On a hot, humid summer morning, a driver reports that fog forms on the outside of the windshield when the A/C is set to maximum cold with air directed at the glass. The wipers clear it. What is the correct explanation?

Cold conditioned air is chilling the glass below the outside air's dew point, which is normal condensation. Fog that forms on the outside and clears with the wipers comes from humid outside air condensing on glass cooled by the A/C air. It is normal. Change the mode or raise the temperature slightly. Heater core leaks, plugged drains, and a stuck recirculation door cause fog on the inside of the glass.

A driver complains that the windshield develops a greasy film on the inside that smears when wiped, especially with the defroster on, and the cab has a sweet odor. The surge tank has needed coolant twice in a month. What is the most likely cause?

A leaking heater core, or a coolant leak inside the HVAC case. An oily film, a sweet smell, and unexplained coolant loss together point to coolant escaping into the HVAC case from the heater core or its connections. The defroster blows the vapor onto the cold glass. Pressure-test the cooling system and inspect the case drain and floor for coolant.

Technician A says a plugged evaporator drain can cause window fogging because water in the case evaporates back into the airstream. Technician B says an A/C compressor that will not engage in Defrost on a 50°F rainy day can contribute to fogging because the air is not dehumidified. Who is right?

Both Technician A and Technician B. Both are right. Standing water in the evaporator case re-humidifies the air, especially at start-up. Many systems run the compressor in Defrost, above the ambient lockout temperature, to strip moisture from the air before it is reheated. A compressor that does not engage leaves the air wetter and fog clears more slowly.

Diagnosing Outlet Air Temperature Problems & | Free Guide 2026

Key Takeaways

ASE T7 tasks C1 and C2 require diagnosing outlet air temperature control problems and window fogging problems in the HVAC system.
Heat-side outlet temperature complaints are isolated by checking coolant level and temperature, heater hose temperatures, heater control valve position, and blend door travel and calibration in that order.
An oily film on the inside of the windshield with a sweet odor and unexplained coolant loss points to a leaking heater core.
Interior fogging is commonly caused by a fresh/recirculation door stuck on recirculation, a compressor that does not run in defrost, a plugged evaporator drain, or water leaking into the cab.
Fog on the outside of the windshield on hot, humid days, when cold air is aimed at the glass, is normal condensation that the wipers clear.

Two Heating-Area Tasks

The first two heating tasks are diagnostic reasoning tasks. They ask you to diagnose outlet air temperature control problems and window fogging problems. Both depend on how three systems interact: engine coolant heat, refrigerant dehumidifying, and the doors and blower that mix and deliver air.

Outlet Air Temperature Problems

Step-by-step for too little heat

Coolant level and temperature: low coolant starves the heater core first, because it is the highest point in its loop. A thermostat stuck open, or the wrong thermostat, keeps coolant below normal temperature, so heat is weak, especially at highway speed in winter. Compare the scan-tool coolant temperature with an actual measurement.
Heater hose temperatures: hot inlet and hot outlet means coolant flows, so move on to the air side. Hot inlet and cool outlet means a restricted core or an air lock. Both cool means no flow: a closed shutoff valve, stuck heater valve, failed auxiliary pump, or air lock.
Heater control valve: confirm it receives the command (vacuum, air, or electrical signal) and actually opens.
Blend door: confirm it reaches full heat. Check the actuator, linkage, calibration relearn, and door seals.
Airflow: a plugged cabin filter, weak blower, or disconnected duct reduces heat delivered, even with a hot core.
Controls: for automatic systems, check the setpoint, the in-cab and outside air sensors, and DTCs.

Too much heat, or not enough cooling

Heater control valve leaking or stuck open during A/C operation reheats cold air. Check the downstream hose temperature on full cold.
Blend door stuck toward heat, a broken linkage, or failed door seals.
In-cab sensor reading falsely cold on automatic systems, so the controller keeps calling for heat.
Sleeper: an open bunk shutoff valve, or a bunk blend door stuck in the heat position.

Temperature swings or poor control

A blend actuator losing position, often with clicking from a stripped gear, or needing a relearn.
An intermittent sensor or connector.
Air or vacuum control leaks that let doors drift.
Heater valve hunting on a PWM or stepper valve.
Low coolant causing heat to surge and fade as air passes through the core.

Window Fogging

Start by deciding what kind of fog it is:

What the Driver Sees	Most Likely Cause
Fog on the inside that clears slowly, cab feels humid	Excess moisture inside: recirculation door stuck, no A/C dehumidifying, wet carpet or insulation, plugged evaporator drain, water leaks, wet clothing
Oily, greasy film on the inside that smears when wiped, sweet smell	Heater core leak, or coolant from a hose or fitting inside the case
Fog on the outside on hot, humid days	Cold A/C air chilling the glass below the outside dew point; normal, cleared by wipers or by changing the mode or temperature
Fog only in one area	Disconnected or blocked defroster duct, or objects on the dash covering the outlets
Hazy film that returns after cleaning	Plastic off-gassing or smoke residue on the glass that attracts condensation; clean the glass thoroughly

Causes of inside fogging to check

Fresh/recirculation door: most systems force outside air in Defrost because cab air holds moisture from occupants. A door stuck on recirculation fogs the windshield quickly in cold, wet weather.
Compressor operation in Defrost: many systems run the A/C compressor in defrost modes to dehumidify the air before it is reheated, as long as outside temperature is above the compressor lockout. If the compressor does not engage, because of low charge, an open outside-air sensor reading very cold, or a clutch fault, air reaches the glass wetter than it should.
Plugged evaporator drain: water standing in the case evaporates back into the airstream after shutdown and on start-up. Look for water sloshing sounds and water on the floor.
Water leaks into the cab: failed door or window seals, cowl or roof leaks, and sleeper window or skylight seals soak carpet and insulation. Wet interiors fog windows every morning.
Heater core leaks add coolant vapor as well as moisture. Pressure-test the cooling system and check for coolant loss.
Weak heat: air that is not warm enough holds less moisture and does not warm the glass above the dew point. Work through the no-heat steps above.
Low blower speed or blocked defroster outlets leave too little air on the glass.

A quick sequence

Look at the glass. Is the fog inside or outside, clear or oily? Smell for coolant.
Select Defrost and confirm outside air and airflow at the defroster outlets.
Confirm the compressor engages in Defrost when the ambient temperature allows it.
Check the evaporator drain and the floor for water, and look for water leaks.
Check coolant level and loss history, then pressure-test if coolant loss is suspected.
Verify heat output and blend door travel.

ASE T7 HVAC (Medium/Heavy Truck)

7.3 Diagnosing Outlet Air Temperature Problems & Window Fogging