TFP Valve Position Switch Circuit/ Transmission 4-Wheel Drive Low Indicator Short Circuit To Battery
The transmission's gear position sensor is detecting an electrical short to the battery, like a wire touching the positive terminal instead of sending the proper signal. Your transmission can't properly read what gear you're in or engage 4WD low.
The ECM monitors the TFP (Transmission Fluid Pressure) switch voltage signal, expecting it to toggle between ground and a reference voltage as the transmission changes position. When the circuit shorts to battery voltage (typically 12V), the ECM detects an abnormal high voltage condition that indicates a wiring fault or switch failure.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| TFP Switch Voltage | 0V to 5V (switching pattern) | Constant 12V or stuck high |
| Signal Ground Path | Intact circuit to ECM | Open or shorted to battery |
Code P1815 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, P1815 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.