Perhaps a bad selector switch If it is a bad selector can I replace the one on the dash (the one for selecting the vent) or is there some control module elsewhere in the vehicle that I can replace to fix the problem?
sounds like your heater core went out
You have hit a cow, please remove cow and continue journey
Sends power to the selector causing the door inside the unit to open and close swapping tanks.
try checking your cabin air filters located inside glovebox. mine were full of leaves causing the same problem.
I had the same issue with my Pontiac Montana. One of the spark plug wires had cracked and water was causing the issues. I replaced the plug wires and the problem was solved.
I had a 1999 Montana in which the door locks would cycle from lock to unlock, also when I turned on (if I remember the right turn signal). Took to dealer and they diagnosed the BCM (body control module) bad. I did NOT have them replace it due to cost. The problem was a bad ground connection for the BCM.
It could be the rear window wiper fluid supply line broken. The wiper fluid supply line run directly over the back dome light. My 2002 Montana had the same problem.
when they changed the heater core on my durango they didnt attach the air conditioner drain back up and it ran through my firewall so it caused my floor to get wet
My 1999 Pontiac Montana would seem to "skip a beat" while driving. It then started stalling and restarting, all while driving at highway speeds. My mechanic found that an electrical cable that carried an engine sensor signal was leaning against a hot part of the engine. The cable insulation had melted or worn through, causing intermittent shorts. The shorts confused the engine computer, which then stalled the engine. He simply rerouted the cable and all was fine after that.
The Optispark distributor is notorious for causing this problem.
The floor is where a lot of the wiring is run. The floor gets wet due to various things..clogged drainage holes on A/C or runoff water under windshield causing water to come into the unit from the firewall. The connectors for the wires get corroded and quit working. They must be cleaned at the connectors.
Fuel Sending Unit is bad. It is a common problem in these vans!