P0794

What it means

P0794 means the transmission control module has detected an intermittent fault in the circuit for intermediate shaft speed sensor A. The signal from the sensor drops out, spikes or becomes erratic from time to time rather than failing outright. The module compares this signal with other speed inputs and expects a steady, plausible reading. Which shaft the sensor monitors and where it sits depends on the transmission design.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Gearbox going into limp mode, often stuck in one gear
• Harsh, delayed or flared gear changes
• Erratic or non-working speedometer or rev reading at times
• Torque converter lock-up not engaging, with worse fuel economy
• Problem comes and goes, often with vibration, heat or damp

Possible causes

• Damaged, chafed or corroded wiring to the intermediate shaft speed sensor, including intermittent open circuit or shortlikely
• Loose, corroded or contaminated connector at the sensor or transmission control modulelikely
• Faulty intermediate shaft speed sensor with an intermittent internal faultlikely
• Metal debris or contamination on the sensor tip, or a damaged or dirty reluctor or tone wheelpossible
• Incorrect sensor position, or a loose or damaged sensor mountingpossible
• Low or degraded transmission fluid, or fluid ingress into the wiring harness or connector, depending on the transmission designpossible
• Fault in the transmission control module or its power and earth suppliesrare

How to diagnose

1. Read all stored and pending codes and record the freeze frame data to see when the fault occurred, such as speed, gear and temperature
2. Check for other speed sensor or gearbox codes, which can point to a shared wiring or supply fault
3. Inspect the sensor wiring and connectors for chafing, heat damage, fluid contamination, bent pins and corrosion
4. Wiggle test the harness and connector with the engine running while watching the live intermediate shaft speed and output speed data for dropouts
5. Check the transmission fluid level, condition and type against the manufacturer specification, and look for metal debris on the sensor tip where accessible
6. Test the sensor circuit for continuity, short circuits, and correct supply and signal voltage using the manufacturer wiring diagram and values
7. Check the sensor resistance or output against the model-specific specification, where one is published
8. Check power and earth supplies to the transmission control module before suspecting the module itself

How to fix

• Repair or replace damaged wiring, and secure the harness away from heat and moving parts
• Clean, repair or replace corroded or damaged connectors and terminals
• Replace the intermediate shaft speed sensor if it fails testing or gives dropouts, ensuring it is seated and fitted correctly
• Correct the fluid level or replace degraded fluid with the specified type, and deal with the cause of any contamination
• Repair or replace a damaged reluctor or tone wheel, which may need transmission disassembly depending on the model
• Repair the module supply or earth, or have the module checked, only after the wiring and sensor are proven good

Can I keep driving?

Nearby codes

• P0793GenericIntermediate Shaft Speed Sensor A Circuit No Signal
• P0795GenericPressure Control Solenoid C