The RV AC unit is running — you can hear the compressor cycling, you feel air moving — but the coach isn't getting cold. This is one of the most common complaints in the SMART-RV case database. It's also one of the most frequently misdiagnosed, with owners (and sometimes untrained techs) jumping straight to "low refrigerant" when the actual problem is almost always something cheaper and simpler.
Here's the order that field-experienced technicians use when diagnosing this problem.
Step 1: Check air flow before anything else
The most common cause of an RV AC that won't cool is restricted airflow — either over the evaporator coil (inside) or over the condenser coil (on the roof). Both scenarios produce the same symptom: the unit runs, the compressor cycles, but the air coming through the vents isn't cold.
Start by pulling the interior shroud off and looking at the air filter. RV AC filters are notorious for being neglected. A clogged filter restricts evaporator airflow, causes the coil to ice over, and blocks heat exchange. If the filter is gray and compacted, clean or replace it and try again before doing anything else.
Next, get on the roof and look at the condenser coil fins. Cottonwood, insects, and road debris pack into these fins over time. Use a garden hose to rinse the coil from the inside out (not outside in — that drives debris deeper). The fins should be visibly open.
Step 2: Measure supply air temperature
Put a thermometer in the supply duct — or hold an infrared thermometer at the vent — and measure the air temperature after the unit has run for 10 minutes. On a functioning unit, supply air should be 16–22°F cooler than the return air temperature (this is called delta T, or temperature differential). A delta T below 14°F indicates a problem. A delta T near zero means the refrigerant circuit is not functioning at all.
This measurement tells you whether you have a partial cooling problem (restricted airflow, low charge, compressor starting to fail) or a complete no-cooling problem (compressor not running, refrigerant loss, failed capacitor).
Step 3: Check voltage at the unit
RV AC compressors are sensitive to voltage. Most require 103–130V to operate correctly. Below 103V, the compressor will struggle to start, run hot, and eventually trip its internal thermal overload — which looks exactly like an AC that "runs but doesn't cool" because the fan is still running while the compressor has shut itself off.
Measure voltage at the AC unit, not just at your shore power pedestal. Voltage drop through a worn 30-amp cord, a damaged twist-lock connection, or a long extension cord is common and significant. If you're seeing below 108V at the unit under load, that's your problem.
Step 4: Listen for the compressor
With the unit running, listen carefully. Can you hear the compressor — a distinct lower-frequency hum layered under the fan noise — or only the fan? If you hear only the fan, the compressor isn't running. Common causes: failed run capacitor (very common), failed compressor contactor, or the compressor itself has tripped its internal thermal overload and needs time to cool before it will attempt to restart.
A failed run capacitor is the single most common reason a compressor stops running while the fan continues. It's a $15–40 part. On Dometic and Coleman units it's accessible from the rooftop unit without removing the shroud on most models. A tech with a capacitor tester can confirm this in 5 minutes.
Step 5: Refrigerant — last, not first
Refrigerant loss in a sealed AC system means there's a leak. RV AC systems don't consume refrigerant — if the charge is low, something caused it to escape. A tech will need to find and repair the leak before recharging, or the problem recurs. This is the correct last step, not the first.
Signs that point toward a refrigerant issue: the compressor runs continuously without cycling, delta T is low but not zero, and the evaporator coil ices over only on one side or unevenly. These patterns in combination suggest a low charge. Confirming requires gauges.
Know when to escalate
If the filter is clean, airflow is good, voltage is adequate, and the compressor is running — but delta T is still low — you're beyond homeowner diagnostics. At that point you need a tech with manifold gauges, a capacitor tester, and experience with your specific unit's service history.
Search the SMART-RV database by your AC brand and model (Dometic, Coleman, Advent, Furrion) plus your symptom. Cases in the database include root cause, parts used, and labor time — so you can arrive at the conversation with a technician already knowing what's likely.