Code Definition

P078A — Shift timing solenoid B circuit open

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P078A

P078A is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P078A is stored, the module has detected an open (broken or disconnected) electrical path in the monitored circuit.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P078A is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The P078A fault code is set when the Engine Control Module (ECM) or Transmission Control Module (TCM) detects an open circuit condition in the shift timing solenoid B circuit. This solenoid is a crucial part of the transmission’s shift control system, regulating the timing of hydraulic fluid flow to enable gear changes. An open circuit in this context suggests a discontinuity or break in the electrical pathway that controls the solenoid. The ECM/TCM recognises this problem when it fails to establish an electrical connection with the solenoid during operation, indicating a lack of electrical ...

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