Low, degraded or contaminated transmission fluid affecting hydraulic response; Shift Timing Solenoid B sticking or worn internally; Blocked transmission filter or contaminated valve body passages; Wiring or connector fault in the solenoid circuit (chafing, corrosion, high resistance); Worn clutch packs or internal leakage causing the gear change to differ from the command; Faulty transmission speed sensor giving incorrect feedback to the control module; Outdated transmission control software where the manufacturer has issued a correction for this fault.

P078B

Shift Timing Solenoid B Range/Performance.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P078B is a generic powertrain code meaning the transmission control module has detected that Shift Timing Solenoid B is not performing as commanded. The module compares the commanded gear change or hydraulic response with the actual gear ratio, input and output speeds, or pressure feedback, and sets the code when the two do not agree within limits. The exact solenoid function and test method vary with the transmission design, so the manufacturer's information is needed to confirm what 'B' controls.

Symptoms

Harsh, delayed or flared gear changes
Gearbox slipping or revving between shifts
Transmission stuck in one gear (limp mode)
Gearbox warning light or malfunction indicator lamp on
Poor acceleration or reduced fuel economy
Shifts occurring at the wrong road speed or engine speed

Possible causes

Low, degraded or contaminated transmission fluid affecting hydraulic responselikely
Shift Timing Solenoid B sticking or worn internallylikely
Blocked transmission filter or contaminated valve body passagespossible
Wiring or connector fault in the solenoid circuit (chafing, corrosion, high resistance)possible
Worn clutch packs or internal leakage causing the gear change to differ from the commandpossible
Faulty transmission speed sensor giving incorrect feedback to the control modulerare
Outdated transmission control software where the manufacturer has issued a correction for this faultrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting gear, temperature and speed when the fault set
Check the transmission fluid level and condition (where a dipstick or check procedure is provided), noting colour, smell and debris
Inspect the transmission wiring harness and connectors for damage, leaks, corrosion and fluid ingress
Check for related codes such as speed sensor, pressure or other solenoid faults, and deal with those first
Measure the solenoid resistance and compare it with the manufacturer's specification
Use live data to compare commanded and actual gear ratio, line pressure and shift times during a road test
Check for applicable technical service bulletins and software updates for the vehicle
Carry out pressure tests or a valve body inspection if the electrical checks pass, following the model-specific procedure

How to fix

Correct the fluid level or replace the fluid and filter using the specified type
Replace the faulty Shift Timing Solenoid B or the solenoid pack, as the design requires
Repair or replace damaged wiring, terminals or connectors
Clean or overhaul the valve body, or replace it if worn or blocked
Replace a faulty speed sensor if live data shows incorrect readings
Update the transmission control software only if the manufacturer lists it as a fix for this fault

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but have it checked soon as slipping or harsh shifts can cause further transmission wear. Stop driving if the gearbox slips badly or will not select gears.

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