P0721

Output Speed Sensor Circuit Range/Performance

Car Fault Codes Transmission

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 3 hours

Typical cost SAR 100 to 900

What it means

An output speed sensor signal is present, but the computer finds it implausible, such as sudden speed jumps or a value that does not agree with wheel speed or input speed and the current gear. The common cause is debris on the sensor or noise on the signal.

Symptoms4

Check engine light on.
Erratic shifts or sudden downshifts while cruising.
Speedometer fluctuating on some vehicles.
Transmission sometimes going into limp mode.

Possible causes5

Metal debris on a magnetic sensor tip.
Electrical noise from damaged wiring or wiring routed near noise sources.
A weak sensor, or a loose sensor that changes the gap to the tone ring.
A damaged or cracked tone ring on the output shaft.
Tire size or final drive ratio not matching the computer's programming (less common).

How to diagnose5

Compare output speed on a scan tool with wheel speed while driving, and look for jumps or mismatch.
Check the sensor signal with an oscilloscope for missing pulses or noise.
Confirm the sensor is mounted tightly and its tip is intact.
Inspect the wiring routing and condition.
Confirm the tire size matches the vehicle specification if no other fault is found.

How to fix5

Clean debris from the sensor tip and mount it securely.
Repair the wiring and reroute it away from noise sources.
Replace the output speed sensor if its signal stays faulty.
Repair a damaged tone ring, which requires internal work on some designs.
Clear the codes and confirm output speed stays steady and matches road speed.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for cleaning the sensor or repairing wiring, and higher when the sensor is replaced or the tone ring needs repair.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

Why does the transmission suddenly downshift?

Because a false jump in the output speed signal makes the computer think speed has dropped, so it commands a lower gear for no real reason.

Can the tires cause this code?

In a few cases, if the tire size is far from specification, the compared speeds may not match, but check the sensor and wiring first.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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