Transmission Fluid Pressure Sensor/Switch C Circuit Range/Performance
The transmission fluid pressure sensor isn't sending the right voltage signal to the engine computer, like a gauge that's stuck or reading wrong. This prevents the transmission from adjusting pressure correctly for smooth shifting.
The ECM monitors voltage from the transmission fluid pressure sensor (typically 0.5–4.5V) to verify actual line pressure matches commanded pressure. If the reading falls outside expected range for current operating conditions, the ECU logs a fault. The sensor should respond linearly to pressure changes.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5–4.5V corresponding to 0–300+ PSI | Out-of-range voltage or no response to pressure demand |
| Pressure Correlation | Voltage matches engine load and gear selection | Voltage static or illogical relative to transmission state |
Code P0871 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, P0871 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.