Heating and Cooling System Diagnosis and Repair
Two fluids, one cab
Heating uses engine coolant through a heater core; cooling uses refrigerant through the evaporator. Both rely on airflow: blower, ducts, doors, and a clean evaporator and heater core face. Diagnose by temperature and airflow: no heat at all suggests coolant flow (thermostat, blend door, plugged core, low coolant); weak heat at speed suggests airflow or blend door; no cooling suggests the refrigerant loop.
Know the blend door actuator and mode doors, the blower resistor or PWM controller for speed control, and the cabin air filter as a common airflow restriction. Heater core replacement often requires dash removal, so confirm the core is actually plugged (inlet hot, outlet cold with flow) before teardown. Check for coolant leaks at the core and for a pressure cap or air pocket causing no-heat complaints.
Traps
Questions often present a no-heat complaint that is actually a blend door or actuator fault, not a bad core. Another trap: a blower that works on high but not low is usually the resistor or controller, not the motor. Also remember that a plugged heater core can mimic a thermostat problem.
Worked example
Customer reports cold air from the vents with the engine at temperature. Feel both heater hoses: if both are hot, suspect the blend door; if the outlet hose is cold, suspect restricted coolant flow through the core. That single check splits the diagnosis before any parts are ordered.