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DTC codes / P09xx · Transmission / P0979

P0979Shift Solenoid 'C' Control Circuit Low

P0979 means the transmission control module (TCM) has detected an abnormally low voltage or current on the control circuit for shift solenoid 'C', which is one of the solenoids that directs hydraulic pressure to engage or release a gear clutch. The TCM commands the solenoid on or off and monitors the circuit, and when the feedback falls outside the expected range for the commanded state, it stores this code and usually illuminates the check engine light.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaTransmission

Symptoms you may notice

  • Check engine light or transmission warning light illuminated, often with no other symptoms
  • Harsh or delayed shifts, especially when the affected gear is engaged
  • Transmission stuck in a single gear or fails to shift into certain gears
  • Slipping or flaring during acceleration as the clutch for the affected gear does not apply fully
  • Limp mode with reduced power and a fixed gear ratio
  • Occasional neutral condition or loss of drive when the solenoid circuit fails intermittently

Common causes, most likely first

  1. Damaged, chafed, or corroded wiring in the shift solenoid 'C' control circuit, including the harness connector at the transmission
  2. Failed shift solenoid 'C' itself, with an open or shorted internal coil
  3. Poor electrical connection at the solenoid or at the TCM, such as loose pins or water intrusion
  4. Faulty TCM driver circuit for that solenoid, though this is less common
  5. Low transmission fluid level or contaminated fluid causing hydraulic issues that mimic an electrical fault
  6. Internal transmission mechanical wear that changes solenoid current draw and sets a circuit code

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data to confirm P0979 is active and not a history code from a past event
  2. Inspect the transmission harness and solenoid connector for chafing, corrosion, or loose pins, and repair as needed
  3. Measure the resistance of shift solenoid 'C' at the transmission connector and compare it to the manufacturer's specification
  4. Check the control circuit for a short to ground or an open circuit with a multimeter or test light while wiggling the harness
  5. Verify TCM output by back-probing the solenoid control wire and observing the command signal with a lab scope or bidirectional scan tool
  6. If the circuit and solenoid test good, perform a transmission fluid level and condition check and inspect the valve body for debris
  7. Replace the faulty solenoid or repair the TCM driver circuit only after electrical and hydraulic tests confirm the fault

What P0979 does not always mean

  • A low circuit code often comes from a simple wiring rub-through or a corroded connector pin rather than a failed solenoid, so inspect the harness before condemning the part.
  • Some transmissions use shift solenoid 'C' for a specific gear only, so a driver may notice a problem only during certain shift sequences and the code may seem intermittent.
  • Low or dirty transmission fluid can change solenoid current draw and set a circuit code, so fluid condition is part of the diagnosis even though the code is electrical.
  • A failing TCM driver can mimic a bad solenoid, and replacing the solenoid without testing the control signal may leave the code unresolved.
  • After repairs, the TCM may need a relearn or adaptive reset before shift quality returns to normal, and the code may reappear briefly until the monitor runs.

Can I keep driving with P0979?

You can usually drive the vehicle to a repair shop, but avoid heavy acceleration and long trips because the transmission may stay in limp mode or slip, which can cause further wear. If the vehicle loses drive or shifts erratically, stop driving and arrange a tow.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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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