Shift Solenoid "C" Control Circuit Low
The Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected that the voltage in the control circuit for Shift Solenoid "C" is lower than the manufacturer's specified range. This typically indicates a short to ground in the wiring or a failure within the solenoid itself, which prevents proper fluid pressure control for gear changes.
This code requires immediate attention as it can cause erratic shifting, transmission slipping, or force the vehicle into limp mode. Continued driving with this condition can lead to severe internal transmission damage.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Inability to shift into specific gears
- Vehicle entering transmission limp mode (stuck in a default gear)
Likely causes
- Short to ground in the Shift Solenoid "C" wiring harness
- Faulty Shift Solenoid "C"
- Corroded or damaged electrical connectors in the transmission circuit
- Low or severely contaminated transmission fluid
- Internal transmission valve body failure
- Faulty PCM or TCM (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Look for low fluid levels or burnt, contaminated fluid that could impede solenoid operation or indicate internal damage.
Visually inspect the wiring harness and connectors for Shift Solenoid "C".
Look for: Look for chafed wires, melted insulation, or corroded pins that could cause a short to ground in the control circuit.
Measure the internal resistance of Shift Solenoid "C" using a digital multimeter.
Look for: Compare the resistance reading to the manufacturer's specifications; an out-of-limits reading (often near zero ohms) indicates a internally shorted and failed solenoid.
Test the control circuit for a short to ground.
Look for: With the TCM and solenoid disconnected, check for continuity between the control wire and the vehicle chassis ground; continuity indicates a shorted wire.
Use a bi-directional scan tool to manually command Shift Solenoid "C" on and off.
Look for: Listen for a clicking sound from the transmission and monitor the circuit voltage to verify if the TCM is capable of sending the correct signal.
This page explains the generic P0979 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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