RV Heat Pump Turns On Then Immediately Shuts Off - 3 Common Causes & Fixes

The Quick Answer

When your rooftop heat pump comes on for just one second then shuts off, you're most likely dealing with a low refrigerant charge, a faulty pressure switch, or a bad start capacitor. This immediate shutdown is typically a safety feature protecting the compressor from damage. The most common culprit is low refrigerant, which causes the low-pressure safety switch to trip almost instantly after startup. Since heat pumps are sealed systems, refrigerant loss usually indicates a leak that needs professional attention.

What's Causing This

The one-second on/off cycle is a classic symptom of several specific issues. Low refrigerant is the leading cause - when refrigerant levels drop, the system can't build proper pressure, triggering the low-pressure safety switch to shut down the compressor immediately. This protects the expensive compressor from running without adequate lubrication.

A failing start capacitor is another common cause. The capacitor provides the initial electrical boost needed to start the compressor motor. When it's weak or failing, the compressor may attempt to start but immediately stall, causing the thermal overload (overload protector) to trip and shut the system down. You might hear a brief humming sound before it cuts off.

Faulty pressure switches can also cause this behavior. Both high and low-pressure switches monitor system pressures and will shut down the unit if readings are outside safe parameters. A stuck or malfunctioning switch might read incorrect pressures and shut the system down prematurely.

Less common but possible causes include a seized compressor, faulty control board, or wiring issues in the start circuit. Outdoor temperature sensors can also cause issues if they're reading incorrectly and telling the system to shut down for temperature protection.

Tools You'll Need

For basic diagnosis, you'll need a digital multimeter to test electrical components and voltage. A clamp-on ammeter is helpful for checking compressor current draw during the brief startup attempt. Have basic hand tools ready including screwdrivers, nut drivers, and pliers for accessing electrical connections.

You'll also want a flashlight or headlamp for working inside the unit, and a small mirror can help you see pressure switch labels and wiring in tight spaces. Safety equipment is essential - turn off the breaker before any electrical work and have non-conductive gloves available.

If you suspect capacitor issues, a capacitor tester or multimeter with capacitance function will help verify the capacitor's condition. Keep in mind that capacitors store electrical charge even when power is off, so you'll need an insulated screwdriver to safely discharge them.

Step-by-Step Fix

Start by turning off the thermostat and the main breaker to the heat pump. Remove the access panel on your rooftop unit to expose the electrical compartment. Look for any obvious signs of damage like burnt wires, corroded connections, or oil stains that might indicate refrigerant leaks.

Check the start capacitor first since it's the easiest component to test and replace. The capacitor is typically a round or oval metal canister with two or three terminals on top. Turn off all power and carefully discharge the capacitor using an insulated screwdriver across the terminals while keeping yourself isolated from the discharge path. Remove the wires (taking a photo first) and test the capacitor with your multimeter. The capacitor should read within 10% of its rated value (for example, a 35 microfarad capacitor should read within 10% of that value) - if it's significantly low or shows no reading, replace it with an identical part.

Next, examine the pressure switches, usually mounted on the refrigerant lines near the compressor. These small switches have electrical terminals and are connected to the refrigerant system. First test the switches with power off to check for proper continuity, then with power restored, use your multimeter to check if the switches are opening and closing properly during the startup attempt. A stuck-open low-pressure switch will immediately shut down the compressor.

If the capacitor and pressure switches check out, you'll need to observe the compressor during startup. Turn the system on and listen carefully - a healthy compressor should start smoothly, while a seized compressor may make grinding noises or draw excessive current before the overload trips. Use your clamp-on ammeter to measure the current draw during startup - excessive amperage indicates mechanical problems.

Check all electrical connections in the start circuit, including the contactor, overload protector, and wiring to the compressor. Loose or corroded connections can cause voltage drops that prevent proper startup. Clean any corrosion and tighten connections as needed.

When to Call a Pro

Call a certified HVAC technician immediately if you suspect low refrigerant, as this requires specialized equipment and EPA certification to handle properly. Refrigerant work involves recovering, evacuating, and recharging the system with precise amounts of refrigerant - not a DIY job. Additionally, finding and repairing refrigerant leaks requires professional leak detection equipment and brazing skills.

If your troubleshooting points to a seized compressor, this is definitely professional territory. Compressor replacement involves refrigerant recovery, cutting and brazing refrigerant lines, and system evacuation - typically costing $800-1500 depending on your heat pump model. Sometimes it's more economical to replace the entire unit.

Control board issues also warrant professional attention, as these electronic components require precise diagnosis and programming. A bad control board replacement can cost $300-600 plus labor, and incorrect installation can damage other system components.

Finally, if you're not comfortable working with electrical components or if your initial checks don't reveal the problem, don't hesitate to call for help. Heat pump systems operate at high voltages and pressures, and improper diagnosis can lead to expensive damage or safety hazards.