P0791–P2749 Intermediate Shaft Speed Sensor A Circuit
The ECU has logged a fault on P0791. The definition below is the exact wording the standard uses for this code.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0791 | Intermediate Shaft Speed Sensor A Circuit | Investigate soon |
P2745 | Intermediate Shaft Speed Sensor B Circuit | Investigate soon |
P2749 | Intermediate Shaft Speed Sensor C Circuit | Investigate soon |
The definition
P0791: Intermediate Shaft Speed Sensor A Circuit.
P0791 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
- Check the connector at the part for corrosion, and that it is fully seated.
- Clear the code and see whether it returns, and under what conditions.
What actually causes this on a VAG
Your gearbox has speed sensors inside it that tell the transmission control unit how fast its internal shafts are turning. The control unit compares those speeds against engine and road speed to know when a shift has completed and how much the clutches are slipping. When that signal goes missing or reads implausibly, the gearbox protects itself - harsh or delayed shifts, refusing to select a gear, or dropping into limp home, which can mean losing drive in traffic. On DSG boxes and most modern VAG automatics these sensors are inside the unit rather than being a bolt-on part you can simply swap, so the diagnosis matters more than usual before anyone commits to a repair.
Most likely causes, in order
- Damaged gearbox connector or loom. Start here, because it is the cheapest thing that produces this fault. The multi-pin connector where the loom meets the gearbox lives in heat and road spray, and a cracked seal or corroded pin upsets the signal. On gearboxes whose control unit sits outside the casing, gearbox fluid can also creep along the loom and contaminate the plug - so look for oil, damp or green corrosion in the connector before assuming the worst.
- A connection that only drops out sometimes. If the car has been behaving itself and this code was simply found stored, don't let anyone replace parts on the strength of it. With the transmission control unit's live data on screen, wiggling the loom and connector while watching for a drop-out, or logging the speed signals on a drive that actually reproduces the fault, tells you whether the signal genuinely disappears or the gearbox is slipping. Those are two completely different bills.
- Recent work in the area. If this appeared soon after clutch, gearbox, subframe or exhaust work, suspect a connector that wasn't latched, a loom pulled tight or a plug left disturbed before anything internal. It costs nothing to check and it is a very common trigger.
- Sensor inside the mechatronic or valve body unit. On DSG boxes and many torque converter automatics the shaft speed sensors are built into the internal control unit, so a failed sensor or its internal wiring means going inside. It shows in the transmission control unit's live data: one speed value sitting at zero or jumping about while the others read sensibly is the giveaway. Confirm it there before agreeing to any internal work.
- Genuine internal wear. Worn clutches or a failing bearing can change how the shafts behave and produce implausible speed readings. It's the least likely explanation for a circuit fault and should only be considered once the wiring, connector and control unit live data have been examined - and normally only if the car is also slipping or shifting badly.
Worth knowing: This code on its own is not a scrapped gearbox, whatever the first quote says. Have the transmission control unit's live speed values read before agreeing to anything - a mechatronic repair or exchange unit is normally far less than a complete replacement gearbox, and a corroded connector is cheaper still.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does P0791 mean?
P0791 means Intermediate Shaft Speed Sensor A Circuit. It is logged when the ECU sees this circuit outside its expected operating window.
Can I drive with P0791?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.