Faulty or sticking shift timing solenoid B; Wiring damage, chafing or a poor connection in the solenoid circuit, including the internal gearbox harness; Low, degraded or contaminated transmission fluid, or a blocked filter or valve body passage; Corroded or loose connector at the gearbox or control module; Worn valve body or internal hydraulic fault affecting shift timing; Low battery or charging system voltage causing erratic solenoid operation; Faulty transmission control module or outdated software calibration, where the manufacturer has a known issue.

P078A

Shift Timing Solenoid B.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
8A: FaultThe specific fault in that area

What it means

P078A is the generic code for a fault with Shift Timing Solenoid B in the automatic transmission. The transmission control module has detected that this solenoid, or its circuit, is not behaving as commanded. This is usually an electrical fault or a mismatch between the commanded and actual shift timing. The solenoid's exact role and the precise setting conditions vary by transmission and manufacturer, so check the service information for the vehicle.

Symptoms

Gearbox warning light or malfunction indicator lamp on
Harsh, delayed or flared gear changes
Gearbox stuck in one gear (limp mode)
Poor acceleration or higher revs than expected
Shift timing feels wrong or erratic
Reduced fuel economy

Possible causes

Faulty or sticking shift timing solenoid Blikely
Wiring damage, chafing or a poor connection in the solenoid circuit, including the internal gearbox harnesslikely
Low, degraded or contaminated transmission fluid, or a blocked filter or valve body passagelikely
Corroded or loose connector at the gearbox or control modulepossible
Worn valve body or internal hydraulic fault affecting shift timingpossible
Low battery or charging system voltage causing erratic solenoid operationpossible
Faulty transmission control module or outdated software calibration, where the manufacturer has a known issuerare

How to diagnose

Read all stored codes and freeze frame data, and note any related gearbox, speed sensor or battery voltage codes.
Check the transmission fluid level, condition and smell, following the manufacturer's procedure for the model.
Inspect the wiring and connectors to the gearbox for damage, chafing, corrosion, water ingress or fluid contamination.
Check battery condition and charging voltage, as low voltage can set solenoid faults.
Measure the solenoid resistance and compare it with the specification in the service information.
Check the solenoid supply, earth and control signal at the connector with the ignition on and during operation.
Use live data or an actuator test to command the solenoid and watch the response and shift behaviour.
Check for technical service bulletins or software updates for the vehicle and gearbox.

How to fix

Repair or replace damaged wiring and clean or replace corroded connectors.
Correct the fluid level, or change the fluid and filter if it is degraded or contaminated, using the specified type.
Replace the faulty shift timing solenoid B, or the solenoid pack, as the manufacturer specifies.
Repair or replace the valve body if there is internal wear or sticking.
Repair the battery or charging system fault if voltage is out of range.
Update the control module software, or replace the module, only where an outdated or faulty calibration or module fault is confirmed.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but shifting may be harsh or limited and the fault may get worse. Book it in soon, and avoid heavy loads or towing to limit the risk of transmission damage.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.