Transmission Fluid Pressure Sensor/Switch D Circuit Intermittent
Your transmission's fluid pressure sensor is sending an unstable signal to the engine computer, like a loose radio connection that cuts in and out. The ECU detects this intermittent dropout and triggers a fault code rather than using bad data.
The ECM monitors voltage from the transmission fluid pressure sensor (switch D) to verify hydraulic pressure is within normal operating range. When the signal becomes intermittent—dropping out or spiking unpredictably—the ECM detects loss of signal continuity. The sensor typically uses a 0-5V analog or digital switch signal that the ECM samples continuously.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5-4.5V (stable, continuous) | Intermittent dropout or erratic spikes lasting >0.5 seconds |
| Signal Continuity | Uninterrupted signal every 10ms | Signal loss detected during driving cycle |
Code P0879 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0879 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.