For 99 sienna code reading p0420,0770,0773,0755

my 99 sienna got code reading p0420,0770,0773,0755, 115k miles. i’ve told to change the transmission, my question is that if i can only change the transmission’s filter and oil? will it work?

Has this transmission ever been serviced? If so, specify - mileage intervals and method (“flush” vs. filter/pan service).

My guess is that it probably hasn’t been serviced or has only been “flushed” without any filter changes. In that case it may be too late. If it were me (but not everyone is me), I would dump some Seafoam TransTune into it, drive it for a couple hundred miles, drop the pan, change the filter, and then have a full fluid exchange done. That might help - but it also might not. If it seems to “hurt” it would only be because things are that far gone already. I would do this before a new trans figuring I had nothing to lose by giving it a go.

What kind of shop has told you that you need a new transmission? Is it a local, independent shop that specializes in transmissions? If not, go to one and ask them what they think.

The P0420, btw, is unrelated to the others which all refer to shift solenoid problems. I would figure that out at the same time as the transmission as it might mean you’re headed for a new catalytic converter. You’d want to know about that first since those aren’t cheap either.