P078D - Shift Timing Solenoid B High
The P078D code indicates that the Powertrain Control Module (PCM) has detected an abnormally high voltage or resistance in the circuit of the Shift Timing Solenoid B. This solenoid controls hydraulic pressure to adjust shift timing in the transmission. A high signal suggests a short to voltage, open circuit, or faulty solenoid, leading to improper gear engagement and potential transmission damage.
01. Most Common Causes
- Faulty shift timing solenoid B
- Short to voltage in solenoid circuit
- Open or high resistance in wiring/connectors
- Corroded or damaged electrical connectors
- Failed PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Harsh or delayed shifts
- Transmission slipping or stuck in gear
- Reduced fuel economy
- Transmission overheating
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Inspect wiring and connectors for damage, corrosion, or shorts.
Step 3: Test solenoid B resistance with a multimeter (compare to specs).
Step 4: Check voltage at solenoid connector with ignition on (should be battery voltage).
Step 5: If wiring and solenoid are OK, test PCM output or replace PCM.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. Driving with a faulty shift solenoid can cause erratic shifting, transmission damage, or complete failure. Have it diagnosed and repaired promptly.
How much does it cost to fix?▼
Repair costs typically range from $50 to $150 for parts (solenoid) plus labor. If wiring repair is needed, costs may be higher. A new solenoid is usually $20-$80.