What does it mean?
Based on my personal experience, when a P0780 code is stored, it means that the Powertrain Control Module (PCM) has detected a problem with automatic transmission shifting. This code can be due to a mechanical or electronic malfunction but is limited to vehicles with automatic transmissions.
Automatic transmissions in vehicles equipped with OBD-II are controlled by the PCM or a standalone Transmission Control Module (TCM). Input signals from a network of engine and transmission sensors are used by the transmission controller to calculate the transmission shift strategy. Electronic solenoids are then activated by the controller, allowing high-pressure fluid to flow into the appropriate hydraulic circuit (in the valve body) and enabling the transmission to upshift or downshift as needed.
A powerful pump (integrated into the transmission case and driven by the engine) is used to create the hydraulic pressure necessary to accomplish smooth and efficient shifting functions. Since different degrees of hydraulic pressure are required at different engine speeds and loads, the transmission controller regulates the pressure via an electronic pressure regulator. The transmission pressure sensor and transmission temperature sensor help the controller monitor hydraulic pressure and make necessary adjustments.
The transmission controller is programmed to compare input signals from the input speed sensor and output speed sensor to determine if the transmission is shifting effectively. If the PCM detects that the transmission is not shifting correctly, a P0780 code will be stored, and a Malfunction Indicator Lamp (MIL) may be illuminated. On some models, this type of code requires multiple drive cycles for the MIL to light up. In this case, the code may initially be displayed as a pending code.
Symptoms and Severity
A P0780 transmission shift malfunction code should be treated as urgent. A minor condition could turn into a catastrophic one if left unattended for an extended period.
Symptoms of a P0780 code may include:
Irregular transmission shifting patterns
Delayed transmission engagement
Harsh transmission shifts (especially if the controller has been placed in limp mode)
Transmission slipping
Transmission pump whining noise
Causes
Possible causes for setting this code are:
Short-circuited or open circuits in the transmission control harness
Low transmission fluid
Expired or burnt transmission fluid
Faulty shift solenoid(s)
Faulty input or output speed sensors
Bad electronic pressure regulator
Faulty transmission pump
Mechanical failure; including torque converter failure, clutch failure, or wear/breakage of internal components
A faulty PCM or PCM programming error
Diagnostic and Repair Procedures
You will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), a transmission pressure gauge, and a vehicle service manual (or All Data DIY) to diagnose a P0780 code.
A thorough visual inspection of the transmission and all associated wiring harnesses is the best place to start your diagnosis of this code. Pay close attention to wiring routed near hot exhaust components and sharp edges. Keep in mind that part of the transmission control harness is located inside the transmission case. You may choose to wait and see if you need to enter the transmission case (for other purposes) to inspect this part of the harness. Internal transmission harnesses are prone to failure due to the harsh temperatures and sharp edges found inside the automatic transmission.
Connect the scanner to the vehicle’s diagnostic connector and retrieve stored codes. Note the codes (along with any associated freeze frame data), then clear them from the PCM memory.
Since this code can be caused by an electronic or mechanical malfunction, you will also want to inspect the transmission, transmission cooler, and transmission cooling lines for signs of leakage. If the transmission is low on fluid, it will cause transmission slipping, improper shifting, and delayed engagement.
Either of these conditions can contribute to the storage of this code. If the transmission is slipping and there is delayed engagement (with normal fluid level), remove the dipstick (if applicable) and see if the fluid smells and looks burnt. If the fluid appears burnt, suspect a mechanical transmission failure that will require a rebuild, along with a new torque converter and cooler.
If the transmission fluid is significantly low (more than 2 quarts), you will need to locate and repair the leak, refill the transmission with the recommended fluid, and test the vehicle to see if the problem persists and the code is reset. If the code is reset, use the transmission pressure gauge to check the pump pressure. If the pump has run without enough fluid, it may have been damaged, resulting in a low-pressure condition that will also require a transmission overhaul.
If the symptoms are limited to a faulty shifting function, suspect a faulty wiring harness or shift solenoid. Follow the manufacturer’s recommendations for testing the transmission control wiring and shift solenoids.
Additional Diagnostic Notes:
Some models require the simultaneous replacement of all shift solenoids. If this applies to your vehicle, do not try to determine which shift solenoid is faulty; simply replace the solenoid harness.
Often, transmissions are rebuilt when an electrical malfunction is the cause of storing a P0780.