Good catch by JayTee2014 about the Blend Door Actuator. Forgot all about that possibility. Surprised, too, because I've had to replace TWO of those on my Taurus.
The following is a description on my Taurus, which *may* be similar to your vehicle:
The Blend Door Actuator (BDA) is just a 3" square X 3/4" thick plastic box (held in by 4 screws under the dash, toward the firewall, in the center). It has a single electrical plug connection and fits into the Blend Door Housing (BDH) to blend hot and cool air. You change the temp setting on the dash and the BDA turns to move the air Blend Door. It has a ~1.5" protruding plastic grooved shaft sticking out at a 90-degree angle (this is the part that plugs into the BDH). Inside is a mess of tiny plastic gears. Failure mode is cracked teeth on tiny interior gear which causes the BDA to lift the blend door only so far ... then it falls down (repeatedly making a clunking or clicking noise). As temporary workaround, was able to prop the Blend Door UP (in the HEAT position) by fashioning a piece of plastic and jamming it into a thin, metal "arm" that moves on the exterior of the BDH. Replaced the Actuator later.
So try moving your temp setting and watching under the dash to see if you can see (and hear) the BDA working. Feel the air coming out the vents. If the engine is hot, and you can feel *some* heat, and there's no "clicking" or "clunking" going on, your BDA is probably OK. I'd try flushing the Heater Core next. Then maybe the Water Pump after that.
Of course, I suppose it's also possible that your BDA has failed (or frozen) in a partial-heat position. Not sure if you'll be able to determine this without pulling it out (still electrically-connected), and watching the shaft turn as you turn the temp knob on the dash. And then even if it turns, is it turning the *full* range?? Might be hard to tell without something to compare it to. Can't remember how my my Taurus shaft turned, but it was probably at least 90 degrees?
2011 Equinox 1LT (new Jan 2011): Summit White / 3.0 L V6 / FWD / Pioneer Stereo / 18" Wheels / 138.2k miles [05-NOV-2019]
2002 Impala 1LT (new Jun 2002): Summit White / 3.4 L V6 / FWD / Cassette-CD Combo /225.5k miles [05-NOV-2019]
Last edited by Colt Hero; 12-12-2017 at 10:13 PM.