P078D

What it means

P078D is a generic transmission code meaning the control unit has detected a high signal on the control circuit for shift timing solenoid B. The transmission control unit commands the solenoid and monitors the circuit voltage. It set the code because the voltage was higher than expected, which points to a short to voltage, an open circuit or a driver fault. Which solenoid is 'B', and how it is used, depends on the model and transmission.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Harsh, delayed or flared gear changes
• Transmission stuck in one gear (limp mode)
• Reduced performance or economy
• Gearbox not shifting up or down as expected

Possible causes

• Wiring fault to solenoid B, such as a short to voltage, chafing or an open circuitlikely
• Corroded, damaged or loose connector at the solenoid, transmission harness or control unitlikely
• Faulty shift timing solenoid B (internal open circuit or high resistance)possible
• Contaminated or degraded transmission fluid, or debris affecting the solenoid or valve bodypossible
• Low or unstable system voltage from a weak battery or charging faultpossible
• Faulty transmission control unit or internal driver circuitrare
• Outdated control unit software where the manufacturer has a known fixrare

How to diagnose

1. Read all stored and pending codes and note any other transmission or electrical codes
2. Review freeze frame data for gear, fluid temperature, road speed and battery voltage when the code set
3. Check battery condition and charging voltage, as low voltage can cause false solenoid faults
4. Check transmission fluid level, condition and smell, following the manufacturer's procedure for your model
5. Inspect the transmission wiring harness and connectors for chafing, heat damage, water ingress and corrosion
6. Using the wiring diagram for your model, test solenoid B's supply, control wire and earth for shorts to voltage, open circuits and high resistance
7. Measure solenoid B resistance against the manufacturer's specification
8. Use a scan tool to activate the solenoid if supported, and check whether the control unit reads the response correctly

How to fix

• Repair or replace damaged wiring and terminals in the solenoid B circuit
• Clean, secure or replace corroded or damaged connectors
• Replace shift timing solenoid B if it is out of specification, with fluid and filter service as the manufacturer requires
• Correct fluid level or replace contaminated fluid, and clean or repair the valve body if debris is found
• Repair the battery or charging system if voltage is unstable
• Update the control unit software only if the manufacturer lists it as a fix, or repair or replace the control unit once other causes are ruled out

Can I keep driving?

Nearby codes

• P078CGenericShift Timing Solenoid B Low
• P078EGenericShift Timing Solenoid B Intermittent