# If the Roof Air Runs but Stops Cooling, Check These in Order > A rooftop unit can run all day and still not cool. Work airflow, then the temperature split, then supply voltage, before blaming the compressor. Source: https://ocrv.info/blog/hvac-roof-air-runs-but-stops-cooling Published 2026-07-23. Category: How-To Guides. About 6 minutes. Check airflow before anything else: return filter, ceiling assembly, divider gasket and evaporator coil. Then take a temperature split at the return and the supply. Then read supply voltage under load at the unit itself. Capacitors, thermistors and boards are the repairable faults. Sealed system leaks and seized compressors mean replacement. ## The short answer A rooftop unit that spins but will not pull the box down is failing in one of three places: airflow, supply voltage, or the sealed refrigerant system. Work them in that order. Airflow faults are the most common and the cheapest to correct. Voltage faults show up at startup and punish the compressor. Sealed system faults get diagnosed last, because they are the ones that usually end in a replacement unit rather than a repair. ## Separate not cooling from not cooling enough A unit that produces no cold at all and a unit that produces some cold but loses ground are two different problems, and treating them as one wastes an hour of bench time. No cooling at all means the compressor is not running, or is running without pumping. Stand under the shroud and listen. A fan that turns while the compressor never engages sounds nothing like a compressor that starts, labors, and drops out after twenty seconds on its internal overload. Not cooling enough means the compressor is pumping and the system is losing the load. That points at airflow, at heat gain through the shell, or at a duty cycle the unit was never sized to hold. Write down which one you have before you touch anything, along with the ambient reading and how long the unit had been running when the complaint started. That single note changes the test plan. It also changes the estimate, because a no-cool complaint often ends at a component while a weak-cool complaint often ends at a ceiling assembly. Our [rooftop air conditioner diagnostics](/services/plumbing-appliances-and-hvac/air-conditioner-service) begin with that split. ## Airflow first, always Four airflow faults account for most weak-cool complaints, and all four are visible without gauges. - The return air filter. A foam or mesh filter loaded with dust and pet hair chokes the evaporator and drops supply volume long before anyone notices the temperature. - The ceiling assembly. Registers partially closed, a damper left in the wrong position, or a shroud reinstalled with the wrong screw pattern all restrict the path. - The divider gasket. This is the one people miss. On a ducted setup, a foam gasket separates the supply plenum from the return opening. If it is crushed, torn, or missing after a roof reseal, chilled supply air loops straight back into the return. The unit cools its own return air, the split at the shroud looks reasonable, and no register in the coach ever gets cold. - The evaporator coil. Fins packed with lint act like a second filter. Comb them, clean them, recheck. Pull the interior shroud and look at that gasket before you condemn anything electrical. We have opened units where the entire complaint was a two-dollar strip of foam compressed flat by an over-torqued mounting bolt. ## The temperature split test and what it means Put a probe thermometer in the return grille and a second one in the closest supply register. Let the unit run at least ten minutes, then subtract. That difference is the split. On a healthy rooftop unit you are generally looking for roughly 16 to 20 degrees Fahrenheit at moderate humidity. Below about 12 degrees, something is wrong. Above about 25 degrees, airflow is restricted and the coil is running colder than it should, which is its own problem and will eventually ice the fins. Two things move the number, and both get misread. High humidity pushes the split down, because the coil spends its capacity condensing moisture instead of dropping air temperature. A high ambient reading at the condenser also pushes it down, because the compressor has less head room to reject heat. A unit reading a 13 degree split with soaked interior air is not necessarily faulty. The same unit reading 13 degrees with dry air and a cool condenser inlet is. Take the reading twice, ten minutes apart. A split that starts strong and decays points at a coil icing over or a condenser fan slowing under load. ## Voltage sag, hard starts and the compressor Measure voltage at the unit while the compressor is trying to start, not at the pedestal and not at rest. The number that matters is the one under load at the far end of the wire. A compressor drawing locked rotor current for a fraction of a second pulls a large slug of amperage. If the supply is a long cord, an undersized cord, a shared pedestal, or a corroded connection, voltage collapses during that instant. The compressor cannot break away, the internal overload opens on heat, and the unit sits there with the fan turning and nothing cooling. Repeat that a few hundred times and the windings pay for it. Sustained readings below roughly 105 volts under load deserve attention before anything gets replaced. Coaches feeding the roof air from an inverter add a layer, and we chase those through the [battery and charging system](/services/electrical-power-and-generators/battery-and-charging-systems) rather than the appliance. A hard start kit, meaning a start capacitor and a potential or PTC relay, shortens the startup window and cuts how long the compressor holds locked rotor current. It helps a marginal supply. It does not repair a shorted winding, it does not fix a crushed cord, and it adds no capacity. ## Capacitors, thermistors and control boards Three electrical parts account for most repairable no-cool calls. The run capacitor comes first. A dual can serves the fan and the compressor from one body, commonly something like 35 microfarads on the compressor leg and 5 on the fan leg. Discharge it, pull the leads, read it on a meter. More than about six percent off the stamped value and it gets replaced. A bulged top or a weeping base is a decision already made. A weak capacitor is exactly what makes a compressor hum, draw locked rotor, and trip. The thermistor is second. On board-controlled units a thermistor senses return air and tells the board when to cut out. If it drifts, cracks, or falls out of its clip in the return grille, the board reads a temperature that does not exist and shuts the compressor down early. Check resistance against the chart, and check that the sensor is physically where it belongs. The board is third, and it is third for a reason. Boards get blamed for capacitor and thermistor faults constantly. Look for burned relay contacts and heat-darkened traces first. Board pricing follows the posted [parts and materials policy](/prices/parts-and-materials). ## When the unit gets replaced instead of repaired Three findings end the repair conversation. A sealed system leak is the first. Rooftop units are brazed closed without service ports. Oily residue at a joint, a system that shows no pressure differential, and a compressor that runs without producing a split all point the same direction. There is no recharge on an assembly that was never built to be opened. A seized compressor is the second. It reads a short to ground or an open winding, or it draws locked rotor amperage with a known good capacitor and clean supply voltage. That is the end of the unit. The third is arithmetic. Diagnostics bill at $285 per hour and mechanical and electrical work at $260 per hour under our posted [labor rate schedule](/prices/labor-rates). Once a repair stacks diagnostic time, a board, a capacitor and a compressor against the delivered cost of a complete unit, replacement usually wins. Run that comparison before ordering parts. The [repair range estimator](/tools/repair-range-estimator) gives you a bracket to work from. Eastvale owners bring units to the Yorba Linda shop, roughly 20 miles out SR-71 to SR-91 and 25 to 35 minutes on a normal run. Everything happens in the bay. --- OCRV Center. 23281 La Palma Ave, Yorba Linda, CA 92887. (949) 799-3387. info@ocrvcenter.com. Serving Eastvale, California in Riverside County, about 20 miles from the shop. All work is performed in shop at the Yorba Linda facility. No mobile, roadside or on-site service is offered. Scope is body, paint, structural, fiberglass, interior and vehicle systems work. Engine rebuilds, transmission rebuilds, drivetrain work, DOT inspections and emission testing are not performed.