What it means
The control module detects a range or performance problem in the shift solenoid C control circuit.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Shift solenoid C is an electrically operated transmission valve that routes hydraulic fluid to control gear changes. The control module monitors the C circuit and the solenoid’s response, setting P0978 when the circuit behavior or response falls outside its expected range. The code identifies the C control path but does not by itself distinguish a solenoid fault from wiring trouble.
Common causes
Most likely first.
- Faulty shift solenoid C
- Open, shorted, or high-resistance wiring or connection
- Transmission control module fault
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Record freeze-frame data and check transmission live data for commanded shifts and solenoid C operation at the time of the fault.
- Check fluid condition and level using the vehicle’s specified procedure, since fluid problems can alter shift response.
- Inspect solenoid C’s connector and harness for damaged insulation, loose pins, corrosion, or fluid contamination.
- Test the C solenoid coil and wiring for continuity, shorts, and resistance against the manufacturer’s specification.
- Compare the module command with solenoid C’s electrical response using an appropriate scan-tool test.
- Investigate the transmission control module only after solenoid C and its wiring have passed testing.
Common fixes
- Repair a loose, corroded, or contaminated solenoid C connector.
- Repair open, shorted, chafed, or high-resistance wiring in the solenoid C circuit.
- Replace shift solenoid C if it fails the specified coil or response test.
- Repair or replace the transmission control module if testing confirms it is not controlling the C circuit correctly.
Can I keep driving?
If shifting is harsh, delayed, slipping, or the vehicle enters limp mode, avoid towing and hard acceleration, and limit driving to a safe place for diagnosis. Continued abnormal shifts can cause further transmission damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.