Damaged, chafed or corroded wiring to the pressure sensor or switch; Loose, corroded or contaminated connector at the gearbox or control module; Fluid ingress into the wiring harness or connector, with fluid wicking up the loom; Failing pressure sensor or switch giving a dropout signal; Low, degraded or contaminated transmission fluid causing unstable pressure readings; Internal valve body or solenoid problem affecting pressure; Fault within the transmission control module or its supply and earth.

P0879

Transmission Fluid Pressure Sensor/Switch D Circuit Intermittent.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P0879 is a generic code meaning the transmission control module has detected an intermittent fault in the circuit of transmission fluid pressure sensor or switch 'D'. The signal has dropped out, jumped or gone implausible for short periods rather than failing outright. Which sensor or switch is 'D', and where it sits, depends on the vehicle and gearbox.

Symptoms

Gearbox warning light or malfunction indicator lamp on
Occasional harsh, delayed or flared gear changes
Gearbox dropping into a limp-home gear now and then
Shift quality changing unpredictably, often with vibration or temperature
Transmission not engaging a particular gear at times
Fault appearing and clearing without a clear pattern

Possible causes

Damaged, chafed or corroded wiring to the pressure sensor or switchlikely
Loose, corroded or contaminated connector at the gearbox or control modulelikely
Fluid ingress into the wiring harness or connector, with fluid wicking up the loompossible
Failing pressure sensor or switch giving a dropout signalpossible
Low, degraded or contaminated transmission fluid causing unstable pressure readingspossible
Internal valve body or solenoid problem affecting pressurerare
Fault within the transmission control module or its supply and earthrare

How to diagnose

Read all stored and pending codes and freeze frame data to see gear, temperature, speed and voltage when it set, and note any related pressure or solenoid codes
Check for manufacturer service bulletins and the wiring diagram for this model to identify which sensor or switch is 'D' and where it is
Check transmission fluid level and condition, following the procedure for that gearbox, as some are sealed and need a specific temperature
Inspect the wiring and connector at the sensor and the control module for chafing, corrosion, bent pins, fluid contamination and poor pin tension
Wiggle test the harness and connector with live data displayed, watching for the pressure signal dropping out or spiking
Check the sensor supply voltage, signal and earth against specification, and test the circuit for resistance and short circuits
Check the battery and charging voltage and the control module supply and earth connections
Clear the code and road test to see whether it returns, comparing live pressure readings with specification

How to fix

Repair or replace damaged wiring, with proper soldered and sealed joins where permitted
Clean or replace corroded or loose connectors and repair terminal tension
Replace the pressure sensor or switch if it fails testing, then follow any adaptation or relearn procedure
Correct the fluid level or renew degraded fluid with the specification the manufacturer requires
Investigate and repair the valve body or internal fault if pressures are confirmed wrong
Repair the control module supply or earth fault, or replace or update the module only if confirmed faulty

Can I keep driving?

Medium severity

Short, gentle journeys are usually possible, but the gearbox may enter limp mode or shift harshly without warning. Get it checked soon, as continued driving with a pressure fault can accelerate gearbox wear.

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