P0991

What it means

P0991 is a generic powertrain code meaning the transmission control module has detected an intermittent fault in the circuit of transmission fluid pressure sensor or switch 'E'. The signal drops out, spikes or becomes erratic, then returns to normal, so it does not match the pressure the module expects. Which pressure sensor or switch is 'E', and where it sits in the valve body or case, depends on the make, model and gearbox.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Harsh, delayed or flared gear changes that come and go
• Gearbox dropping into a fixed gear (limp mode)
• Occasional slipping or loss of drive
• Gear selection or shift quality varying with temperature or road bumps
• Transmission fault stored only some of the time

Possible causes

• Damaged, chafed or corroded wiring to the pressure sensor or switch, including internal gearbox harnesslikely
• Poor connection at the sensor, solenoid body or transmission control module connector (loose pin, water or fluid ingress)likely
• Failing transmission fluid pressure sensor or switch giving an intermittent signallikely
• Low, degraded or contaminated transmission fluid affecting pressure and the sensor readingpossible
• Valve body or hydraulic fault causing unstable pressure (varies by gearbox)possible
• Low battery voltage or poor earth or supply to the transmission control modulepossible
• Fault inside the transmission control module or an outdated calibration known for this fault on the specific modelrare

How to diagnose

1. Read all stored and pending codes and freeze frame data, noting gearbox temperature, speed and gear when the fault set, and check for other pressure, solenoid or voltage codes.
2. Check for manufacturer service bulletins or known issues for the specific gearbox, since the position of sensor 'E' varies by model.
3. Check transmission fluid level and condition using the correct procedure for that gearbox, looking for burnt smell, debris or metal.
4. Inspect the wiring and connectors from the sensor to the transmission control module for chafing, corrosion, fluid ingress, bent or pushed-back pins.
5. With live data, watch the pressure sensor or switch reading while wiggling the harness and connectors to provoke a dropout.
6. Check battery condition, charging voltage, and the module's power and earth supplies.
7. Test the sensor circuit for supply, signal and earth with a multimeter against the manufacturer's specification, looking for resistance or voltage drops.
8. Test the sensor or switch against specification and, if the fault stays, assess the valve body and hydraulic pressures as the manufacturer directs.

How to fix

• Repair or replace damaged wiring and terminals, securing the harness clear of heat and rubbing points.
• Clean, dry and reseat connectors, or replace corroded or loose terminals and sealed connectors.
• Replace the faulty transmission fluid pressure sensor or switch, with fluid and filter service if the manufacturer requires it.
• Correct fluid level or replace the fluid with the specified type if it is low or degraded.
• Repair or replace the valve body or related hydraulic components if pressure faults are confirmed.
• Update the transmission control module software, or replace the module, only if a fault or outdated calibration is confirmed as the cause.

Can I keep driving?

Nearby codes

• P0990GenericTransmission Fluid Pressure Sensor/Switch E Circuit High
• P0992GenericTransmission Fluid Pressure Sensor/Switch F Circuit