You said diverter door. I'm wondering if you mean the actuator that opens and closes it? The actuators last a pretty long time. I put 2 new ones on my Boxxer in about 15 years. Under heavier use ( a lot of hours per year) I could see you needing to replace maybe every 5 years or so. They do wear out but aren't really expensive to replace. I have had that problem caused by old vacuum lines too which is the very first thing you should look at or replace. I just replaced all my vacuum lines for under 10 bucks. Unfortunately, I think my problem is one or both of the vacuum solenoids. I seriously doubt the solenoids are your problem unless the machine is getting pretty old.
How old is the machine? How long since you put new vacuum lines on it? If the actuator is over say 5 years old that could be the whole problem or it could just be a cracked or loose vacuum line. Make sure none are cracked and that all are on nice and tight.
To clarify it is the door that diverts the exhaust for heating. I’ve had the bushing wear out (which causes the door to not move freely and either stay open or stay close) and thus had to buy new bushings every year or so... has this happened to anyone else?