Transmission Fluid Pressure Sensor/Switch "C" Circuit
P0870 indicates the PCM/TCM has detected a problem with the Transmission Fluid Pressure Sensor/Switch C circuit. This sensor monitors transmission fluid pressure, and a fault prevents the transmission control module from accurately adjusting shift points and pressure regulation.
What You Might Notice
- Transmission shifting erratically or harshly
- Transmission staying in limp mode or default gear
- Delayed or soft shifts
- Check engine light illuminated
- Possible transmission overheating if pressure regulation fails
Most Common Causes
- 1
Open circuit in sensor wiring
A break or poor connection in the signal wire between the TFPS sensor and the transmission control module prevents proper signal transmission.
- 2
Failed transmission fluid pressure sensor
The TFPS sensor itself has failed electrically or mechanically, causing it to send incorrect or no signal to the TCM.
- 3
Short to voltage or ground in signal circuit
The sensor signal wire is shorted to battery voltage or ground, causing the TCM to receive an invalid reading.
- 4
Internal transmission mechanical failure
Actual loss of fluid pressure due to worn pumps, seals, or internal transmission damage can trigger this code if the sensor detects low pressure.
How to Diagnose It
- 1
Check Open circuit in sensor wiring — A break or poor connection in the signal wire between the TFPS sensor and the transmission control module prevents proper signal transmission.
- 2
Check Failed transmission fluid pressure sensor — The TFPS sensor itself has failed electrically or mechanically, causing it to send incorrect or no signal to the TCM.
- 3
Check Short to voltage or ground in signal circuit — The sensor signal wire is shorted to battery voltage or ground, causing the TCM to receive an invalid reading.
- 4
Check Internal transmission mechanical failure — Actual loss of fluid pressure due to worn pumps, seals, or internal transmission damage can trigger this code if the sensor detects low pressure.
How to Fix It
- 1
Inspect and repair sensor wiring
Check the signal wire harness between the TFPS sensor and TCM for breaks, corrosion, or loose connections. Repair or resolder connections as needed and verify continuity with a multimeter.
- 2
Replace transmission fluid pressure sensor
Remove and replace the failed TFPS sensor with a new OEM or quality aftermarket unit. Ensure proper installation and that the connector is fully seated.
- 3
Repair shorted wiring in sensor circuit
Locate the short to voltage or ground in the signal wire using a multimeter, then repair or replace the damaged wiring section. Inspect for chafing against metal or engine components.
- 4
Diagnose internal transmission pressure loss
If wiring and sensor are confirmed good, the transmission may have internal damage. Perform a transmission fluid pressure test with a gauge at the TFPS location to verify actual pressure levels match sensor readings.
Need a deeper diagnosis?
Search for related repair guides or browse by system.