Shift Solenoid Control Circuit Low Voltage
The P0979 code indicates that the Powertrain Control Module (PCM) has detected a low voltage condition in the control circuit of one of the transmission shift solenoids. This solenoid is responsible for directing hydraulic fluid to engage specific gears. A low voltage reading suggests a short to ground, open circuit, or faulty solenoid, which can prevent proper gear engagement and cause transmission performance issues.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifting
- Transmission stuck in a single gear (limp mode)
- Decreased fuel economy
2. Most Common Causes
- Short to ground in the shift solenoid 'C' control circuit wiring
- Faulty shift solenoid 'C' with internal short circuit in the coil
- Corroded, loose, or poor continuity electrical connections in the transmission harness
- Failed transmission control module (TCM) or powertrain control module (PCM)
- Internal mechanical issues in the valve body that overload the solenoid
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect wiring and connectors for damage, corrosion, or loose connections.
Step 3: Test solenoid resistance with a multimeter (typically 10-30 ohms).
Step 4: Check for voltage at the solenoid connector with the ignition on (should be battery voltage).
Step 5: If wiring and solenoid are good, test the PCM output or replace the PCM.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the transmission may shift poorly or enter limp mode, potentially causing further damage.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, depending on whether the solenoid, wiring, or PCM needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.