A/C Compressor Doesn’t Turn On: 8 Common Reasons

It can be alarming when you switch on the air conditioner, and the a/c compressor doesn’t turn on. You wonder what could cause the problem, how much it will cost to repair or replace the compressor, and if the entire AC unit needs to be replaced. Unfortunately, repairing or replacing the compressor or the whole unit are possibilities. Fortunately, more opportunities exist. A technician may be able to perform maintenance or a less expensive repair on the system to get it running again. You may even be able to troubleshoot the problem yourself by learning what a compressor does, issues that can prevent a compressor from turning on, and how to remedy the problems.

The compressor is often referred to as the “heart” of the air conditioner. It is a vital component in the outdoor compressor unit, and the AC system can’t function without it. The HVAC compressor circulates the refrigerant, which cools the air in the house, through the AC system.

The refrigerant entering the house is in a liquid state. The refrigerant removes the heat and humidity from the air in the home at the evaporator coils, located in the indoor air handler unit. The refrigerant absorbs the heat and changes its state to a gas. The refrigerant flows out of the house to the compressor, which pressurizes and increases the temperature of the refrigerant. This process allows the condenser coils to remove the heat and humidity from the refrigerant. The refrigerant then changes back to a liquid state, and the cycle repeats.

There are a variety of issues that could lead to the AC compressor not turning on:

• Power supply issues
• Faulty wires
• Capacitor and starter relay problems
• Dirty coil and filter
• Circuit breaker tripping
• Refrigerant levels
• Suction line issues
• Dead compressor

Checking the power supply is the first thing to do when the compressor doesn’t turn on. The switch on the thermostat should be set to “cool,” and the temperature setting should be lower than the room temperature. Try turning the temperature down a few notches to get the system to kick on.

The system switch, located on or near the indoor air handler unit, must also be switched on. The air handler unit is typically located near the furnace in a basement or utility room. The system switch resembles a light switch.

If the fan in the outdoor unit is running, but the compressor is not, the issue is not with the power supply and likely resides within the unit containing the compressor.

Faulty wires inside the outdoor unit can keep a compressor from turning on. To check the wires, first, turn off the power supply to the unit. Carefully open the unit by removing the metal cage. Look for disconnected and burnt wires, but it is best to let an experienced technician handle electrical repairs. Performing electrical repairs yourself could lead to damaging the unit, harming yourself, or death.

The capacitor and starter relay are two essential parts of the AC unit that can cause the compressor to malfunction or prevent it from turning on. These two parts supply power to the compressor. Like a battery, the capacitor stores energy and provides an initial jolt of electricity to the unit’s motors to get them running. It continues to supply power to a lesser degree to the compressor and other parts throughout the cooling cycle. The starter relay transmits power from the capacitor to the compressor.

One sign of a bad capacitor or starter relay is the compressor not turning on or taking unusually long to begin the cooling cycle. Other signs include an abnormal humming noise while the unit is operating, smoke or a burning smell from the unit, warm air from the vents, and extraordinarily high electric bills.

Replacing the capacitor or starter relay are not jobs you can do yourself. If you suspect the capacitor or starter relay are bad, turn off the system and call a trained HVAC technician. Luckily, capacitors and starter relays are relatively inexpensive parts.

Dirt and debris on the condenser coils or the air filter reduce airflow and cause the compressor to overheat and shut down. The condenser coils release the warmth from inside the house. The coils are located in the outdoor unit, so they quickly become covered in dirt, bird droppings, and other filth, which affect their operation; debris like leaves, weeds, and vines around and inside the unit will block the outflow of warm air. Similarly, airflow is reduced if the indoor air filter is clogged with dust, pet hair, and other contaminants inside your house. When the air conditioning unit can’t expel the warm air from the home, it runs more often, and the compressor can overheat. Continual overheating of the compressor will cause it to break.

How often you change the air filter depends on the quality and number of contaminants within the house, but as a general rule of thumb, it should be replaced every 45-90 days. The area around the outdoor unit should be cleared of tall grass, weeds, vines, and branches. There are a few ways to clean the condenser coils. Always turn off the power to the unit first. The metal fins around the unit can be vacuumed with a shop vac. Bent fins can be straightened with a fin comb or a butter knife; do not insert the knife into the unit more than ½ inch. If you feel comfortable removing the metal cage surrounding the unit, the coils can be sprayed with condenser coil cleaner or a garden hose. Place the hose inside the unit, and spray the water through the fans toward the outside. Alternatively, call an HVAC technician to clean and service the AC unit.

A circuit breaker connected to the AC unit can be tripped if you operate too many appliances simultaneously. The breaker panel is typically located in a basement or utility room. Each switch on the panel should be labeled and designate what it controls. Flip the breaker switch that controls the AC from “off” to “on.” If the switch is hot, immediately call an HVAC technician. You should also contact a technician if the breaker is repeatedly tripped to avoid damage to the compressor if the system turns off and on.  

Refrigerant is a gas that circulates through your AC system and cools the warm air from your house. Too much or too little refrigerant affects the entire system; like Goldilocks from the children’s story, the amount of refrigerant must be just right. A technician may have filled the unit with the wrong or too much refrigerant, damaging the compressor. There could also be a crack or hole in a refrigerant line, causing the Freon to leak and the level to be too low. The system would have to work harder to cool the house, and the compressor could fail. Most newer AC systems contain a fail-safe switch that will turn the system off if it has too little or too much refrigerant.

Inspect the areas around the units for puddles, which could indicate a refrigerant leak. Leaks are potentially hazardous to your health and the environment. Puddles can also indicate a blocked, or broken drain pipe, a minor issue. However, drain pipe issues should be fixed or mold or rust could develop and cause more severe problems .

A professional should handle all refrigerants issues since only licensed technicians may take it.  

The suction line is a hose that pulls gaseous refrigerant from the house’s indoor unit to the outdoor compressor. There, the heat is released, and the refrigerant turns back into a liquid before returning to the home through the liquid line. The suction line is typically the larger of the two hoses and is insulated.

A clogged, bent, kinked, or otherwise damaged line will affect the unit’s ability to cool the house and can overheat the compressor. Occasionally, a technician installing or servicing a unit will use a line that is the wrong size. A line that is too large or small can damage the compressor. You can visually inspect the suction line outside your house for damage. Frost on the line is another indication of a problem with the system. Call an HVAC technician if you find or suspect an issue with the suction line.

A dead compressor is longer able to function. If the compressor dies, the entire air conditioning system goes down. As mentioned, the compressor is the “heart” of the system and needs it to function. If you’ve inspected your system for all other issues contributing to the compressor not turning on, your compressor is probably dead.

Regular air conditioner maintenance is the key to catching and fixing the problems that raise these red flags and preventing future AC compressor failures.

Call an HVAC technician if the compressor is not turning on after you’ve checked the power supply and circuit breaker, changed the air filter, and cleaned the coils. A technician will check for other issues that affect the compressor’s functionality such as faulty wires, capacitor and relay problems, refrigerant levels, and suction line issues. A professional HVAC technician must inspect, repair, and replace the compressor.