If the thermostat is plugged or stuck closed, the vehicle will usually run very hot or overheat quickly. But sometimes, if the thermostat is stuck open, the vehicle will not heat up to normal operating temp.(hard to tell in summer and not particularly harmful, unless using air conditioning, but very noticeable in winter when the heater won't heat and the engine runs constant high idle). A more visual way to check for certain is to remove the thermostat from the engine (usually a fairly simple procedure involving only partial draining of the radiator, a couple of bolts, a gasket, and some sealant). Put the thermostat in a pot of water on the stove and boil it. Using a thermometer, check the temp. at which the thermostat opens and closes (most vehicles 185 - 215 degrees Fahrenheit). If no thermometer is available, then just the fact that the thermostat opens and closes around boiling tells you it's working (just not the specific operating temp. which matters in some vehicles and environments). However, as inexpensive (usually) as thermostats are, if one goes to the trouble of removing it from the car, one might as well buy a new one of the correct temp. range, install it in the vehicle and be done with it.
Your coolant level is low.
Check coolant level, and radiator fan operation.
THE CAUSE for this is that when your cold air is on the petrol is used to create air. and that is how cars create cold air
gas leak or wrong wiring
My blower would blow cold air at idle & engine would start to overheat, I replaced my thermostat & problem solved.
Compressor not operating.
I had a similar problem with Chevy Lumina when the thermostat went out. It was stuck open so the car never got up to a temp high enough to blow hot air. Kerry
Air in the water system.
the blown head gasket would make it over heat. but it wouldn't cause the heater to blow cold, it should actually blow hot if that is wat happened
Needs a recharge
If the unit started it`s probably not the stat.
There are several causes that an air conditioner is only blowing cold air for a short period of time then only blow hot air and then not going back to cold. This is especially common in the hotter weather including in the 90s and above. It could be from the air conditioner freezing over. You can fix this by simple trouble shooting and if that doesn't work call a repair man.