• Reduced engine power or "limp home" mode
• Unpredictable engine RPMs (surging or idling high)
• Engine hesitation or stalling
• Difficulty accelerating
• No throttle response (pedal is pressed but engine RPMs do not increase)
• Vehicle may go into "failsafe" mode, limiting speed

Common causes

1. Faulty Throttle Position Sensor (TPS) or Accelerator Pedal Position (APP) Sensor: The sensor itself may be internally shorted, sending a high voltage signal regardless of throttle position.
2. Short to voltage in the TPS/APP sensor 'A' circuit: A wiring harness chafed or damaged, causing the signal wire to come into contact with a power wire.
3. Corroded or loose electrical connectors: Poor connection at the TPS/APP sensor or PCM can lead to intermittent or constant high voltage readings.
4. Faulty PCM: While less common, a defective PCM can misinterpret sensor data or have an internal fault causing a high voltage reading. This is usually diagnosed after all other possibilities have been ruled out.

How a technician diagnoses it

1. Verify the complaint and perform a visual inspection: The technician will first confirm the P0123 code using an OBD-II scan tool. They will then visually inspect the TPS/APP sensor, its wiring harness, and connectors for any obvious signs of damage, corrosion, or loose connections.
2. Check for Technical Service Bulletins (TSBs): The technician will check for any known TSBs related to the P0123 code for the specific vehicle make and model.
3. Monitor live data with a scan tool: The technician will connect a scan tool and monitor the TPS/APP sensor 'A' voltage while moving the throttle pedal (if APP sensor) or manually moving the throttle plate (if TPS). A healthy sensor should show a smooth, varying voltage from low (e.g., 0.5V at closed throttle) to high (e.g., 4.5V at wide open throttle). If the voltage is constantly at 5V or jumps erratically to 5V, it indicates a problem.
4. Perform voltage drop tests and continuity checks: Using a multimeter, the technician will check for proper reference voltage (typically 5V) and ground at the TPS/APP sensor connector. They will also perform continuity checks on the signal wire between the sensor and the PCM to check for opens or shorts to voltage/ground. A high input indicates a short to voltage on the signal wire or an internal sensor fault creating a high voltage on its output.
5. Test the TPS/APP sensor directly: If accessible, the sensor may be disconnected and its resistance or voltage output tested directly with a multimeter according to manufacturer specifications. Some sensors, specifically APP sensors, might have multiple redundant circuits for safety; the