Transmission

P0721 — Transmission Output Speed Sensor Range/Performance

Severity: medium Drive with care, to a shop

What P0721 means

The transmission control unit compares the output shaft speed sensor signal with input shaft speed, wheel speed and the selected gear. If the signal is out of range or doesn't match those values, P0721 is stored. This sensor helps time shifts and feeds the speedometer.

Symptoms

  • Check Engine light on
  • Erratic or inaccurate speedometer
  • Harsh or delayed shifts
  • Transmission stuck in one gear (limp mode)
  • ABS or traction control light may come on
  • Torque converter lockup not working or flaring

Possible causes

  1. Failed output speed sensor
  2. Damaged, broken or corroded wiring or connector
  3. Metal debris or dirt on the sensor's magnetic tip
  4. Damaged or worn tone ring on the shaft
  5. Internal transmission problem
  6. Faulty transmission control unit (uncommon)

How to diagnose

  1. Scan all codes and freeze frame data, and look for related wheel speed or ABS codes
  2. Compare output speed sensor data with wheel speed and speedometer on a scan tool
  3. Inspect the sensor, harness and connector for damage, oil, corrosion or rubbing
  4. Check fluid level and condition, and look for metal debris
  5. Remove the sensor and inspect its tip for debris or damage
  6. Test sensor resistance, power and ground per the factory diagram, and check wiring continuity
  7. Scope the signal while driving to check its pattern
  8. If the sensor and wiring are good, inspect the tone ring and diagnose the transmission

Common fixes

  • Replace the output speed sensor
  • Repair the wiring or replace the connector
  • Clean the sensor and change fluid if contaminated
  • Replace a damaged tone ring
  • Repair the transmission if an internal fault is found

Questions and answers

Can I drive with a P0721 code?

For a short time, yes, but the transmission may shift harshly or go into limp mode. Get it diagnosed soon to avoid transmission damage.

Why is the ABS light on with P0721?

Several systems use speed signals. A speed circuit fault or shared wiring problem can affect ABS too, so check the wheel speed sensors as well.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.