What does it mean?
P1791 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T Vehicle Speed Sensor Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and the speedometer is erratic or reads zero
- The transmission is stuck in one gear or shifts at completely wrong road speeds
- Harsh shifts and no torque converter lockup
- Cruise control will not engage
- ABS and traction control warning lamps may light at the same time
- Odometer may stop counting on some vehicles
Likely causes
- Failed output or vehicle speed sensor with a weak, noisy or absent signal
- Metal debris built up on the sensor tip from internal transmission wear
- Damaged, cracked or contaminated tone ring, or the wrong air gap after a repair
- Open or shorted sensor wiring, or a corroded connector at the transmission
- Sensor wire routed against an ignition lead or other noise source, corrupting the signal
- Fault in the module or component that supplies the speed signal to the transmission, such as the wheel speed source
- Control module input circuit fault
How to fix it
- Compare vehicle speed and individual wheel speeds in live data during a road test
- Remove and inspect the sensor for debris on the tip and check the tone ring teeth for damage
- Measure sensor resistance or check its output signal while a wheel is rotated, according to the sensor type
- Confirm supply voltage and ground for a Hall-effect type sensor with the ignition on
- Check wiring for continuity, shorts and damaged shielding, and confirm the loom is routed away from ignition components
- Fit a new sensor once wiring is proven, then verify a stable speed reading through the full speed range