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Odyssey's heat/air conditioning temp changes constantly

Our 2003 Honda Odyssey changes temperature constantly via the heat/air conditioning system. If I leave the thermostat at 72, it will blow very hot air, even if it is 85 degrees out. Then it’ll start working normally. And then it goes back to hot air.

We’ve taken it to the mechanic. The freon (or whatever coolant they use) is full and fully charged. My mechanic took the whole system apart and put it back together twice, and everything checks out – except that it continues to behave oddly.

Couple of other things that might help. First, the system occasionally burbles gently. Second, when it’s blowing hot air, I can get a usable temperature by turning the temp down to about 63.

Of course, 10 minutes later, it starts working normally, so I have to turn the temp back up. And then back down 5 minutes after that, and so on.

It’s very aggravating. Any ideas/insights are most appreciated.

Thanks in advance –

Joel from Roswell, GA

Looking at some data for the vehicle it seems to show that the air blend doors are controlled electrically. Some systems use vacuum air to control the doors and if there is a vacuum problem the doors can be effected. If the air blend doors are working ok (and it kind of sounds like they are) then it would seem to me that the AC system itself is the problem somehow. If the hot air comes in real quickly then I would check the air blend doors to see if they are moving and changing the air temperature by mixing in hot air.

Perhaps the temperature sensor inside the vehicle is malfunctioning and causing the control unit to adjust the air temperature. There is also a sunlight sensor at the base of the windshield.

The temperature of the air coming from the vents is controlled by a flap that directs air through either the heater core, which is always hot, or the AC evaporator, which is cold when the AC is on or the HVAC system is set to “Automatic.” Warm and cold air is blended to achieve the desired temperature.

The temperature control flap is moving, so the motor which moves it must be working. The question is, why is the control unit moving the flap? Has anyone tested the sensors?

The control unit itself could also be bad, but that’s the last resort.