What Does P078C Mean?
The TCM has detected that the Shift Timing Solenoid B circuit voltage or current is below the expected threshold. This low-signal condition indicates the solenoid is receiving insufficient power due to high resistance in the circuit, a partial open in the solenoid coil, or a wiring fault. The solenoid cannot energize fully, which degrades its ability to precisely control shift timing.
Common Causes
30%
Shift timing solenoid B with high internal coil resistance or developing open circuit
25%
High resistance in the wiring harness from corrosion or damaged wire
25%
Corroded or damaged connector pins creating high resistance connection
12%
Poor ground connection reducing current flow through the solenoid circuit
8%
TCM driver circuit providing insufficient output
Diagnostic Steps
1
Measure shift timing solenoid B resistance at the transmission connector — compare to spec and note if it is significantly higher than expected.
2
Check voltage at the solenoid connector while the TCM commands it on — compare to battery voltage to identify excessive voltage drop.
3
Inspect the transmission connector for corrosion, water damage, oxidized pins, or compromised weatherpack seals.
4
Perform a voltage drop test across each connection point in the circuit (TCM output, harness connectors, solenoid) to isolate the high-resistance point.
5
Check the ground circuit — measure voltage drop between the solenoid ground pin and the battery negative terminal.
6
If all external wiring tests good, remove and bench-test the solenoid for internal coil integrity.
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
The TCM has detected that the Shift Timing Solenoid B circuit voltage or current is below the expected threshold. This low-signal condition indicates the solenoid is receiving insufficient power due to high resistance in the circuit, a partial open in the solenoid coil, or a wiring fault. The soleno...
The most common cause of P078C (Shift Timing Solenoid B Low) is: Shift timing solenoid B with high internal coil resistance or developing open circuit
Typical repair costs for P078C range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle is drivable but shift timing will be impaired, causing degraded shift quality. The low-signal condition may worsen over time as the connection degrades further. Schedule repair to prevent complete circuit failure and more serious shifting issues.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P078C to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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