What does it mean?
P1778 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T Lockup Solenoid Valve 'C' Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and the transmission may hold a single gear
- Torque converter lockup does not apply, or applies and releases erratically
- Higher than normal engine speed at a steady motorway cruise
- Shudder or a pulsing sensation under light acceleration
- Fuel consumption rises noticeably
- Fluid temperature climbs on longer drives
Likely causes
- Solenoid C winding open or shorted, with resistance outside the specified band
- Corroded transmission case connector, a very common cause once fluid reaches the terminals
- Solenoid drive wire shorted to ground where it passes the bellhousing or a bracket
- Short to battery on the same wire from insulation damage near a power feed
- Varnish, clutch debris or a blocked solenoid screen holding the valve spool
- Deteriorated fluid or an overdue service reducing solenoid response
- Failed output driver in the control module
How to fix it
- Check the fluid level, colour and smell, and look for debris on the magnet or in the pan
- Watch converter slip and the solenoid command in live data during a steady-speed road test
- Measure solenoid C resistance at the case connector and compare with specification
- With the solenoid disconnected, check the drive wire for continuity and for shorts to ground and to battery
- Inspect the case connector terminals for fluid wicking, corrosion and spread contacts
- Replace the solenoid or valve body when the electrical value is wrong, service the fluid and filter, then confirm lockup on a road test