The fan is blowing, the unit on the roof is running, and the inside of the rig keeps getting warmer. Most of the time this is something you can fix yourself in under half an hour: a dirty filter, a blocked coil on the roof, a thermostat setting, or low voltage from the campground pedestal, a long cord or a small generator. Some of the time it’s a real fault in the compressor or the sealed refrigerant system, and that’s a technician’s job. And some of the time nothing is wrong at all. The unit is doing everything it can on a very hot day. Here’s how to tell which one you’ve got, in the order the makers tell you to check.
Start with the easy stuff: power, settings and a short wait
Before you go anywhere near the roof, check the breaker. Dometic’s Air Command manual puts “Check circuit breaker is on” at the top of its list for a unit that won’t cool, and Houghton’s manual asks the same question. Then look at the thermostat. Furrion’s first check is that it’s set to cool and set lower than the temperature in the room. It sounds obvious, and it’s the cheapest fix on this page.
Then wait. The compressor has a built-in delay that protects it from restarting too soon. Houghton’s is three minutes, and Dometic says to wait at least two minutes before restarting a compressor you’ve switched off. During that delay the fan blows room-temperature air on purpose. If you’ve been flipping the unit off and on to test it, you’ve been restarting that clock every time.
Last, turn the fan up. Dometic and Houghton both tell owners to run the fan on high to get the unit’s full cooling capacity. Low fan on a hot afternoon leaves cooling on the table.
How often should you clean an RV AC filter?
More often than most people do, and it’s the first cause on almost every maker’s list. Coleman-Mach puts dirty or clogged filters at the top of its poor-cooling causes. Furrion says a dirty filter “restricts airflow and can lead to both odors and cooling issues.”
Furrion’s schedule is once a month in travel season and every two weeks in peak season, and more often if you’re in dusty country or running the unit hard. The filter sits behind the return-air grille on the ceiling unit. Pull it, hold it up to a light, and wash or replace it. Don’t run the unit with the filter out while you wait for a new one. Dometic’s manual says never to run it without the return-air filter in place.
While you’re inside, check the supply louvers. Coleman-Mach lists “Supply air louvers may be shut off” as its own cause. A closed vent in the bedroom can be enough to make the front of the coach feel like the unit is failing.
Is the outside of the unit blocked?
Your AC moves heat from inside the RV to the air outside, and it does that through the condenser coil in the rooftop shroud. If air can’t get through that coil, the heat has nowhere to go.
Coleman-Mach says airflow through the outdoor coil can be blocked by debris, or the cooling fins can be bent, and its maintenance interval is to clean that coil at least once a year. Dometic’s Harrier manual says to clear leaves and dirt from the unit’s ventilation grilles regularly, and to make sure the condensate drain channels at the sides are clear so the water can escape. Leaves are the example Dometic gives, so a season parked under trees is a good reason to look.
This is the one check on the list that means climbing on the roof. If you’re not comfortable up there, that’s a fair reason to hand this part to a mobile tech, who can clean the coil on site.
Why is my RV AC freezing up?
Ice on the coil, or a unit that keeps shutting itself off, is a clue worth reading. Furrion says frozen coils are “usually caused by restricted airflow or low refrigerant levels.” Dometic’s FreshJet manual lists a tripped icing sensor as a reason a unit keeps switching off, and points at closed air outlets or a cold outdoor temperature.
The airflow half is yours. Switch the unit to fan only so the ice melts, then fix the filter and open every vent. If it freezes again with a clean filter and every louver open, the airflow explanation is used up, and what’s left is the refrigerant side. That’s a technician.
One cool-weather note. Some Dometic manuals, including the FreshJet and Harrier, say cooling only works above 16 degrees C, which is about 61 degrees F. Test the unit on a cold spring morning and it may refuse to cool by design.
Test it with a thermometer
This is the most useful two minutes in the whole guide. Coleman-Mach says a healthy unit should put out air 16 to 22 degrees cooler than the air it takes in at the filter. Hold a thermometer at the return-air grille, then in the nearest supply vent, once the unit has been running a while on high fan.
If the difference lands in that range, the unit is cooling. The heat coming into the RV is outrunning it, and the answer is in the next section, not in a repair. If the difference is well short of that range with a clean filter and a clear coil, something inside the unit isn’t right, and you now have a number to give a technician.
Humidity changes the reading. Coleman-Mach calls humidity a significant factor in that temperature split, alongside outdoor temperature and airflow, so a muggy day won’t read the same as a dry one.
Hot weather, long cords and generators
Heat makes every one of the problems above worse, and it brings two of its own: the power supply sags, and the unit hits its ceiling.
Low voltage at the pedestal, the cord or the generator
Coleman-Mach is specific. The unit is built for 115 volts and “may be run safely between 105 volts and 125 volts.” Outside that range, it warns, serious component damage may result. Coleman-Mach also names the loads inside your own rig that pull voltage down on a 30-amp supply: the refrigerator, the converter, the coffee pot, the microwave, the electric water heater.
Cords count too. Coleman-Mach says “light gauge and/or long extension cords tend to cause significant voltage drops.” A long run to a far pedestal, or a household cord off a garage outlet in the driveway, is the setup that warning describes.
A plug-in voltage meter costs less than a service call and answers the first question a tech will ask. Plug it into an outlet, turn on the AC and your usual loads, and watch it. If it drops under 105, shed loads or fix the cord before you blame the compressor.
Running the AC on a generator
Starting the compressor takes far more power than running it. Coleman-Mach’s rule for sizing: find the running amp draw on the unit’s data tag and multiply it by 2.5 to get the startup capacity the generator needs. Houghton’s manual adds that a “0” or a jumbled display while on generator power may mean unstable voltage. If the generator itself is the trouble, see why an RV generator won’t start or shuts down.
When nothing is broken and it’s hot
Rooftop units have a limit. Furrion’s rule of thumb is that an RV AC “can usually only cool about 20 degrees below the outside temperature.” On a 100-degree afternoon, a coach sitting around 80 is a unit doing its job. Dometic’s FreshJet and Harrier manuals list a ceiling of 52 degrees C, about 125 degrees F, above which the unit isn’t designed to cool at all.
In the desert Southwest that limit shows up for weeks at a time. Rowley White RV in Phoenix lists reduced air conditioner efficiency among the effects of prolonged temperatures above 100 degrees F, and says a 110-degree day may only get the inside down to somewhere between 90 and 95.
The fixes for heat aren’t parts. Close every window, door, hatch and skylight, and use the shades and the awning, which is Dometic’s own list. Furrion adds: park in shade where you can, start the AC in the morning before the rig heats up, pick a realistic setting in the 72 to 78 degree range, and run the bathroom and kitchen roof fans to pull out warm, humid air. More of the same is on the extreme heat maintenance checklist.
Is there an error code on the thermostat?
Newer units report faults as a code on the wall thermostat. The makers’ keys are on their own page: what RV air conditioner error codes mean. The short version: sensor codes are technician work, while voltage codes and a high pressure lockout are yours to rule out first.
What needs a technician
The makers draw this line plainly. Coleman-Mach, after its own checklist: “If you have eliminated the problems listed above, you may have a more serious issue that would require evaluation by a trained technician.” On refrigerant, the thing owners most often guess at, it says to have a suspected low charge evaluated by a qualified service technician. The system is closed and sealed and ideally should never need recharging. There’s no port for an owner to top up the way there is on a car.
Furrion’s list of repairs that need a pro is electrical work beyond basic wiring, compressor and sealed-system work, and refrigerant, which it notes is regulated and requires certified techs. Dometic’s instructions say the unit “must be serviced by qualified service personnel only.”
Age matters to the repair-or-replace call. Furrion treats a unit over 10 to 12 years old as near the end of its typical life, and says major sealed-system failures in the compressor, coils or refrigerant are a reason to replace. A mobile tech can do most of this work on the roof at your site. For what that tends to cost, see what mobile RV repair costs.
Fix it yourself or call a tech?
Yours: the breaker, the thermostat, the restart wait, the fan speed, the filter, the louvers, the shades and awning, the voltage, and the outside coil if you’re comfortable on the roof. That covers most of the reasons an RV AC stops keeping up, and none of it costs more than a filter and a meter.
Call a tech when:
- The fan runs but the compressor never starts, even after the restart delay and with voltage in range
- The coil ices up again with a clean filter and every vent open
- The thermometer split is well short of 16 degrees with a clean filter and a clear coil
- The thermostat shows a sensor or control code
- The unit is under warranty. Dometic’s troubleshooting pages send owners to an authorised service agent, so ask the tech whether they’re authorized for your brand before they open it up
Your next step, in the next hour: pull the filter and look at it. Then plug a voltage meter into an outlet with the AC and your normal loads running, and take the thermometer reading at the return grille and a supply vent. Write down all three numbers. If the AC still isn’t keeping up, those numbers, plus the brand and model off the data plate and any code on the thermostat, are exactly what a tech needs in the first minute of the call. It shortens the diagnostic and it changes the quote.
The techs listed below say air conditioning is among the systems they work on.