What does it mean?
P1123 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Accelerator Pedal Position Sensor ‘D’ Circuit High Input .
Symptoms
- Malfunction indicator lamp on with the vehicle dropping into reduced power
- Pedal feels dead or delayed; engine speed does not follow pedal movement
- Unexpected idle rise or surging right after start-up
- Cruise control refuses to engage or drops out
- Hesitation and flat spots during acceleration
Likely causes
- Short to battery voltage or to a 5 V reference line inside the pedal harness
- Corroded, spread or water-soaked terminals in the pedal sensor connector
- Open or high-resistance sensor ground return, which drags the signal line upward
- Worn resistive track or internal failure inside the pedal position sensor assembly
- Chafed harness between the pedal bracket and the bulkhead, rubbing through the insulation
- Wrong or poorly matched replacement pedal assembly fitted previously
- Rarely, a failed analogue input or reference driver inside the PCM itself
How to fix it
- Read freeze frame data first, then log APP sensor 'D' voltage at rest and through full pedal travel with a scan tool
- Perform a wiggle test on the pedal connector and harness while watching that live value for spikes
- Unplug the connector and inspect for moisture, green corrosion, pushed-back or spread terminals
- Measure signal voltage at the PCM connector and compare it with the reading at the sensor to expose voltage drop
- Check sensor ground resistance to chassis; anything above a few tenths of an ohm needs repair
- With the sensor unplugged, check the signal wire for a short to battery or to the reference line
- Replace the pedal assembly only after the tracks prove out of specification, then clear codes and verify the learned pedal values