Faulty shift timing solenoid B (open or shorted internally); Damaged or corroded wiring harness or connector at the solenoid; Poor electrical connection at the transmission control module; Low transmission fluid level or contaminated fluid causing solenoid malfunction; Internal transmission mechanical issues affecting solenoid operation; Transmission control module software or hardware fault (less common)

DTC codes / P07xx · Transmission / P078C

P078CShift Timing Solenoid B Low

Q: What does code P078C mean?

Shift Timing Solenoid B Low. P078C indicates that the transmission control module has detected an abnormally low current or voltage on the circuit for shift timing solenoid B, which is responsible for controlling the timing of a specific gear shift. The computer sets this code when the electrical signal from the solenoid falls outside the expected range, often due to a faulty solenoid, wiring issue, or poor connection.

P078C indicates that the transmission control module has detected an abnormally low current or voltage on the circuit for shift timing solenoid B, which is responsible for controlling the timing of a specific gear shift. The computer sets this code when the electrical signal from the solenoid falls outside the expected range, often due to a faulty solenoid, wiring issue, or poor connection.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaTransmission

Symptoms you may notice

Illumination of the check engine light or transmission warning light
Harsh or delayed gear shifts, particularly when engaging or disengaging certain gears
Transmission may enter limp mode, limiting vehicle speed and gear selection
Slipping or flaring during shifts
Often no symptoms besides the warning light, especially in early stages

Common causes, most likely first

Faulty shift timing solenoid B (open or shorted internally)
Damaged or corroded wiring harness or connector at the solenoid
Poor electrical connection at the transmission control module
Low transmission fluid level or contaminated fluid causing solenoid malfunction
Internal transmission mechanical issues affecting solenoid operation
Transmission control module software or hardware fault (less common)

How to diagnose and fix it

Verify transmission fluid level and condition; top off or replace if necessary
Perform a visual inspection of the solenoid B wiring and connector for damage or corrosion
Use a scan tool to command solenoid B on and off and listen for activation; check resistance with a multimeter
Check voltage and ground signals at the solenoid connector to rule out wiring faults
If wiring is intact, replace shift timing solenoid B
Clear codes and perform a relearn or adaptation procedure if required
If the code persists, diagnose the transmission control module or internal transmission faults

Don't overlook the wiring harness

A common oversight is replacing the solenoid without checking the wiring harness for chafing or corrosion, which can mimic a failed solenoid.
Low or dirty transmission fluid can cause solenoid performance issues and set this code, so fluid condition should always be checked first.
The code may be intermittent and only set under specific temperature or load conditions, making it important to test the circuit when the fault is active.
Sometimes a faulty transmission control module or a software glitch can cause a false P078C, so module diagnostics should be considered before major repairs.
Related codes such as P078A or P078B may also be present if multiple solenoids are affected, indicating a shared power or ground issue.

Can I keep driving with P078C?

Driving with P078C is generally not recommended because shift timing issues can lead to transmission damage or unsafe driving conditions. Avoid heavy loads and high speeds, and have the vehicle diagnosed promptly.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.