Diagnose an RV Air Conditioner That Won't Cool

1 Power & Thermostat Verification

Start here — a surprising number of "dead" AC units turn out to be a power or thermostat issue rather than a mechanical failure.

• Verify Campground Voltage: Plug a digital voltmeter into an outlet inside the RV. If the voltage reads below 108V AC under load, the compressor will overheat and trip its internal thermal overload switch.
• Inspect the Breaker Panel: Locate the 120V breaker labeled "A/C." Turn it completely off, then flip it back on — a breaker can be tripped even if it doesn't look like it.
• Test Thermostat 12V DC Power: Remove the thermostat faceplate. Using a multimeter set to DC volts, test between the red wire (12V+) and black wire (ground) — you should see 12V to 14V DC. If you see 0V, look for a blown 12V fuse in your main RV fuse panel.
• Jumper the Thermostat: To rule out a broken thermostat, bypass it — use a small jumper wire to connect the red wire (12V power) directly to the yellow wire (compressor) and green wire (high fan). If the AC kicks on, the thermostat is broken and needs replacement.

2 Airflow & Freezing Diagnostics

Low airflow drops the evaporator coil's temperature below 32°F, turning condensed humidity into ice that blocks airflow completely.

• Inspect and Wash Filters: Remove the return air grille from the ceiling inside the RV, pull out the foam filters, wash them with warm soapy water, squeeze them completely dry, and reinstall.
• Check the Evaporator Coil: Look up into the return air plenum with a flashlight. If you see a solid block of ice, turn the thermostat to "Fan Only – High" and leave it running for 2 to 3 hours to melt the ice before doing anything else.
• Inspect the Freeze Sensor: Locate the small plastic probe clipped into the aluminum fins of the evaporator coil — it should sit about 1/3 of the way up from the bottom. If it fell out and is hanging in the air, the unit will freeze up because it can't detect when the coil is getting too cold.
• Fix the Plenum Divider: RV manufacturers use a thin piece of foam or cardboard to separate cold air dropping down from warm air being sucked in. If this divider falls, cold air gets sucked straight back into the intake and causes instant freezing — use foil HVAC tape to permanently reseal it.

3 Rooftop Component Testing

If the unit has power and good airflow but still won't cool, it's time to go up on the roof — remove the plastic shroud screws to access the unit.

• Look for signs of wasp or mud dauber nests blocking the fan blades. Check the aluminum condenser fins at the back of the unit — if they're packed with dirt or cottonwood seeds, spray them clean with a garden hose (never a pressure washer, which will bend the fins).

• Locate the metal control box on the roof unit and open the cover — you'll see one or two cylinder-shaped capacitors. If the top of one is bulging, domed, or leaking fluid, it's dead.

1. Turn off all RV power.
2. Slide an insulated screwdriver across the capacitor terminals to discharge any stored electrical energy.
3. Take a picture of the wires, then disconnect them.
4. Set your multimeter to Microfarads (µF).
5. Place your meter leads on the terminals (e.g., HERM and COM for the compressor side).
6. Compare the reading to the label on the capacitor (e.g., 55+5 µF ±5%). If the reading is more than 5% below rating, replace it.

1. Remove the small plastic terminal cover on the side of the compressor.
2. Pull off the three wires labeled C (Common), S (Start), and R (Run).
3. Set your multimeter to Ohms (resistance).
4. Measure resistance between the pins: C-to-S plus C-to-R should roughly equal S-to-R. An open circuit reading (OL) means the internal winding has melted.
5. Test from each individual pin to the copper ground tubing. Any resistance reading at all means the compressor is shorted to ground and must be replaced.

4 Refrigerant & Sealed System Check

Most RV air conditioners are completely sealed at the factory with no service ports to check refrigerant pressure directly — these two tests infer it indirectly.

• Run the AC on high cool for 15 minutes. Hold a digital thermometer directly against the return air intake grille and record the temperature, then hold it directly inside the closest cold air discharge vent. Supply air should read 16°F to 22°F cooler than return air — a drop of less than 15°F indicates either low airflow or low refrigerant.

• Clip an amp clamp meter around the main black power wire feeding the AC inside your breaker box. Check the data tag on your rooftop AC shroud for the RLA (Rated Load Amps) — usually around 11 to 13 amps.
• If the fan and compressor are both running but the unit blows warm air and only draws 2 to 4 amps, the compressor is spinning with essentially zero load because the refrigerant has completely leaked out.
• Because these are sealed systems, fixing a leak requires a certified HVAC technician to weld in piercing valves, vacuum the system, patch the leak, and recharge it. In most cases, replacing the entire rooftop unit is more cost-effective than a refrigerant repair.