The intermediate shaft speed sensor is not reporting correctly, which upsets shift timing on a modern multi-speed automatic or DSG.
P0791 — also written PO791, P 0791, 0791 — is the OBD-II code for intermediate shaft speed sensor circuit. Same code, same fix: this page covers every spelling of it.
P0791 is a transmission fault code, typically costing £180–£1600 to put right in a UK garage.
What causes P0791?
- Failed speed sensor
- Mechatronic unit fault on DSG boxes
- Wiring or connector damage
- Low fluid level
How P0791 is fixed
- Read the gearbox module for related codes
- Replace the sensor where separately serviceable
- Mechatronic repair or replacement on DSG
P0791 repair cost in the UK
Most P0791 repairs land between £180–£1600 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 P0791?
Expect limp mode with a fixed gear. Recover if speed is severely limited.
Urgency: high.
What P0791 is telling you
Gearbox faults are logged when the shift the control unit asked for is not the shift it measured. Fluid condition and level affect that measurement as much as any electrical part, which is why the fluid is checked before anything is dismantled.
On P0791 specifically — intermediate shaft speed sensor circuit — 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 P0791 is diagnosed properly
Each of the known causes is ruled in or out in cost order, cheapest and most likely first. On P0791 that means working through them like this:
- Failed 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.
- Mechatronic unit fault on DSG boxes — confirmed with live data before any part is bought.
- Wiring or connector damage — 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 level — checked for level and condition first, as the cheapest possible cause and a common one.
An hour of diagnostic time on P0791 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 P0791
- Read the gearbox module for related codes
- Replace the sensor where separately serviceable
- Mechatronic repair or replacement on DSG
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 P0791 is ignored
This is a stop-and-sort fault rather than a wait-for-the-service fault. Ignoring it risks turning a sensor-money repair into a component-money one.
For reference, P0791 typically lands at £180–£1600 all-in at an independent garage in the UK — main-dealer quotes for the same job usually sit above that.
Getting P0791 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 P0791 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.