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 is responsible for controlling hydraulic pressure to adjust shift timing in the transmission. A low signal suggests an electrical fault such as a short to ground, open circuit, or a failing solenoid. This can lead to harsh or delayed shifts, reduced fuel economy, and potential transmission damage if not addressed.
01. 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
- Internal transmission mechanical failure
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifts
- Transmission slipping
- Poor fuel economy
- Transmission stuck in limp mode
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: Test solenoid B resistance with a multimeter; compare to manufacturer specs (typically 10-30 ohms).
Step 5: Check for voltage at solenoid connector with key on; should be battery voltage. If not, repair open circuit.
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 it can cause further transmission damage. Limited driving to a repair shop is acceptable if shifts are still manageable.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for DIY solenoid replacement, or $200 to $500 if professional labor is required, depending on vehicle make and model.