Possible causes

– Low transmission fluid level
– Dirty transmission fluid
– Faulty pressure control solenoid valve ‘C’
– Pressure control solenoid valve ‘C’ harness is open or shorted
– Pressure control solenoid valve ‘C’ circuit poor electrcial connection

Tech notes

There is a factory service bulletin for the 2004 Toyota Sienna
2004 Toyota Sienna Factory Service Bulletin OBDII Code P0798

When is the code detected?

Engine Control Module (ECM) detectes an open or shorted condition on the solenoid circuit

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Shift shock

P0798 TOYOTA Description

The Engine Control Module (ECM) uses signals from the vehicle speed sensor to detect the actual gear position (1st, 2nd, 3rd, 4th or 5th gear). Then the ECM compares the actual gear with the shift schedule in the ECM memory to detect mechanical troubles of the shift solenoid valves and valve body.

TOYOTA Technical Service Bulletins for P0798 code