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P0878 is a diagnostic trouble code (DTC) for "Transmission Fluid Pressure Sensor/Switch D Circuit High". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
When the Powertrain Control Module (PCM) detects a transmission pressure that is out of the normal manufacturer's range, a P0878 trouble code is stored and the Check Engine Light illuminates. In addition to the storage of a trouble code and the illumination of the Check Engine Light, the PCM can also put the vehicle into Limp-in mode. Related codes include P0876, P0877, and P0879.
The P0878 code means that the PCM has detected an abnormal pressure reading from the transmission fluid pressure sensor or switch. The pressure detected can be either too high or low when compared to manufacturer specifications.
The causes of a P0878 trouble code include a low transmission fluid level, a defective pump, and a faulty or slipping transmission. In addition, some other causes include a faulty transmission temperature sensor, a bad PCM, and an overheating engine.
Once the Check Engine Light comes on with the storage of this code, drivers should detect the smell of overheating transmission fluid. Smoke might also come from the transmission, and the transmission might fail to disengage or slip when the driver puts the vehicle into gear.
Successfully diagnosing a P0878 trouble code requires the mechanic to have an advanced OBD-II scanner, digital volt/ohmmeter, and a manufacturer's wiring schematic for the vehicle.
The mechanic should start by checking the transmission fluid level and condition.
In particular, the mechanic is looking for transmission fluid that has a burnt smell or that looks abnormal.
If either is detected, the mechanic should drop the transmission pan and inspect it for debris, including debris from the clutch.
After the transmission has been rebuilt, if damage is detected, the mechanic should fill the transmission fluid to a level to check for any leaks.
Once the leaks have been fixed and all faulty wiring, connectors, and components replaced, the mechanic should fill the transmission fluid to the appropriate level, start the vehicle, and allow it to reach a normal operating temperature.
The mechanic should then test-drive the vehicle after clearing the code to see if it returns.
If the problem persists, the mechanic should download any stored codes and freeze frame data to help diagnose an intermittent condition if needed.
If the trouble code comes back, unplug the electrical connector from the transmission temperature sensor. Test the sensor for reference voltage and compare it to the manufacturer's specifications to see if they match up.
If a signal is present when the ignition is in the On position, check the ground wire for the sensor.
If the ground signal is present, check the sensor resistance in comparison to the manufacturer's specifications.
If the transmission temperature sensor checks out on all fronts, disconnect the electrical connector to the PCM and test that component for fault. This includes testing for circuit continuity and the resistance between the transmission temperature sensor and the PCM. Clear the trouble code and retest the system to see if the condition returns.
Common mistakes made by mechanics when diagnosing a P0878 trouble code include not considering that internal damage could have occurred with the transmission due to the low-fluid condition, in addition to assuming the damage to the transmission requires a rebuild when in actuality the transmission only overheated and does not require the mechanic to rebuild it.
When a P0878 trouble code is stored, it is important that the vehicle owner have it checked out before operating the vehicle again. Doing so can help prevent further damage to the transmission and prevent the transmission from failing altogether if left undiagnosed.
To fix a P0878 trouble code, mechanics should take the following steps:
If required, rebuild the transmission and replace the torque converter. Damage to the transmission can occur if the transmission fluid is left too low for an extended amount of time.
In addition to rebuilding the transmission, if needed repair any leaks before proceeding with any further repairs.
Replace the transmission temperature sensor if it is faulty.
Replace and reprogram the PCM if it's the source of the trouble code.
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