What does it mean?
P1639 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Accelerator Pedal Position Sensor 1+2 Range/Performance .
Symptoms
- MIL (check engine lamp) on with a reduced power warning
- Engine limited to a low rpm ceiling, with the pedal partly or completely ignored
- Sudden loss of power that clears after switching the ignition off and on
- Hesitation, surging or a flat spot at part throttle
- Cruise control disabled
- Uneven idle and occasional stalling when stopping
Likely causes
- One of the two sensor tracks worn or contaminated, so the channels drift apart
- Water and corrosion in the pedal connector affecting only one channel
- Shared reference voltage or ground with high resistance, shifting one signal relative to the other
- Harness chafed or pinched under the carpet or through the bulkhead
- Pedal assembly mechanically damaged, sticking, or obstructed by a floor mat
- Pedal adaptation never carried out after replacement or a battery disconnection
- Module input circuit fault, the least likely cause
How to fix it
- Graph both pedal signals together through a slow full sweep; they should track in a fixed relationship with no steps or dropouts
- Wiggle the pedal harness and connector while watching for a glitch on either channel
- Measure the reference voltage and both grounds at the pedal connector and check for voltage drop
- With the connectors unplugged, test each signal wire for continuity and for shorts to ground and to power
- Check that the pedal moves freely through its full travel and remove any obstruction such as a bunched floor mat
- Replace the accelerator pedal assembly when one track is proven faulty
- Carry out the pedal adaptation and confirm the two channels agree across the whole range before returning the vehicle