I've got this plan, when I've got some time and it's not freezing cold outside, to get at the coolant lines going to the heater core and cut / remove them from the core and somehow rig up a couple of garden hoses and just flush the poopies out of the core, have the out-flow run to a sewer grate. Then reconnect the lines. In the middle of summer, if I set the interior temperature to "hi" (which is all the way up), the air the comes out of the vents is very very hot. If, say a week ago when the ambient air is 40f, the vent air is also hot, or reasonably hot. Certainly enough to bring the cabin up to room temperature quickly. But today, the ambient is 25f (way below normal) and the engine temp gauge is right up to operating temp but the air coming out of the vent is luke warm, or maybe a bit less. Where is the heat? I know there is a self-diagnostic test you can do by pressing the right buttons and the fan and the various internal doors/gates open and close and every time I've done it I get a pass (haven't done it this year yet, but I suspect it will pass). Last year I struggled to get the blend door motor out (what a pain!) and watched it operate during a test and it seemed to work fine. So I'm back to plan-A which is the flush the core before winter sets in, but I really can't figure out why I get plenty of heat out of this thing in the summer and really any time the ambient outside air is over 40f (or maybe even 35f) but below that the heat really drops off fast. The only other thing I can think of is that the electronics is malfunctioning at low temps and keeping the blend door open too far?