P0777 indicates that the automatic transmission's pressure control solenoid 'B' is stuck in the 'on' position, or its actual hydraulic response does not match what the transmission control module commanded. The code is set when commanded pressure modulation fails, which can cause incorrect clutch/band apply pressure and abnormal shifting.
The solenoid coil or pintle can wear, stick, or fail internally, preventing it from regulating hydraulic pressure as commanded. This is the most common direct cause of P0777.
Burned, degraded, or debris-laden fluid can clog solenoid screens or valve body passages, causing the solenoid to mechanically hang in the on position even when electrically released.
An open, short-to-power, short-to-ground, or corroded connector on the pressure control solenoid B circuit can hold the solenoid energized or produce a false stuck-on reading.
Excessive wear in the valve body bore or sticking valves can prevent the solenoid's hydraulic output from changing state, mimicking a stuck solenoid.
A control module commanding the solenoid incorrectly is possible but uncommon; it should be considered only after the solenoid, wiring, fluid, and valve body have been ruled out.
Usually no. The fault most often involves a single solenoid, wiring issue, or contaminated fluid/valve body, not a complete transmission failure.
It is best to limit driving. Continued operation can cause abnormal clutch apply pressures, accelerated wear, and possible overheating.
That is rarely the correct fix. Most P0777 cases are caused by a stuck solenoid, fluid contamination, wiring problem, or valve body issue. Module replacement should be a last step after everything else is ruled out.
A fluid and filter change can help if the cause is contamination or a clogged solenoid screen, but it will not repair a failed solenoid, damaged wiring, or a worn valve body.