P078C

What it means

P078C means the transmission control module has detected a low voltage or low signal in the circuit of shift timing solenoid B. The module monitors the solenoid's drive circuit and sets the code when the voltage or current is lower than expected for the commanded state. Which gear changes or functions solenoid B controls depends on the transmission, so check the model-specific information.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Harsh, delayed or flared gear changes
• Gearbox stuck in one gear (limp mode)
• Reduced performance or restricted gear selection
• Poor fuel economy
• Shift timing feels wrong or inconsistent

Possible causes

• Wiring fault in the solenoid B circuit, such as chafed insulation or a short to earthlikely
• Corroded, loose or damaged connector at the solenoid, the transmission harness or the control modulelikely
• Faulty shift timing solenoid B with a low internal resistance or internal shortlikely
• Transmission fluid contamination or metal debris affecting the solenoid or its internal wiringpossible
• Low or degraded transmission fluid affecting solenoid operation and the readings seenpossible
• Low system or battery voltage, or a poor power or earth supply to the control modulepossible
• Fault in the transmission control module or its driver circuitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including battery voltage, gear and temperature when it set.
2. Check for other transmission, battery or charging codes that may point to a common cause.
3. Check the transmission fluid level, condition and smell, following the manufacturer's procedure for that model.
4. Inspect the transmission harness and connectors for chafing, heat damage, fluid ingress and corrosion.
5. Check the battery condition, charging voltage and the module's power and earth connections.
6. Measure the solenoid B resistance and compare it with the manufacturer's specification for that transmission.
7. Check the circuit for a short to earth and for continuity between the solenoid and the control module, with the connectors disconnected.
8. Command the solenoid with a bidirectional scan function if available and watch the circuit response.

How to fix

• Repair or replace the damaged wiring section and secure it away from heat and abrasion.
• Clean or replace corroded or damaged connectors, and renew any fluid-contaminated harness plug.
• Replace the faulty shift timing solenoid B or the solenoid pack, as the design requires.
• Correct the fluid level, or service the fluid and filter if it is contaminated, and investigate the source of any debris.
• Repair the battery, charging or earth fault found.
• Replace or repair the transmission control module only after the wiring and solenoid have been proven good, and apply any updated calibration the manufacturer specifies.

Can I keep driving?

Nearby codes

• P078BGenericShift Timing Solenoid B Range/Performance
• P078DGenericShift Timing Solenoid B High