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 a shift solenoid. This solenoid is responsible for regulating hydraulic pressure to engage specific gears in the automatic transmission. A low voltage reading suggests a short to ground, open circuit, or faulty solenoid, which can disrupt transmission operation and lead to harsh shifting, slipping, or failure to shift.
01. Most Common Causes
- Faulty shift solenoid
- Short to ground in the solenoid control circuit
- Open or damaged wiring/connectors
- Low transmission fluid level or dirty fluid
- Failed PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or erratic shifting
- Transmission stuck in gear or limp mode
- Delayed engagement when shifting
- Decreased fuel economy
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect transmission fluid level and condition; top off or replace if necessary.
Step 3: Visually check wiring and connectors for damage, corrosion, or loose connections.
Step 4: Use a multimeter to test the solenoid resistance and compare to manufacturer specs.
Step 5: Check for shorts to ground or power in the control circuit; repair as needed.
Step 6: If circuit and solenoid are good, test the PCM output; replace PCM if faulty.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the transmission may shift erratically or fail, potentially causing further damage. Limp mode may limit speed and drivability.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, including parts and labor. Solenoid replacement is often $100-$250, while wiring repairs may be less. PCM replacement can exceed $500.