P078C - Shift Timing Solenoid B Low
The P078C code indicates that the powertrain control module (PCM) has detected a low voltage or resistance condition in the shift timing solenoid B circuit. This solenoid controls hydraulic pressure to adjust shift timing in the automatic transmission. A low signal suggests a short to ground, open circuit, or faulty solenoid, leading to improper shift timing and potential transmission damage.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifts
- Transmission slipping
- Poor fuel economy
- Transmission stuck in limp mode
2. Most Common Causes
- Faulty shift timing solenoid B
- Open or shorted wiring in solenoid circuit
- Corroded or damaged connector
- Low transmission fluid level or dirty fluid
- Faulty PCM (rare)
3. 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 solenoid B resistance (typically 10-30 ohms); compare to specs.
Step 5: If solenoid is faulty, replace it; if wiring is bad, repair or replace as needed.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. Driving with a faulty shift solenoid can cause harsh shifting, transmission damage, or complete failure. Have it diagnosed and repaired promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for DIY solenoid replacement, or $200 to $500 at a shop including labor and parts.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.