Transmission Fluid Pressure Sensor/Switch "D" Circuit High
Code P0878 indicates the transmission fluid pressure sensor/switch D circuit is reading higher than expected voltage, suggesting an electrical fault in the sensor circuit. This typically means a short to power, open ground circuit, or failed sensor that prevents accurate transmission fluid pressure monitoring.
What You Might Notice
- Transmission shifting erratically or harshly
- Check Engine light illuminated
- Transmission may operate in limp mode or default gear
- Possible transmission slipping or delayed engagement
- No noticeable symptoms in some cases if PCM can compensate
Most Common Causes
- 1
Failed Transmission Fluid Pressure Sensor
The TFPS sensor has internally shorted or failed, causing it to send incorrect voltage signals. This is the most common cause and requires sensor replacement.
- 2
Short to Power in Signal Circuit
The wiring between the sensor and PCM/TCM has a short to the positive power supply, causing the sensor circuit to read high voltage continuously.
- 3
Open Ground Circuit
The ground wire from the transmission fluid pressure sensor is broken or disconnected, preventing proper signal return and causing high voltage readings.
- 4
Faulty PCM/TCM Module
Rarely, the powertrain or transmission control module itself may be failing and misinterpreting sensor signals, though this is less common than sensor failure.
How to Diagnose It
- 1
Check Failed Transmission Fluid Pressure Sensor — The TFPS sensor has internally shorted or failed, causing it to send incorrect voltage signals. This is the most common cause and requires sensor replacement.
- 2
Check Short to Power in Signal Circuit — The wiring between the sensor and PCM/TCM has a short to the positive power supply, causing the sensor circuit to read high voltage continuously.
- 3
Check Open Ground Circuit — The ground wire from the transmission fluid pressure sensor is broken or disconnected, preventing proper signal return and causing high voltage readings.
- 4
Check Faulty PCM/TCM Module — Rarely, the powertrain or transmission control module itself may be failing and misinterpreting sensor signals, though this is less common than sensor failure.
How to Fix It
- 1
Replace Transmission Fluid Pressure Sensor
Remove and replace the failed TFPS sensor with a new OEM or quality replacement. Ensure proper connector seating and torque sensor to specification. Clear the code after installation and test drive to verify resolution.
- 2
Inspect and Repair Sensor Wiring
Check the wiring harness between the TFPS sensor and PCM/TCM for damage, corrosion, or shorts to power. Repair or replace damaged wire sections and ensure all connections are clean and secure.
- 3
Check and Restore Ground Connection
Inspect the ground wire from the TFPS sensor to the PCM/TCM or engine block for breaks or loose connections. Clean contact points with a wire brush and reattach securely, or replace the ground wire if damaged.
- 4
Reprogram or Replace PCM/TCM
If sensor and wiring are confirmed good, the control module may require reflashing or replacement. This is a specialist procedure requiring programming equipment and should be performed by a qualified technician.
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