RV AC Not Cooling? Use This Temperature Test to Find and Fix the Cause

Quick answer: If your RV AC is not cooling but still runs, measure the air at the return filter and the output. A 16-22°F drop means the unit is likely fine, so check heat load and duct leaks. A lower drop points to airflow, voltage, thermostat faults or a sealed-system problem.

Why is my RV AC not cooling even though it’s running?

An RV AC that runs but blows warm or weak air usually has one of eight problems: a dirty filter, an iced evaporator coil, low voltage, a thermostat fault, dirty coils, leaking ducts, a sealed-system or compressor failure, or more heat than the unit can handle.

This guide is for a unit that runs and moves air but does not cool enough. If your AC only clicks, or hums with no fan, see our separate guides on those start-up symptoms.

Use this table to choose where to start.

What you notice	Likely cause	First action
Warm air from the start	Wrong thermostat setting, low voltage, or a sealed-system or compressor problem	Check the mode and set point, then run the temperature test
Cools, then warms up	Ice building on the evaporator coil	Turn off cooling, look for ice, and let the coil thaw
Shuts off after a few minutes	Low voltage, an overloaded circuit, dirty coils, or a thermostat fault	Check voltage while the AC runs and turn off other large loads
Ice on the coil	Restricted airflow, cold temperatures, low fan speed, a very low set point, or low refrigerant	Thaw the coil, then clean the filter
Cool vents but a warm cabin	Duct leaks or too much heat load	Run the temperature test, then inspect the ceiling assembly

How do you test whether your RV AC is actually cooling?

Compare the air going into the unit with the air coming out. You only need one thermometer.

1. Set the thermostat to cool, at least 5°F below room temperature.
2. Close the windows, doors and roof vents.
3. Let the AC run for 20-30 minutes.
4. Hold the thermometer at the return air filter until the reading settles. Write it down.
5. Use the same thermometer at the output and write down that reading.
6. Subtract the output temperature from the filter temperature.

Published runtime advice ranges from 10-15 minutes to 20-30 minutes. Use the longer time before drawing conclusions.

How many degrees should an RV air conditioner cool the air?

Coleman-Mach calls a 16-22°F drop between the filter and the output ideal. For example, 80°F at the filter and 60°F at the output is a 20°F drop, which is in range.

• Drop is in range: Look at heat load and duct leaks.
• Drop is low: Check airflow first, then voltage, then the sealed system.

High humidity lowers the temperature difference between return and supply air. One borderline reading on a humid day does not prove the unit has failed. Rule out the simple causes and test again.

Could a dirty filter or coil be the problem?

Yes. Check the return air filter first because it is the easiest fix. A clogged filter cuts airflow, which weakens cooling and can lead to ice on the coil.

Clean the filter about every 2-4 weeks in heavy use. In regular use, every 30-60 days is the outer limit. Dusty sites and pets are good reasons to clean it sooner.

What is the difference between the evaporator and condenser coils?

• Evaporator coil: This coil absorbs heat from cabin air. When it is dirty, less air passes through and the coil can ice over.
• Condenser coil: This coil releases heat outdoors. When it is dirty, the unit cannot shed heat well, so cooling drops.

How do you clean the coils?

Clean both coils once a season with a cleaner made for AC coils, and follow the label. Work gently so you do not bend the thin metal fins.

Warning: Shut off all power before opening the shroud. Turn off the breaker, unplug shore power, and turn off any generator or inverter. Take care on the roof, since it may be slippery or not rated to walk on. If you are unsure, hire a technician.

What should you do if the evaporator coil is frozen?

Stop cooling and let the coil thaw completely. Running the AC on cool with ice on the coil will not fix the problem.

Why does my RV AC cool for a while and then blow warm air?

Ice is a common reason. As ice builds on the evaporator coil, it blocks the air moving through it. Cooling fades and airflow from the vents gets weaker.

Why does an RV AC freeze up?

Coleman-Mach and Camping World list these common causes:

• Cold indoor or outdoor temperatures
• Blocked airflow, such as a dirty filter
• Low fan speed
• A thermostat set low enough to keep the compressor running
• Air leaks in the ceiling assembly or a quick-cool vent left open too long
• Possible low refrigerant

How do you thaw it safely?

1. Turn off cooling.
2. Leave the AC off, or run the fan only to help melt the ice.
3. Allow at least 30-60 minutes, and longer if ice remains.
4. Let the ice melt on its own. Do not chip at it, since that can damage the coil.
5. Clean the filter, then restart the AC and repeat the temperature test.

To prevent another freeze, keep the filter clean, use a higher fan speed, and avoid cooling when it is below about 60°F outside.

Decision rule: If ice returns even with good airflow, have a technician check the refrigerant.

Is low voltage or an overloaded circuit weakening your AC?

Low voltage can keep an RV air conditioner from working properly, and it can do lasting harm. Coleman-Mach says its units can run safely between 104 and 125 volts and warns that low-voltage operation may cause serious component damage. Check your manual if you have another brand.

Check voltage at the pedestal, or use a display panel or energy management system (EMS), while the AC is running. A reading with the AC off can hide the problem. If voltage is below the manufacturer’s range, stop running the AC.

Common causes of low voltage include:

• Long or light-gauge cords: Coleman-Mach warns that these can cause significant voltage drops.
• 15-amp outlets and 115-volt cheater plugs: Coleman-Mach does not recommend either one.
• Peak-hour demand: Voltage can sag on hot afternoons when many campers run their AC at once.

How much of a 30-amp service does the AC use?

A 30-amp service provides 3,600 watts. An RV AC can draw about 2,400 watts at startup and about 1,500 watts while running.

That leaves about 2,100 watts for everything else while the AC runs. At startup, only about 1,200 watts remain. Turn off the microwave, electric water heater or electric fridge first, then see whether the AC runs better.

How can you tell if the thermostat is causing the problem?

Compare the thermostat with a separate thermometer placed nearby. Micro-Air says a gap of more than 2-3°F points to a thermostat problem, and some factory analog thermostats can misread by up to 10°F.

Work through these checks in order:

1. Confirm the thermostat is in cool mode, the set point is below room temperature, and the fan setting is correct.
2. Check the 12 V battery voltage and connections, because the controls depend on that side of the system.
3. Reset the system by turning the power off and waiting 3-5 minutes.
4. Restart the AC and repeat the temperature test.

Camping World notes that on some Dometic systems, a thermostat that is not properly signaling the compressor can mimic compressor failure. Rule out the thermostat before paying for a compressor or a new unit.

Why are the vents cool but the RV still warm?

A normal temperature drop with a warm cabin means the problem is the heat load or the ducts, not a failed unit. Either cooled air is being lost, or the RV is gaining heat faster than the AC can remove it.

How do you check for duct leaks?

Roadtrippers says warm attic or return air can leak into cooled air at the plenum or dam area. The plenum is the box that directs air from the unit, and the dam separates return air from cooled air.

1. Shut off power to the AC.
2. Remove the ceiling cover.
3. Look for gaps around the plenum and the dam.
4. Seal the gaps with HVAC foil tape.
5. Replace the cover and test again.

Why won’t my RV AC keep up in hot weather?

Every AC has a capacity limit. Micro-Air says a unit may work correctly but fall behind above roughly 95-100°F, in direct sun, with poor insulation, or with too little BTU capacity for the space.

To reduce the load:

• Park in the shade when you can.
• Close the shades on sun-facing windows.
• Reduce indoor humidity by running vent fans while cooking and showering.
• Pre-cool the RV before you arrive or before the hottest part of the day.

When is it a refrigerant, capacitor or compressor problem?

Suspect these parts when the temperature drop stays low after you have ruled out airflow, voltage and the thermostat. Most of this work belongs with a qualified technician.

Can you add refrigerant to an RV air conditioner?

Not as a DIY job. Coleman-Mach says refrigerant does not wear out and the sealed system should not need recharging. A low charge suggests a leak, so a qualified technician should evaluate it. Do not use DIY recharge kits or open the sealed system.

Could a bad capacitor be the cause?

Capacitors mainly cause start-up problems, so our guides on clicking and humming units are the better place to start. Warning: Micro-Air notes that capacitors can hold high voltage even when the RV is unplugged. Do not touch the terminals unless you are trained to discharge them safely.

How do I know if my RV AC compressor is bad?

You usually cannot confirm it with a thermometer alone. A compressor failure becomes more likely when the fan runs and the air stays warm after the filter, coils, voltage, thermostat and capacitor have checked out. Have a technician confirm it, because a bad compressor generally means replacing the AC unit.

Should you repair or replace the AC?

Lippert advises considering replacement if repair costs approach half the price of a new AC system, especially if the unit is more than 8-10 years old. Get the repair quote in writing and compare it with the installed price of a new unit.

How do you keep an RV AC cooling well?

A short routine prevents many of these problems:

• Check the filter weekly when the AC is in use.
• Clean the filter on schedule.
• Clean the coils each season.
• Watch the voltage at each campsite.
• Book an annual inspection with a certified RV technician to cover the filters, coils, capacitor and compressor.

Write down your temperature drop when the AC is working well. That gives you a baseline to compare against the next time the cabin feels warm.