Chrysler Repair: 99 Concorde Heater Problem, 1999 chrysler concorde, automatic temperature control

Question
I have a 1999 Chrysler Concorde with the 2.7 liter v6.  The car produces heat when I am driving at a speed over 40mph, but only blows out cool air when driving below this speed.  What do you think the problem can be?  

Answer
Hi Tony,
It might have to do with the rate of coolant flow through the heater core not being sufficiently rapid until the rpm of the engine goes high enough to power the water pump at a pressure sufficient to get it thru the core. You could test this theory by feeling the difference in temperature between the incoming and outgoing hoses to the heater core at the firewall of the engine compartment behind the engine. If the heater were working well I would expect the hoses to be fairly similar in temperature, whereas if the coolant were not moving well then the return hose would be a lot cooler than the incoming hose. So do that observation at above and below 40 mph and see if there is any difference.
The other approach is to see if there might be a fault in the heater control system. If you have the automatic temperature control system it does have a self-diagnostic capability. The engine has to be running and the car not in motion to do the test. Simultaneously press the floor, mix, and defrost buttons, (the 1st,2nd and 4th on the top row) and see if the display unit begins to blink. Release the buttons when it has started to do that and then after it blinks for a while longer watch for two digit numbers to appear on the readout. Those are the fault codes. If the first number appears and stays in place, push on the first button in the bottom row (panel) to continue the readout. At the end of the readout the unit will return to normal settings.
Then write back with the number(s) and also tell me more about exactly what is wrong with the function of the unit.*
Roland
*The code numbers range from 23-36. You can also get a decoder list at www.allpar.com/fix/codes_climate.html