What it means
The transmission fluid pressure sensor/switch D reports a circuit high fault. Hydraulic pressure feedback for shift control is unreliable on this channel.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Transmission pressure sensor/switch D supplies the control module with an electrical signal representing pressure feedback for the D channel. P0878 sets when the circuit signal is higher than expected, which can be caused by a signal wire shorted to battery voltage or an open sensor circuit.
Common causes
Most likely first.
- Signal wire shorted to B+
- Open sensor
- Connector pins corroded - high contact resistance
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Record freeze-frame data and check live sensor D data to confirm the high reading and identify the conditions when it appears.
- Inspect sensor D's connector and harness for a disconnected plug, broken wire, damaged insulation, or contact with a power feed.
- Check connector pins for corrosion, looseness, or damage and make sure the connector is fully seated.
- Use the wiring diagram to test the D signal wire for a short to battery voltage and check circuit continuity against the manufacturer's specification.
- If the harness and terminals pass, test pressure sensor/switch D for an internal open using the vehicle's specified procedure.
Common fixes
- Reconnect and secure a loose sensor D connector.
- Repair damaged wiring or a D signal wire shorted to battery voltage.
- Repair corroded or loose terminals interrupting the D circuit.
- Replace pressure sensor/switch D if it has an internal open and the wiring tests correctly.
Can I keep driving?
Avoid towing, hard acceleration, and extended driving if shifting is poor, the transmission slips, or it enters limp mode. Incorrect D-channel feedback can disrupt shift control, and slipping may lead to further transmission damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.