Heater on ZT going cold
 

I have a problem with the heater on my 2005 ZT. Initially it is fine but after about 25 miles it pumps out cold air. Any ideas much appreciated.

are all vents putting out the cold air?

With atc (digital display type ventilation control), after the interior has reached its targeted temperature, the drivers side upper vents will start to blow cooler air, in an effort to keep the driver alert, whilst maintaining heat in the rest of the car. Personally I find it cooler than it needs to be. The footwell vents will remain warm, as will the passenger side upper vents. The central vents will be determined also by the thumb wheel.below them.

If all vents blow cold, then it is likely to be an airlock, at which point, I would ask if the cooling system has had any work done and been refilled. If it has, then the correct bleeding/refill procedure would need to be conducted. If it has not had any work done, check for leaks in the system to see if there is anywhere for that air to get in (and more importantly the coolant to escape!!)

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The first thing to check is the coolant level.
A cold heater is about the first sign of too little coolant.
Apart from that it could be the thermostat not closing properly, the heater being fouled, the flaps in the system not working properly, or a blockage in the heating circuit generally.

I’m guessing at sticky thermostat.