P078A: Shift Timing Solenoid B Malfunction

What it means

General fault on shift timing solenoid B — second timing solenoid for shift events that need a different ramp profile (typically downshifts vs. upshifts, or specific gear pairs).

At a glance

• Usually lights the check-engine light

How it works

Shift timing solenoid B is an electrically controlled transmission valve used for gear-change timing on a second shift-control channel. The TCM sets this general malfunction code when solenoid B or its circuit does not operate as commanded; the specific electrical or hydraulic fault must be identified during diagnosis.

Common causes

Most likely first.

1. Shift Timing Solenoid B electrical failure
2. Damaged wiring or a poor connector
3. Solenoid sticking or restricted hydraulic flow
4. Contaminated transmission fluid

Symptoms

• Check engine or transmission warning light on
• Harsh, delayed, or missing shifts
• Possible limp mode (locked in one gear)
• Slipping or shuddering during gear changes

How to diagnose it

1. Record freeze-frame data and check transmission live data for the commanded state and response of shift timing solenoid B.
2. Inspect solenoid B's transmission connector and wiring for damage, corrosion, loose terminals, or fluid contamination.
3. Check solenoid B circuit continuity and shorts, then test its coil according to the manufacturer's procedure.
4. If the electrical circuit passes, check transmission fluid level and condition using the vehicle's specified method.
5. If fluid and electrical checks do not explain the fault, assess solenoid B operation and hydraulic flow using manufacturer procedures.

Common fixes

• Reconnect or repair a loose or damaged solenoid B connector.
• Repair open or shorted wiring in the solenoid B circuit.
• Replace shift timing solenoid B if its electrical failure or sticking is confirmed.
• Correct the fluid level or service contaminated fluid using the vehicle's specified fluid and procedure.
• Repair a TCM fault only after verifying the solenoid, wiring, and hydraulic path.

Can I keep driving?

If the transmission still shifts normally, limit driving to short, gentle trips and avoid towing or hard acceleration. Harsh shifts, slipping, or missing gears can worsen transmission damage, so stop if they occur.