What does it mean?
P1871 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: APS system (Signal abnormality) .
Symptoms
- Malfunction indicator lamp on and the engine drops into reduced power mode
- Little or no response when the pedal is pressed; the car will only creep
- Hesitation, surging or sudden rpm changes while driving at part throttle
- Unstable idle or the engine settling at a fixed elevated idle
- Cruise control disabled and a warning message displayed
- Fault may appear only in a specific pedal range, such as just off idle
Likely causes
- Worn or contaminated sensor track producing dropouts and spikes as the pedal moves
- Open, shorted or high-resistance wiring in the signal, 5 V reference or ground circuit
- Corroded or loose connector at the pedal assembly, often after a footwell water leak
- Harness chafed against the pedal bracket or crushed under trim
- Reference voltage fault shared with other sensors, pulling the pedal signal off
- Poor sensor ground causing an offset between the two signal tracks
- Failed input circuit in the engine control module
How to fix it
- Read all codes and freeze frame data to see the pedal position when the fault set
- Graph both pedal signals in live data while pressing the pedal slowly through its full travel and look for dropouts or crossovers
- Wiggle the pedal connector and the harness behind the trim while watching the same data
- Measure the 5 V reference and ground at the sensor connector with the key on
- Perform a voltage-drop test on the sensor ground circuit
- Clean or repair corroded terminals; if the footwell has been wet, dry and seal the area
- Replace the pedal assembly if the signals stay erratic with proven-good wiring, then carry out any required pedal relearn