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P0720 to P0722: output speed sensor circuit and no signal
The output sensor feeds the speedometer, the cruise control, the ABS and the gearbox. When it goes quiet, four systems fail at once and none of them are broken.
The output speed sensor measures how fast the transmission output shaft is turning, which is directly proportional to road speed. That makes it one of the most widely consumed signals on the vehicle.
| Code | Meaning |
|---|---|
| P0720 | Output speed sensor circuit malfunction |
| P0721 | Range/performance |
| P0722 | No signal |
Related: P0500 vehicle speed sensor diagnostics.
The multi-system symptom
This is the reason the code is worth recognising quickly. One sensor failing produces:
- Speedometer dead or erratic: speedometer not working.
- Cruise control refusing to engage, because it cannot regulate to a speed it cannot measure.
- Harsh or absent shifts, since the gearbox cannot calculate ratio.
- ABS and stability control faults on vehicles that cross-check this against wheel speeds.
- Limp mode, holding a single gear.
- Odometer not counting on some vehicles, which has consequences beyond diagnostics.
Four warning lamps and one sensor. If that is your picture, look here before believing four separate faults.
Transmission not shifting diagnosis.
Where the speed signal comes from
Worth understanding, because vehicles differ.
Some take road speed from the transmission output sensor. Others derive it from the ABS wheel speed sensors and distribute it over the network. Many use both, and cross-check them.
That cross-check is what produces P0721: the transmission’s own sensor disagrees with the speed the ABS system is reporting. In that case one of the two is wrong, and comparing them tells you which.
ABS wheel speed sensor diagnostics and C0035 to C0050 wheel speed sensor codes.
Causes
The sensor
Often external and accessible on the transmission housing, which makes this one of the cheaper transmission repairs when it is the answer.
The connector
Underneath the vehicle, near the exhaust, in the road spray. Corrosion and heat damage.
The reluctor ring
Debris from clutch wear clinging to the toothed wheel adds phantom pulses. Damage to the teeth removes them. Either produces P0721 rather than a clean loss of signal.
Wiring
The signal is low level and the cable may be shielded. A repair that ignores the shield introduces noise, and noise reads as an erratic signal.
Tyre or wheel size changes
Not a fault, but worth knowing. Fitting a significantly different rolling radius changes the relationship between output shaft speed and actual road speed. On vehicles that cross-check against ABS or GPS, a large enough discrepancy can flag a fault.
Wheel and tyre size change electronics.
What to log
Vehicle speed against engine RPM, on a steady cruise in a known gear.
| Observation | Meaning |
|---|---|
| Speed tracks RPM smoothly | Sensor healthy |
| Speed drops to zero while moving | Signal loss — P0722 |
| Speed erratic or spiking | Reluctor debris or wiring noise |
| Speed reads but disagrees with GPS | Calibration or wrong tyre size |
That last check is easy and free: compare the indicated speed with a phone GPS on a steady road. Most speedometers read slightly optimistic by design; a large discrepancy is a finding.
The odometer angle
On vehicles where the odometer counts from this signal, a period of driving with a dead sensor means mileage was not recorded.
That matters for service intervals, for warranty, and for resale honesty. It is also one of the ways odometer discrepancies arise innocently: how to detect odometer fraud through OBD.
In what order
- Note which systems are complaining — several at once points here.
- Compare indicated speed with GPS.
- Log speed against RPM at a steady cruise.
- Check whether the ABS system reports a plausible speed — if it does and the transmission does not, the transmission sensor is the fault.
- Inspect the connector for corrosion.
- Establish whether the sensor is external before assuming a big job.
- Check the pan for debris if the signal is erratic rather than absent.
Try it on the free tier of the app — pair an ELM327 adapter and log vehicle speed alongside engine RPM on a motorway run. A speed trace that drops out while you are clearly still moving is the fault, captured.
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