Transmission

P0791 — Intermediate Shaft Speed Sensor Circuit Malfunction

Severity: medium Drive with care, to a shop

What P0791 means

The transmission control module (or PCM) reads a speed signal from the intermediate shaft sensor in the automatic transmission, and it has detected that the signal is missing or out of range. The module compares this speed with the input and output shaft speeds to work out the actual gear ratio and control hydraulic pressure. A bad signal makes accurate shift control impossible.

Symptoms

  • Check Engine or transmission warning light on
  • Harsh or abrupt shifts
  • Transmission limp mode (stuck in one gear)
  • Additional incorrect gear ratio codes
  • Erratic or missing speedometer reading
  • Reduced fuel economy

Possible causes

  1. Failed intermediate shaft speed sensor
  2. Damaged wiring inside the transmission or in the harness
  3. Poor connection or corrosion at the connector
  4. Short to ground or to battery voltage in the sensor circuit
  5. Damaged or dirty tone ring on the shaft
  6. Metal debris on the magnetic sensor from internal wear
  7. Faulty TCM or PCM (uncommon)

How to diagnose

  1. Scan all codes and review freeze frame data for other shaft speed or gear ratio codes
  2. Compare input, intermediate and output shaft speed readings on a scan tool while driving
  3. Inspect the connector and harness for chafing, corrosion and fluid leaks
  4. Check transmission fluid level and condition, and look for metal debris
  5. Test the sensor circuit for power, signal and ground and for shorts, following the factory wiring diagram
  6. Measure sensor resistance or check the signal pattern with a scope
  7. Inspect the tone ring and sensor mounting when the pan or transmission is opened
  8. If the circuit tests good, check the TCM/PCM and for software updates

Common fixes

  • Replace the intermediate shaft speed sensor
  • Repair the wiring or replace the internal transmission harness
  • Clean or replace the connector and restore the contact
  • Replace the tone ring or repair the transmission if internal wear is found
  • Change the fluid and filter if contamination is the cause
  • Reflash or replace the control module once it is confirmed faulty

P0791 on different brands

Ford · manuals
On Ford, P0791 may appear with an extended sub-code such as P0791:12, which indicates a short to battery voltage in the sensor circuit; 8-speed transmissions have one such sensor and 10-speeds have two.

Questions and answers

Can I drive with a P0791 code?

Only briefly and carefully, to get to a shop. The transmission is likely in limp mode, and extended driving can speed up wear and raise the repair cost.

What most often causes P0791?

Most often it is the intermediate shaft speed sensor itself, damaged wiring or a poor connector contact. Start by inspecting the harness and connector, then test the sensor.

Can I just clear the P0791 code?

Clearing it only hides the problem for a while. If the cause is not fixed, the code will return and the transmission may go back into limp mode.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.