Damaged, chafed or corroded wiring to the intermediate shaft speed sensor; Poor connection at the sensor or transmission connector (water ingress, corrosion, loose pins); Faulty intermediate shaft speed sensor; Metal debris or contamination on the sensor tip or reluctor ring from internal transmission wear; Low, degraded or contaminated transmission fluid affecting sensor readings or gearbox operation; Damaged or worn reluctor/tone ring or shaft fault; Fault in the transmission control unit or its internal wiring (mechatronic unit on some designs).

P0791

Intermediate Shaft Speed Sensor A Circuit.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
91: FaultThe specific fault in that area

What it means

P0791 is the generic SAE J2012 code for a fault in the Intermediate Shaft Speed Sensor A circuit. The transmission control unit monitors the signal from this sensor, which reads the speed of an intermediate shaft inside the automatic gearbox, and compares it with other speed inputs. It sets the code when the signal is missing, erratic or implausible. Which shaft the sensor reads, and where it sits, varies with the transmission design.

Symptoms

Malfunction indicator lamp or transmission warning light on
Harsh, delayed or flared gear changes
Gearbox stuck in one gear (limp mode)
Speedometer or rev-based gauges behaving erratically
Torque converter lock-up not engaging, with higher fuel consumption
Poor or no gear selection under load

Possible causes

Damaged, chafed or corroded wiring to the intermediate shaft speed sensorlikely
Poor connection at the sensor or transmission connector (water ingress, corrosion, loose pins)likely
Faulty intermediate shaft speed sensorlikely
Metal debris or contamination on the sensor tip or reluctor ring from internal transmission wearpossible
Low, degraded or contaminated transmission fluid affecting sensor readings or gearbox operationpossible
Damaged or worn reluctor/tone ring or shaft faultrare
Fault in the transmission control unit or its internal wiring (mechatronic unit on some designs)rare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting vehicle speed, gear and fluid temperature when the fault set
Check for related codes, such as other speed sensor or ratio errors, which point to a shared wiring or power fault
Check the transmission fluid level and condition using the correct procedure for the model, as some are sealed units
Inspect the sensor wiring and connector for chafing, heat damage, corrosion and fluid ingress
Check live data to see whether the intermediate shaft speed reads plausibly against input and output speed while driving
Test the sensor circuit for supply, earth and signal, and measure sensor resistance or signal pattern against the manufacturer's specification
Remove the sensor, if accessible, and inspect the tip for debris and damage, and check the fluid for metal particles
Where the sensor is internal to the gearbox, consult model-specific procedures before dismantling anything

How to fix

Repair or replace damaged wiring and terminals in the sensor circuit
Clean, dry and secure the connector, or replace it if corroded or damaged
Replace the intermediate shaft speed sensor if it fails testing
Correct the fluid level or change the fluid and filter to the manufacturer's specification if it is degraded
Repair the transmission internals if debris or a damaged reluctor ring is found
Repair or replace the control unit or mechatronic unit only after the wiring and sensor have been proven good

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but limp mode and poor shifting can strain the gearbox and leave you stranded. Get it checked soon and avoid heavy loads or towing until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.