My heater doesn't work. The blower comes on but no heat. Regardless of the setting (60-90 degrees) only cold air. Defrost works to clear windows but no heat.

A/C Switch Assembly
The A/C switch assembly integrates the A/C request button, RECIRC request button and rear DEFOG button into a single assembly.
The A/C request button is enabled by the blower motor switch. The A/C switch assembly sends a reference signal to the blower motor switch. When the reference signal is grounded (blower motor switch turned ON) the A/C request button is enabled. The A/C request button is disabled when the blower motor switch is in the OFF position. When the A/C request button is activated (indicator ON), an A/C request signal is sent to the PCM. The A/C request button indicator can be toggled on and off when in the FLOOR/DEFROST or DEFROST mode, but an A/C request signal will be sent to the PCM regardless of the indicator status.
When the RECIRC request button is activated (indicator ON), the air inlet mode door actuator will drive to the recirculated air inlet position. When the RECIRC request button is deactivated (indicator OFF), the air inlet mode door actuator will drive to the fresh air inlet position. The RECIRC request button is disabled when in the FLOOR/DEFROST or DEFROST mode.

well the ac compressor will come on if you have it in defrost mode,if heater is blowing cold ,after you just had thermostat replace ,you could possibly have air in system.now if your heater motor is not working which is called the blower motor under dash it could be the resistor that controls it also these rigs have been known for wire harness problems at heater control unit and the blower motor connector.good luck,mike

Depending on the vehicle, it could be a vent door actuator

check the controls for the vent doors.

These vehicles had a problem with casting sand getting trapped in the block and over time plugging the heater core. the way to fix this is to flush the heater core. this can be done by disconnecting the hoses one is on passengers side near the firewall the other is on the drivers side also by the fire wall and following them to the clamps on the engine and disconnecting them and then hooking a water hose up to them one st a time untill clean water comes out. in some cases the heater core will need replaced

The cooling fan should come on when the coolant temp reaches around 200 degrees F. regardless of what position the heater switch is in.

The fan comes on when the defrost is on because the defrost automatically turns on the A/C. The A/C dries the incomming air to help clear the windshield.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

The heater resistor is probably faulty, its located near the blower fan

Most likely a faulty heater tap not allowing engine coolant to flow into heater core, check the heater tap for correct operation.

Sounds like the air "blend" door might be inoperable. This door opens and closes a specified amount to regulate the amount of either cooled or heated air, as requested by the climate control unit. If the door is continually in the position which is bypassing the heater core (cold setting), the air will not get warm regardless of the condition/operation of the heater core. I'd suggest having this component checked.

after checking the heater core hoses they were both hot I went in to look at the blend door and noticed it wasn't moving on hot or cold so I simply disconnected the lincage and moved to hot and its working ok. but its not completely repaired