What Does P078D Mean?
The shift timing solenoid B is reporting a high voltage or signal condition. This solenoid controls the precise timing of gear shifts in the automatic transmission, and a high signal indicates the solenoid is receiving excessive voltage or is stuck in an energized state. This can cause harsh or delayed shifts, particularly in specific gear ranges controlled by this solenoid.
Common Causes
35%
Shift timing solenoid B internal short circuit or failure
25%
Wiring harness short to power in solenoid B circuit
20%
TCM (Transmission Control Module) driver circuit failure
12%
Contaminated transmission fluid causing solenoid sticking
8%
Corroded or damaged connector at transmission case pass-through
Diagnostic Steps
1
Monitor shift timing solenoid B command vs actual duty cycle using live data PIDs to confirm the high signal condition.
2
Inspect the transmission wiring harness and connector at the case pass-through for damaged, shorted, or corroded wires.
3
Measure solenoid B resistance with an ohmmeter; compare to manufacturer specifications (typically 5-15 ohms).
4
Check for voltage on the solenoid B control circuit with the key on and solenoid commanded off — any voltage indicates a short to power.
5
If wiring checks pass, remove and inspect the solenoid for debris, scoring, or internal short; replace if out of spec.
6
If solenoid and wiring are good, suspect TCM driver failure and test or replace the transmission control module.
Estimated Repair Cost
$200 - $900
Parts + labor, varies by vehicle and location
The shift timing solenoid B is reporting a high voltage or signal condition. This solenoid controls the precise timing of gear shifts in the automatic transmission, and a high signal indicates the solenoid is receiving excessive voltage or is stuck in an energized state. This can cause harsh or dela...
The most common cause of P078D (Shift Timing Solenoid B High) is: Shift timing solenoid B internal short circuit or failure
Typical repair costs for P078D range from $200 to $900, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Driving is generally safe at reduced speeds, but harsh or unexpected shifts can occur. Avoid heavy towing or aggressive driving. Have the vehicle diagnosed soon to prevent further transmission damage.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P078D to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Generic (SAE)
Recommended Tools
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