The output speed signal from the gearbox is erratic. Gear selection and torque converter lock-up depend on it, so shifts become unpredictable.
P0721 — also written PO721, P 0721, 0721 — is the OBD-II code for output speed sensor circuit range/performance. Same code, same fix: this page covers every spelling of it.
P0721 is a transmission fault code, typically costing £150–£650 to put right in a UK garage.
What causes P0721?
- Failed output speed sensor
- Damaged reluctor teeth
- Chafed wiring under the car
- Low fluid
How P0721 is fixed
- Compare output speed with wheel speed on live data
- Replace the sensor
- Repair the loom
P0721 repair cost in the UK
Most P0721 repairs land between £150–£650 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Can you drive with P0721?
Drive gently to a garage.
Urgency: high.
What P0721 is telling you
Transmission codes come from the gearbox control unit comparing commanded gear, clutch or solenoid state against the speeds its sensors report. A mismatch triggers a protection strategy, which is what the driver feels as limp mode or a locked gear.
On P0721 specifically — output speed sensor circuit range/performance — the stored code records the moment that check failed, not the moment the part gave up. That gap is why two cars with the same code can need completely different repairs.
How P0721 is diagnosed properly
Each of the known causes is ruled in or out in cost order, cheapest and most likely first. On P0721 that means working through them like this:
- Failed output speed sensor — compared against live data and a known-good reference before it is condemned — a sensor reading the wrong thing accurately is a different fault from a dead one.
- Damaged reluctor teeth — confirmed with live data before any part is bought.
- Chafed wiring under the car — checked with the loom flexed and the engine warm, because a wiring fault that is intermittent when cold shows itself under heat and vibration.
- Low fluid — checked for level and condition first, as the cheapest possible cause and a common one.
An hour of diagnostic time on P0721 routinely saves more than it costs, because the alternative is buying the most expensive part on that list and hoping.
The repairs that actually clear P0721
- Compare output speed with wheel speed on live data
- Replace the sensor
- Repair the loom
After the repair the code is cleared and the car is driven until the relevant readiness monitor completes, rather than handed back as soon as the light goes out. A code that has only been erased will come straight back on the next drive cycle.
What happens if P0721 is ignored
Leaving this one is the expensive choice. The protection strategy the ECU falls into is there to limit damage, and every mile driven with the fault active is a mile the engine or gearbox spends outside its designed operating window.
For reference, P0721 typically lands at £150–£650 all-in at an independent garage in the UK — main-dealer quotes for the same job usually sit above that.
Getting P0721 looked at
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0721 is showing on your car, bring the scan results with you — we will read the live data ourselves, tell you which of the causes above it actually is, and price the repair before any work starts.