Fault codeP0791

MODERATE

Intermediate Shaft Speed Sensor Circuit

What this code means

Intermediate Shaft Speed Sensor Circuit.

P0791 is a generic code: it means the same thing on every make. P0791 belongs to family P07xx — Transmission. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit has detected an electrical anomaly on this circuit without saying exactly which. Diagnosis starts with a visual check of the wiring and connector, then measuring the component — resistance, supply, ground — with a multimeter.

Gearbox speed sensors

Automatic gearboxes measure the speed of their input shaft (turbine), their output shaft and sometimes an intermediate shaft. By comparing them with the engine speed, the control unit knows which gear is engaged and whether something is slipping.

What you notice

Harsh or erratic shifts, the gearbox staying in one gear (limp mode), a speedometer that misbehaves.

How to check it

Check the sensors' connectors on the gearbox and the fluid level. The tips of these sensors are magnetic and collect metal particles: cleaning them helps, and a large amount of debris says something about the gearbox's wear. Klaro's engine speed and vehicle speed show whether the ratio is consistent in each gear.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Vehicle speed
Engine RPM

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
7Transmission
91P0791