Heater system

With the engine at full operating temperature, locate the two heater hoses where they go into the firewall and feel them to see if they are both hot to the touch. If they are both hot, the heat in that vehicle is controlled by a blend door that regulates heated air flow. It is operated by an electric motor/actuator. The problem can be that the actuator is stripped or inoperative or the door itself could be damaged. This is what needs to be determined by examining the actuator and see if it is responding to heat change commands or not.

Why no engine size? With no other details, test results, clues, or observations to go on, the thermostat is the best suspect. If it is cold air you want, the AC system is likely low on refrigerant.