Definition of Code P0977
Fault code P0977 indicates a problem with the shift solenoid B control circuit.
What Does Code P0977 Mean?
Code P0977 is a standard OBD fault code that detects an issue with the shift solenoid B control circuit. Shift solenoids are used to help achieve smooth gear shifts in automatic transmissions. The various shift solenoids control the brakes, bands, and clutches within the transmission, with solenoid B varying by vehicle make and model. The Transmission Control Module (TCM) manages the shift solenoids, activating and deactivating them as needed to adjust gear ratios and facilitate smooth shifting. If the TCM records an abnormal voltage reading from the shift solenoid B control circuit, this circuit will be disabled, and the Powertrain Control Module (PCM) will log code P0977.
What Causes Code P0977?
Several factors can trigger code P0977:
- Faulty shift solenoid B
- Defective control circuit
- Damaged or faulty electrical components, such as wires, connectors, and fuses
- Low transmission fluid levels
- Dirty or contaminated transmission fluid
- In rare cases, a faulty PCM, TCM, or Engine Control Module (ECM)
What Are the Symptoms of Code P0977?
Fault code P0977 is typically accompanied by the “Check Engine Soon” light illuminating on the dashboard. The vehicle will often experience transmission issues, such as harsh shifting, delayed or abrupt acceleration, and an inability to access certain gears. It is also likely that the vehicle will have reduced fuel economy.
How Does a Mechanic Diagnose Code P0977?
Code P0977 will be addressed using a standard OBD-II fault code scanner. A qualified mechanic will use the scanner to review freeze frame data and assess the code. Any additional fault codes should also be checked and, if found, addressed in the order displayed by the scanner. The fault codes will then need to be reset, and the vehicle restarted so the technician can see if the code remains detected. If not, a false trigger or intermittent error is the most likely culprit.
If code P0977 returns, the mechanic will check the transmission fluid. If the fluid is low, it should be refilled, and the mechanic must carefully look for signs of leaks. If the fluid is dirty, it can be flushed and replaced, but if it is contaminated, the transmission is likely severely damaged. This will probably require a full replacement or rebuild. Next, the mechanic should visually inspect the electrical components and replace any wires, connectors, or fuses that are damaged or compromised.
If the issue is still not found, a digital volt/ohmmeter can be used to help identify the circuit corresponding to shift solenoid B. Once the correct circuit is verified, the solenoid, wiring harness, and circuit should all be inspected.
Whenever the technician replaces or repairs a component, the fault codes must be reset and the vehicle restarted to check if the code reappears. This helps the technician determine when the problem is resolved.
Common Mistakes When Diagnosing Code P0977
The most frequent mistake when diagnosing this code is failing to follow the OBD-II diagnostic protocol. It is crucial for technicians to always adhere to the step-by-step protocol to ensure thorough and efficient diagnosis and repair.
Shift solenoids are often unnecessarily replaced when the only issue is something simpler, such as a shorted wire or dirty transmission fluid.
How Severe Is Code P0977?
A vehicle with fault code P0977 will still be operable, though it may experience transmission problems severe enough to make driving impractical. If code P0977 is not addressed, it can cause further damage to the transmission, so it is important to have the vehicle inspected as soon as the code is noticed.
What Repairs Can Fix Code P0977?
Repairs for fault code P0977 include:
- Replacing shift solenoid B
- Replacing electrical components
- Replacing or adding transmission fluid
- In very rare cases, replacing the PCM, ECM, or TCM