What does it mean?
P1721 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Vehicle Speed Signal .
Symptoms
- MIL on with the speedometer reading zero or jumping erratically
- Cruise control will not engage, or drops out while driving
- Harsh or wrong shift points, with the transmission holding a gear
- ABS and stability control warning lights on
- Odometer stops counting distance
- Idle instability when coming to a stop
Likely causes
- Faulty vehicle speed sensor or transmission output speed sensor
- Metal debris collected on the sensor tip, or a damaged reluctor ring with chipped teeth
- Open, shorted or high-resistance wiring in the speed signal circuit
- Corroded sensor connector or harness damage near the transmission
- Wheel speed sensor or ABS module fault where the speed value is shared over the network
- Incorrect tyre size or final drive ratio after a modification, producing an implausible speed
- Receiving module input circuit fault
How to fix it
- Compare the vehicle speed value in live data with the actual road speed during a road test
- Check whether the speedometer and the scan tool value agree, which separates a sensor fault from a cluster fault
- Inspect the speed sensor for metal debris on the tip and check the reluctor ring for damaged or missing teeth
- Measure the sensor resistance or its output signal, and check the air gap where it is adjustable
- Test the signal circuit for continuity and shorts, and inspect the connector for corrosion and moisture
- Scan the ABS module for wheel speed sensor codes if the speed value is shared over the network
- Replace the speed sensor if the signal is missing or erratic, then confirm the reading on a road test