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DTC codes / P08xx · Transmission / P0878

P0878Transmission Fluid Pressure Sensor/Switch 'D' Circuit High

P0878 means the transmission control module (TCM) has detected an abnormally high voltage on the signal circuit for transmission fluid pressure sensor or switch 'D', which is typically the sensor monitoring the 'D' clutch pack or a related hydraulic circuit. The TCM expects a voltage within a calibrated range and sets this code when the signal stays above the upper threshold, indicating an open circuit, a short to voltage, or a faulty sensor.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other symptoms besides the warning light
  • Harsh or delayed gear shifts, especially when engaging drive or reverse
  • Transmission may enter limp mode and stay in a single gear
  • Slipping or flaring during acceleration as the transmission struggles to maintain pressure
  • Erratic shift points or failure to downshift when coming to a stop

Common causes, most likely first

  1. Faulty transmission fluid pressure sensor or switch 'D' itself, due to internal failure or contamination
  2. Open or damaged wiring in the sensor signal circuit, such as a broken wire or corroded connector
  3. Short to voltage on the signal circuit, often from chafed insulation contacting a power wire
  4. Poor electrical connection at the sensor or TCM connector, including loose pins or water intrusion
  5. Low transmission fluid level or degraded fluid causing pressure irregularities that trigger the sensor
  6. Internal transmission hydraulic issues, such as a stuck valve or worn clutch pack, affecting the 'D' circuit pressure
  7. Rarely, a faulty TCM that misinterprets a valid sensor signal

How to diagnose and fix it

  1. Check transmission fluid level and condition; top off or replace fluid if contaminated or low
  2. Inspect the sensor 'D' connector and wiring for corrosion, damage, or loose pins; clean and secure as needed
  3. Use a scan tool to monitor live data for sensor 'D' voltage and compare to manufacturer specifications
  4. Perform a voltage drop test on the signal circuit to detect open or high-resistance wiring
  5. Test the sensor 'D' with a multimeter for resistance and voltage output; replace if out of spec
  6. Check for short to voltage on the signal wire by disconnecting both ends and measuring to ground
  7. If all electrical checks pass, perform hydraulic pressure tests and inspect valve body for stuck valves or worn components

Sensor D code often misdiagnosed

  • A common oversight is replacing the sensor 'D' without checking the wiring harness for chafing where it routes near hot exhaust components or sharp edges, which can cause an intermittent short to voltage.
  • Technicians sometimes miss that low fluid level or a clogged transmission filter can cause pressure fluctuations that set this code, so always verify fluid condition before condemning electrical parts.
  • The code can be set by a faulty TCM in rare cases, but only after all wiring and sensor tests pass should you suspect the module itself.
  • On some vehicles, sensor 'D' is part of a pressure switch manifold that cannot be serviced separately, requiring replacement of the entire unit, which is often overlooked during diagnosis.
  • A scan tool that can read transmission live data is essential; without it, you may misdiagnose a sensor fault when the real issue is a mechanical pressure loss.

Can I keep driving with P0878?

You can often drive with this code, but the transmission may shift harshly or enter limp mode, so avoid heavy acceleration and towing until repaired. If the vehicle feels unsafe or refuses to shift, stop driving and seek professional diagnosis.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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