Sable blows warm air temp guage stays on cold side

SOURCE: cold air out of heater

check the both heater hose from the heater core to the block. In one of the hoses you may find a valve either vacuum or electric. Try to check for air bubble. You could check this by taking loose the top radiator hose.

SOURCE: 2002 Mercury Sable, Heating Problem

Hello. You are in luck. I just had this same issue in my shop this week. We changed the thermostat, and replaced the coolant. Fixed it right up. Since you did not mention a new thermostat, that would be my next move, if I were you.

SOURCE: i have a 2002 mercury sable that heater will not get hot

Since your blower is working and it is a heat issue,thats where we'll start,First thing is have you checked the coolant level in the radiator when engine is cold,make sure it is filled to top,next verify you have coolant in reserve ,Start your van and get it up to normal operation temperature,If your heat gauge on the dash works correctly it should be 190 degrees,Now lift the hood and locate the two heater hoses going into the heater assembly at the fire wall,Both hoses should be hot to hold on to,if both are hot and no heat inside your problem will be the blend air door in heater assembly.
if neither hose is that hot you need to replace the thermostat in the engine to get it up to 195 degrees.
if one hose is hot and one is luke warm then the heater core is blocked and to blame. hope this helps you.

SOURCE: 99 mercury sable 3.0l

have you checked the coolent level?

SOURCE: 99 sable 3.0ll

Here are a couple of things I would try.

Make sure your coolant level is at the full mark in the reservior, ( plastic tank)

Start your engine and let it run 6 to 8 minutes, so it reaches it's operating temperature.
With the engine still running, turn your heater on full blast. You should hear a light clicking noise from the thermostat opening up to cycle the coolant from the radiator. ( sometimes if a sealant is used, and it shouldn't be in most cases, when installing a thermostat excess sealant can prevent the thermostat from operating properly )
If you hear the click from the thermostat then you know that's working. then.......;

Turn the engine off, look back behind the engine at the firewall you should see 2 small hoses, about 1 inch in diameter. While the engine is still hot, carefully reach back and feel both hoses, both should be hot. If only 1 is hot then the coolant isn't circulation through the heater core and you could have blockage in your system.

Another good possibility is, the trap door that moves between the heater core duct, and the a/c evaporator duct is not moving back and forth like it should. To check this you will need to remove a few parts from the lower dash to gain access to inspect.

I hope this helps you.