5 Common Semi HVAC Problems (and How to Fix Them)

Keeping your semi-truck's HVAC system in top shape is a matter of safety, productivity, and even fuel efficiency. When you're driving long hauls through the scorching valleys or climbing mountain passes in winter, a properly working HVAC system is a must. It keeps you comfortable and reduces fatigue, which is the cause of 91,000 road accidents each year in the US alone.

Below, we’ll dig into five common HVAC problems in semi-trucks and how you can fix them before they leave you sweating (or freezing) on the side of the road.

1. Weak or No Airflow

If you’ve cranked up the AC or heater and you’re still not comfortable, you might have an airflow issue. This problem usually involves a clogged cabin air filter, a failing blower motor, or a busted motor resistor.

How to Fix It

• Start by checking your cabin air filter—these should be replaced every 15,000 to 30,000 miles, especially in dusty regions like Fontana.
  
  
• If the filter looks good, listen for the blower motor when turning the system on. No sound? That motor needs to be replaced, and you’ll need professional HVAC repairs to fix it.
  
  
• If the fan only works on certain speeds, your blower motor resistor is likely the culprit.

2. No Cold Air (AC Malfunction)

An HVAC system that blows nothing but warm air on a hot Inland Empire afternoon is brutal. This typically traces back to a low refrigerant level, a leaking A/C line, or a bad compressor.

How to Fix It

• Use a refrigerant pressure gauge to test the system. Low levels usually mean there’s a leak somewhere.
  
  
• Inspect hoses and fittings for signs of oil residue—this can be a tell-tale sign of a refrigerant leak.
  
  
• If the system is charged and still not cooling, you might be looking at a compressor failure. That’s a job best left to a qualified tech.

3. HVAC Only Works on One Setting

When your HVAC only works on high or low and refuses to adjust, you could be dealing with a faulty blend door actuator or control module. These parts regulate the temperature and airflow mix.

How to Fix It

• If your truck has electronic climate controls, scan for fault codes using a diagnostic tool. This can pinpoint the actuator.
  
  
• In manual systems, listen for clicking sounds behind the dash, which could indicate stripped gears in the actuator.
  
  
• Replacement usually involves removing part of the dash, so make sure you’re up for the task—or call a pro.

4. Strange Smells or Musty Air

That funky gym-bag smell coming from your vents? It's likely due to mold and bacteria growing in your evaporator core. This happens when condensation sits too long without drying out, which is common in high-humidity or stop-and-go driving situations.

How to Fix It

• Use an AC disinfectant spray through the cabin filter housing or vents to kill bacteria.
  
  
• Consider adding an evaporator cleaner foam, which gets sprayed directly into the core via the drain tube.
  
  
• Always run the blower fan for a few minutes without A/C at the end of your drive to help dry things out.

5. Heater Isn’t Working

When winter rolls into the San Bernardino Valley, you’ll want your heater to work like a charm. If it doesn’t, you could be looking at issues with the heater core, thermostat, or coolant levels.

How to Fix It

• Check your coolant reservoir—low levels might mean there's a leak or air pocket in the system.
  
  
• If the heater blows cold but the engine is warm, your thermostat might be stuck open.
  
  
• A clogged heater core can also block hot coolant from reaching your cabin. If coolant flow is restricted, try flushing it out.

Preventive HVAC Maintenance Tips

• Change your cabin air filter regularly.
  
  
• Run the A/C at least once a week, even in winter, to keep seals lubricated.
  
  
• Use your defrost setting often—it activates the A/C compressor, which helps dehumidify the system.
  
  
• Schedule a coolant flush every 60,000 miles to protect the heater core.

Final Thoughts

Your semi-truck HVAC system works hard, day in and day out, to keep you comfortable behind the wheel. Whether you're barreling down I-10 or waiting out traffic near the Cajon Pass, a working climate control system is non-negotiable. By watching for early signs and staying on top of maintenance, you can avoid downtime and discomfort.