Quote:
Originally Posted by exsailor66
Unplugged the mono valve...still blasts out cold air. Will order new mono valve (good one) and change it out myself. Still, if the fail safe position is full on heat, there must be something else wrong.
HELP!
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The aftermarket valves are reputed to have a spring that's weak enough that the engine coolant pump can suck them closed. Considering the relatively weak flow through the heater core, that's a pretty wimpy spring.
Another possibility is a failed or clogged auxiliary water pump if your vehicle has one, though you should still get heat even if it is seized.
Start with the Monovalve cartridge. It is the most common cause of no-heat and one of the cheapest parts you can replace. DO NOT waste money on the aftermarket brands. Go with OEM or don't bother.