P0721Output Shaft Speed Sensor Circuit Range/Performance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- The output shaft speed sensor (Hall or inductive) reports the speed of the transmission output; it gives the gear ratio together with input speed and on many vehicles also the road speed for speedometer and cruise control. P0721 means the output speed is present but implausible: too high (Ford 6R80 above 7450 rpm), not matching wheel or input speed in the engaged gear (ZF 6HP26 more than 200 or 250 rpm apart, Ford's F21 more than 15 % for 10 s), or jumping (Allison 1000: 500 rpm within 0.15 s). On GM's 10-speed with a direction-sensing sensor, P0721 means the reported direction of rotation does not fit – since 2025 after two trips, earlier after one. Wrong tyre sizes or a faulty wheel speed sensor can also falsify the comparison.
- What you may notice
- Speedometer jumps
- Warning lamp
- Wrong shifts, limp mode
- ABS/ESP warning possible if wheel speed is affected
- Likely causes
- LikelyOutput speed sensor faulty or dirty (metal debris)
- PossibleContact problems or interference in the sensor circuit
- PossibleWrong tyre size or axle ratio, faulty wheel speed sensor
- RareReluctor wheel damaged or sensor mounted wrongly
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €80 – €450
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.