What it means
The transmission fluid pressure sensor/switch D reports a circuit intermittent 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 provides hydraulic pressure feedback to the control module. P0879 sets when the D-circuit signal comes and goes, often because a loose connector or chafed harness loses contact with vibration or movement.
Common causes
Most likely first.
- Connector not seated
- Harness chafing intermittently
- Connector with intermittent contact under vibration
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
- Save freeze-frame data and review stored or pending faults with live sensor D data to see when the signal drops out.
- Inspect the D sensor connector for incomplete seating, loose or corroded pins, and fluid contamination.
- Trace the D-circuit harness and check for chafing, stretched sections, or damage near hot or moving parts.
- Display live sensor D data and gently move accessible parts of the harness and connector to see whether the reading changes.
- If the fault remains, test the D signal, reference, and ground circuits for intermittent opens or resistance changes using the manufacturer's procedure.
Common fixes
- Fully seat and secure the sensor D connector.
- Clean corroded terminals or repair loose connector pins.
- Repair or replace chafed wiring in the D circuit.
- Replace pressure sensor/switch D if it intermittently loses output after connector and harness faults are ruled out.
Can I keep driving?
Keep trips short and avoid towing or demanding driving while the D signal is intermittent, especially if shifts become harsh or limp mode occurs. Unreliable D-channel feedback can cause unpredictable shifting, and continued slipping can damage the transmission.
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.