Transmission Fluid Pressure Sensor/Switch D Circuit High
The transmission fluid pressure sensor is sending a signal that's too high, like a gauge needle stuck on the maximum reading. This tells the engine computer that transmission pressure is abnormally elevated, which usually means a sensor or wiring problem rather than actual high pressure.
The ECM monitors voltage from transmission fluid pressure sensor D, expecting a signal that correlates to actual fluid pressure. When the voltage reading exceeds the maximum threshold (typically 4.8-5.0V), the ECM triggers a fault for excessive pressure signal, indicating either sensor failure, wiring short to power, or internal sensor malfunction.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Output Voltage | 0.5–4.5V (proportional to pressure) | >4.8V or open circuit condition |
| Fluid Pressure Signal | 0–300 PSI range | Signal reads abnormally high or maxed out |
Code P0878 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, P0878 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.