The Car air conditioner blows cold then warm if the warn air blows only when the engine is hot or stressed, it means the compressor is automatically disengaging and it's normal.
If the ONLY time warm air starts to blow from the vents is when the engine is either hot or under a lot of stress. This is no coincidence!! It clearly means this vehicle has a safeguard feature that shuts off the A/C's compressor when the engine is hot/stressed.
Why target the compressor? Because it places a heavy load on the engine. Therefore, by disengaging it, the engine is relieved of that extra load. So, in summary, there's actually no problem here (i.e. this is normal behavior for this vehicle).
What to do?
Do nothing. No Remedy Necessary.
If the warm air blows even when the engine is not hot or stressed, then it's abnormal. If this happen the problem could be that the expansion valve freezes up thereby temporarily blocking the flow of refrigerant throughout the AC lines. if this is the case the pattern would be cold air initially for quite a while followed by warm air.
An A/C system has something called an "expansion valve". It controls the amount of refrigerant flowing into something else called the "evaporator" (i.e. the part where the cold air comes from). If the expansion valve is blocked, the refrigerant can't flow to the evaporator.
Consequently, the A/C system will only produce/blow warm air. As indicated above, cold air initially blows but, after a while, the air turns warm. An explanation for this is the following... Soon after the A/C is turned on, the expansion valve starts to freeze. In time, it totally freezes up, thus blocking the flow of refrigerant to the evaporator. This results in warm air. Now then, an expansion valve will freeze if there's moisture in the refrigerant. This moisture collects in the expansion valve, freezes there, and blocks/closes the valve. The expansion valve itself is probably fine, it's just that moisture (probably entering the A/C system via a leak) is causing it to freeze up.
What to do?
Take this vehicle to a garage specializing in A/C repair. The mechanic will probably have to do the following: locate any leaks (this is probably how moisture got into the refrigerant), evacuate the A/C system (i.e. remove the refrigerant), repair any leaks found, replace a critical A/C component known as the "receiver/dryer" (see below), and then "charge" the A/C system (i.e. put new refrigerant in).
As was mentioned in the explanation for this diagnosis, the expansion valve itself is probably fine... it's just the moisture in the refrigerant that's causing it to freeze up. Further, the moisture probably got into the refrigerant by way of a leaking A/C system hose or seal. Incidentally, though, the purpose of the receiver/dryer is to remove such moisture from the refrigerant. Clearly, though, it's not doing a very good job at removing this moisture since it was suspected that the expansion valve is freezing up as a result of this moisture.
If the warm air blows randomly without any pattern, then this could be a problem of a failing compressor or AC electrical problem. this A/C system probably produces warm air in a random fashion. Let's look at the possibilities.
"Failing Compressor" - The compressor is the "driving force" of an A/C system. Specifically, it moves the refrigerant through the system's components. If the compressor occasionally fails, the refrigerant will stop flowing during the down/failed time. Consequently, the A/C system will then "sometimes blow warm air" (i.e. during the downtime).
"A/C Electrical Problem" - Specifically, if an A/C-related wire is loose, it will cause the affected A/C components to stop working during the time the wire has lost contact or "broken the circuit". Consequently, the A/C system will blow warm air during this time. However, once the wire regains contact, the affected A/C components start working again (and soon,cold air blows).
What to do?
Take this vehicle to an A/C repair facility and explain to the mechanic exactly what we explained to you. An inspection of the compressor and A/C-related wiring will, of course, be necessary.