Why Is My RV AC Not Cooling?
The fan runs. The air coming out is warm, or barely cool. You're losing the weekend. We engineer repairs so the same failure doesn't come back — not parts-swap guesses that leave you hot again in a month.
First, Is It Actually Not Cooling?
A healthy RV rooftop AC does two things you can measure. It drops the inside cabin temperature about 15 to 20 degrees below the outside ambient temperature — that's the whole-RV result. And it pulls the air across the evaporator coil 16 to 22 degrees colder than the return air going in — that's the coil's delta-T, which tells you the unit itself is working.
Before you call, check both. For the whole-RV result, compare your cabin thermostat reading to the outside temperature. For the coil delta-T, hold a thermometer or your phone's temp sensor at the supply vent, then at the return air intake — the supply should be 16 to 22 degrees colder than the return. If your delta-T is under 16 degrees, the unit has a real problem or it's dehumidifying in high-humidity conditions (an air conditioner acts as a dehumidifier first — until the air dries out, it can't cool efficiently).
If your coil delta-T is in range but the RV still isn't cool enough, the unit is working and you're losing the fight against sun load, window area, or insulation. If your delta-T is low, keep reading — we'll walk you through the six real causes and how we diagnose each one.
The 6 Real Causes of an RV AC That Runs But Won't Cool
These are the failures we see every week on Dometic, Coleman-Mach, Furrion, Advent, and GE units. Not all six apply to every call — but the diagnostic process rules them in or out one at a time.
1. Failed Run Capacitor
The capacitor is what starts and sustains the fan or compressor. It's rated in microfarads (μF) — typically 5 to 108 μF depending on whether it's a fan capacitor or a compressor capacitor. As capacitors age they lose capacitance. When a capacitor can't deliver the startup torque, the fan or compressor won't run.
How we diagnose: Capacitance is tested with a capacitance-specific meter, not just a continuity check. This is a major cause of "AC runs but won't cool" calls and one of the most affordable fixes in RV service when caught before the compressor burns out trying to start against a dying capacitor.
2. Dirty Evaporator or Condenser Coils
The evaporator coil (inside) absorbs heat from the cabin air. The condenser coil (outside, on the rooftop unit) dumps that heat to outside air. Both coils need clean fins and clean surfaces to transfer heat. Dust, pollen, cottonwood fluff, and tree debris on the condenser — and dust and mold / algae buildup on the evaporator — limit heat transfer dramatically.
How we diagnose: Visual inspection of both coils with the shroud removed. Evaporator coil condition is checked from above the filter, because cleaning the filter is not the same as cleaning the coil.
3. Low Refrigerant Charge
Rooftop RV AC units are sealed, factory-charged systems — no service ports. If refrigerant has leaked out, it's because a tube or solder joint has failed. "Just adding freon" is not a repair path on these units because there is no port, and the leak itself is the underlying problem.
How we diagnose: Suction line temperature, current draw, cooling output, and icing pattern are the tell-tale signs. If refrigerant has escaped a sealed rooftop system, full unit replacement is the right answer — the cost of locating and repairing a small leak exceeds the cost of a new unit. We don't pretend otherwise.
4. Failing Compressor
Compressor failure can be sudden or gradual. Gradual failures show up as reduced cooling that gets worse over weeks. Sudden failures usually announce themselves with a breaker trip and a smell. Either way, the compressor is the most expensive single component — and on sealed RV rooftop units it's not a field-replaceable part.
How we diagnose: Winding resistance measurement, startup amp draw, and capacitor condition testing together confirm compressor status. When the compressor has failed, we present the replacement-vs-repair math honestly — on most rooftop units, full unit replacement is the correct call.
5. Airflow Restriction
The AC can be mechanically perfect and still fail to cool the RV if the air isn't moving. Common culprits: a clogged return air filter, a disconnected flex duct in the ceiling cavity, a crushed duct run, a missing damper, or a failed plenum gasket leaking conditioned air into the attic area instead of the ducts.
How we diagnose: Airflow volume at each vent is measured. If vents are uneven, the ceiling assembly comes apart and we inspect the full duct path — which is how we find the disconnected ducts and gasket failures that shops who just swap parts never catch.
6. Thermostat or Controls Failure
Modern RV AC systems use a thermostat that communicates with a control board in the rooftop unit. Failures on either end produce the same symptom: the compressor never gets the signal to run, so the fan blows and the air stays warm. Dometic, Coleman, and Furrion each use different communication protocols, so a thermostat from one brand generally won't drive another brand's unit without conversion.
How we diagnose: Signal is traced from the thermostat through the wiring to the control board. Control board outputs are verified against the call for cooling. This is where being brand-certified across Dometic, Coleman, and Furrion matters — the protocols aren't interchangeable.
Why We Don't Just Swap Parts
In every situation our service department studies and if need be engineers a better way to install or build the repair. We do this because we care about the outcome of the RV lifestyle of all RV owners — and we don't want you to have the same failure over and over.
A parts-swap shop pulls the capacitor, bolts on a new one, and sends you out the door. And you're back — because the original failure was never actually determined or fixed.
Our process is different. We measure and verify. Check the compressor startup amp draw, check the voltage, inspect the condenser coil, and test the thermostat signal. Then we tell you what failed, why it failed, and what it will cost to repair. We say so — because we'd rather see you camping than back with the same problems.
That's what RVTI Level 4 Master certification means. It's not the badge. It's the diagnostic discipline that comes with it.
RVTI Certified Technicians
Is Your AC Sized Right for Your RV?
Before assuming your AC is broken, it's worth checking whether it was ever strong enough for the job. A single 13,500 BTU rooftop unit cools a small travel trailer adequately in moderate climates. We find most RVs would benefit from a 15,000 BTU unit — a truly well-insulated RV is hard to come by. Any RV in direct Alabama sun with big south-facing windows really needs a second AC or a larger primary unit.
The difference between 13.5K and 15K is about 11 percent more cooling capacity. That's meaningful — but it's not magic. At 95°F outside ambient, it can be tough to hold your RV at 73–78°F interior. We have vent insulators for overhead crank vents and window screens to block some of the sun. If you're struggling to cool off, we're here to help.
When we do a cooling diagnostic, we factor BTU sizing, insulation, window area, and shade exposure into the conversation. Sometimes the right answer is a second rooftop unit, not a repair on the one you have.
Ducted System Blowing Warm Air From Some Vents?
Uneven cooling across ceiling vents is one of the most misdiagnosed problems in RV service. The symptom looks like an AC problem — but the AC is almost always fine. The failure is in the ductwork.
Ducts inside the ceiling cavity come loose at a collar from road vibration, or from poor original construction. The duct drops, and cold air pours into the ceiling insulation instead of into the cabin. Crushed ducts, displaced dampers, and failed plenum gaskets all produce the same result — cold air going somewhere other than the vents you're standing under. Not only does this cause a cooling problem, it will shorten the life of your air conditioner too.
We open the ceiling assembly to verify the full duct path. That's how we find and repair the disconnected duct, or the gasket that's blowing conditioned air into the ceiling cavity. Many other shops are incentivized not to fix the ducting. With the shortened life of the A/C, a shop that doesn't open the ceiling will often sell you a new AC for what is really a duct problem.
Coleman-Mach to Dometic Swaps (and Other Brand Conversions)
The 14-inch roof cutout is an industry standard, so the unit itself usually drops in. That's where the "easy swap" story ends. Coleman, Dometic, and Furrion each use different thermostat communication protocols and different control board wiring. A Coleman-to-Dometic swap almost always requires a new compatible thermostat and sometimes a new control harness run from the roof to the thermostat location.
Done right, a brand swap is a clean install that works every time and doesn't surprise you two weeks later with a control board error. Done wrong — or done as a parts-only job — it produces intermittent failures that are hard to diagnose after the fact.
For the full replacement discussion including costs, heat pumps, quiet-runs, and soft-start options, see our main RV AC Repair page.
RV AC Cooling Problems: Common Questions
A fan that runs while the air stays warm means the compressor isn't doing its job. The most common causes are a failed capacitor, dirty evaporator or condenser coils that can't transfer heat, a refrigerant charge that has leaked down, or a compressor that's failing internally. We test capacitance under load, check coil cleanliness, and verify compressor draw before recommending a repair path.
A healthy RV rooftop AC in good condition will pull the inside cabin temperature about 15 to 20 degrees cooler than the outside ambient temperature. On a 95-degree day, a well-running unit gets you to the mid-70s inside. If your unit is only dropping the temperature 10 degrees or less on a hot day, it may be an insulation or heat-load issue that an air conditioner can't fix on its own.
A single 15,000 BTU rooftop unit is generally adequate for a well-insulated 30-foot travel trailer in most climates. In the Southeast, large windows and parking in direct sun push that math. If your trailer runs hot even with a working AC, the unit isn't undersized — the heat load is too high. Shade, reflective awnings, and window coverings matter more than people think.
A tripping breaker usually means the unit is pulling more current than the circuit can supply. The common causes are low campground voltage forcing the compressor to draw excess amps, a failing compressor that's hard to start, a capacitor that lets the compressor labor on startup, or running too many loads on the same circuit. We test voltage at the pedestal, startup amp draw, and capacitor condition to find the real cause.
A 15,000 BTU unit produces roughly 11 percent more cooling capacity than a 13,500 BTU unit but draws more amps on startup and runtime. The 13.5K is lighter, quieter, and easier on generators and low-amp pedestals. The 15K is the right call for larger RVs, hot-climate use, or units that are the only AC on the coach. Physical footprint is usually identical so swap compatibility is mostly an electrical question, not a roof-cutout question.
Uneven cooling across vents almost always points to a ductwork problem, not the AC itself. The common causes are disconnected ducts inside the ceiling cavity, a crushed duct run, a missing or displaced duct damper, or the plenum gasket between the unit and the ceiling assembly leaking conditioned air into the cavity instead of the ducts. We open the ceiling assembly and inspect the duct run — which is how we find problems many shops miss.
Signs of compressor failure include the fan running without any temperature drop, the unit humming or clicking without starting, tripping the breaker on startup even with a good capacitor, or a burnt electrical smell. The definitive test is measuring compressor winding resistance and startup amp draw with proper equipment. If the compressor has failed on a rooftop RV unit, a full unit replacement is almost always the only repair.
Physical swap between rooftop brands is often possible because the 14-inch roof cutout is an industry standard. The wiring is where it gets complicated. Coleman, Dometic, and Furrion use different thermostat communication protocols, control board wiring, and in some cases different refrigerant lines on split systems. A Coleman-to-Dometic swap typically requires a new compatible thermostat and sometimes a new control harness. We handle the full conversion including the thermostat side, not just the rooftop unit.
Evaporator coils should be inspected at least once per camping season and cleaned when dust buildup is visible. Full-time RVers or RVs stored in dusty or pollen-heavy environments need cleaning more often. A dirty evaporator is one of the most common reasons we see for reduced cooling — and because the coil is above the filter, many owners never look at it. Cleaning the filter once a week during use is recommended by most manufacturers.
Yes. Low refrigerant drops the evaporator temperature below freezing, which causes ice buildup that blocks airflow and then melts into the pan. The size of the coils and the amount of airflow cause almost all RV AC units to freeze up at some point. Ducted systems use a thermistor on the controls — it shuts off the compressor for a few minutes to allow the coil to defrost. A thermistor is rare in non-ducted systems, though some units do include one. Rooftop RV AC units are sealed systems — if refrigerant has leaked out, it's because something failed, and "just adding freon" is not a real repair. The right answer on most sealed RV rooftop units is full unit replacement, because they were never intended to be repaired for a refrigerant leak.
Low fan speed moves less air across the evaporator coil, which means the coil stays colder longer. In humid conditions, that cold coil freezes condensation into ice, which blocks airflow and stops cooling entirely. The fix is running fan speed high any time humidity is elevated. If your AC only cools properly on high fan, that's a sign the unit is borderline — the margin for airflow loss has shrunk.
A soft-start module reduces the startup amp surge of an RV AC compressor by roughly 65 to 75 percent, which is the difference between being able to run on a 2000-watt generator or a smaller inverter and not. If you boondock, run off a portable generator, or use a solar and inverter setup, a soft-start is one of the best investments you can make. The newer inverter-style AC units are slow-start by design and can be run with very small generators. If you're always on 30- or 50-amp shore power, it's optional. We install soft-starts for two reasons: they extend compressor life, and they allow better off-grid use.
RV AC Diagnosis & Repair Near You
Our shop is in Guntersville, Alabama. RV owners drive to us from across North Alabama and beyond for AC diagnostics they can trust.
Also serving Union Grove, Morgan City, Blountsville, Langston, South Sauty, Lacey's Spring, New Hope, Owens Cross Roads, Hampton Cove, Madison, and Athens.
Stop Guessing. Let's Find What's Actually Wrong.
We'll diagnose the real cause, explain your options, and get your RV air conditioner cooling the way it should.