Heat will stay on high until turn engine off and back on

the automatic module for the system needs checking or replacing

as far as the temp of the air blowing...it in direct proportion to engine coolant temp...try a hotter thermostat....as for the faan running continuously that is a seperate problem and sounds like a faulty rheostat or fan switch...rheostat is elect, switch on the fan motor that enables different speeds and may beinternal on your truck. only parts store will know . if it is replace fan motor. if not , test switch and then test rheostat

Usually a sign of a faulty blend door.
The blend doors direct air thru the heater core and out thru the vents.

Please see my tip at http://www.fixya.com/cars/r6235876-no_heat_fan_blowing_cold_air . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

My truck just did the exact same thing -- and it was a Sensor -- cost me $40.00 to get repaired at a local auto repair shop.

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

Probably your left temp actuator failed internally. Check the right instrument panel junction block-right side, end of dash-to make sure connectors are fully seated. But thisis usually caused by a faulty temp actuator.

your heat/cool and vent selection options are controlled by engine vacuum through small clear tubes attached to the rear of your climate control interface. Other vehicles have small steel pull wires that operate the desired changes. You might try removing the trim and any fasteners affiliated with the interface/console. GENTLY pull outward and look to see if any of these tubes are pulled off or cut/damaged. Did anyone recently install a new stereo in the vehicle? If so, the vacuum tubing and or pull wires that change your outlets could have been damaged when the stereo unit was pushed into place. Hope this helps

check your coolant level on the radiator

After replacing the thermostat I pulled the heater hoses off and blew air through the heater core at firewall. Can only blow in left side as directional valves will prevent opposite. Chunks of stuff came out with coolant. I then put a piece of hose on the left side and shot water through until it came out clear.
Put it all back together and heat works fine.