Possible causes

– Low transmission fluid level
– Dirty transmission fluid
– Faulty shift solenoid ‘D’ valve
– Shift solenoid ‘D’ valve harness or connectors
– Shift solenoid ‘D’ valve circuit is open or shorted

Tech notes

There is a factory service bulletin for the 2008 Toyota Tundra:

2008 Toyota Tundra Service Bulletin OBDII Code P0766

When is the code detected?

The P0766 code is detected when the gear required by the Engine Control Module (ECM) does not match the actual gear when driving

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Transmission will not shift gears

P0766 TOYOTA Description

The Engine Control Module (ECM) uses signals from the output shaft speed sensor and input speed sensor to detect the actual gear position (1st, 2nd, 3rd or O/D gear). Then the ECM compares the actual gear with the shift schedule in the ECM memory to detect mechanical problems of the shift solenoid valves, valve body or automatic transaxle (clutch, brake, gear, etc.).

TOYOTA Technical Service Bulletins for P0766 code