P0791: Intermediate Shaft Speed Sensor A Circuit
What to do
Expect restricted shifting or a fixed gear. Worth repairing before any internal diagnosis is attempted, since this sensor is one of the things that would make such a diagnosis cheaper.
What this code means
A fault in the third of the transmission's speed sensors, positioned between the input and the output on gearboxes with enough planetary sets to warrant one.
Its value is in what a third measurement makes possible. With only input and output speeds, a ratio error tells the module that something between them is wrong but nothing about where. Adding a measurement in the middle splits the transmission into two halves that can be assessed separately — so a ratio fault confirmed by the intermediate sensor is localised to the front or rear section before anything is opened. On a transmission where opening it is the expensive part, narrowing the search by half is worth having.
Lose the sensor and that advantage goes with it. The transmission still runs, but its self-diagnosis becomes considerably coarser, which is why the module treats the loss seriously enough to restrict what it will do.
Likely causes
Start here
- Faulty intermediate shaft speed sensor · specialist
- Damaged transmission wiring harness · moderate
- Corroded transmission connector · moderate
Less common
- Metal debris on the sensor tip · specialist
Rare
- Faulty transmission control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P0791.