Damaged wiring, such as a short to voltage or chafed loom, in the sensor signal circuit; Corroded, loose or contaminated connector at the sensor or transmission control module; Faulty transmission fluid pressure sensor or switch D; Incorrect, degraded or contaminated transmission fluid, or a wrong fluid level; Hydraulic fault such as a sticking valve or solenoid in the valve body causing a real pressure error; Open circuit in the sensor ground return or a fault in the 5 volt reference supply; Faulty transmission control module or outdated software.

P0878

Transmission Fluid Pressure Sensor/Switch D Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P0878 is a generic code meaning the transmission control module has detected a high voltage or high signal in the circuit of transmission fluid pressure sensor/switch 'D'. The signal is above the expected range for the current gear, solenoid commands or operating conditions, or it stays high when it should change. Which pressure circuit 'D' refers to, and where the sensor sits, depends on the model and transmission.

Symptoms

Gearbox warning light or malfunction indicator lamp on
Gearbox stuck in a single gear (limp mode)
Harsh, delayed or flared gear changes
Slipping or no engagement of certain gears
Torque converter lock-up not working, with poorer fuel economy

Possible causes

Damaged wiring, such as a short to voltage or chafed loom, in the sensor signal circuitlikely
Corroded, loose or contaminated connector at the sensor or transmission control modulelikely
Faulty transmission fluid pressure sensor or switch Dlikely
Incorrect, degraded or contaminated transmission fluid, or a wrong fluid levelpossible
Hydraulic fault such as a sticking valve or solenoid in the valve body causing a real pressure errorpossible
Open circuit in the sensor ground return or a fault in the 5 volt reference supplypossible
Faulty transmission control module or outdated softwarerare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting gear, fluid temperature and speed when the fault set
Check the transmission fluid level, condition and type against the manufacturer specification, where the transmission has a means of checking it
Inspect the sensor, its connector and the harness for damage, chafing, leaking fluid, corrosion and bent or pushed-back pins
View live data for the pressure sensor or switch and compare it with the specification while changing gear and selector position
Check the sensor reference voltage, ground and signal wire for shorts to voltage or open circuits using the wiring diagram
Check for fluid inside the connector, which can wick along the wires and cause a false high signal
Test or substitute the sensor following the manufacturer procedure, then check hydraulic pressure if the sensor tests good
Check for any applicable software updates or technical service bulletins for the model

How to fix

Repair or replace damaged wiring and fix any short to voltage or open circuit
Clean or replace corroded or damaged connectors and terminals
Replace the faulty pressure sensor or switch, with the correct seals
Correct the fluid level or change fluid and filter to the correct specification if contaminated or degraded
Repair or replace the valve body, solenoid or other hydraulic components if the pressure fault is confirmed
Update or replace the transmission control module only if diagnosis confirms a module or software fault

Can I keep driving?

Medium severity

The car may be driven carefully a short distance if it shifts normally, but limp mode or harsh shifts risk further transmission damage. Have it diagnosed soon and avoid towing or heavy loads until repaired.

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