Compressor Not Starting: Diagnostic Flowchart

For HVAC technicians, a compressor that fails to start is a common yet critical issue requiring a systematic diagnostic approach. This comprehensive guide provides a detailed flowchart and troubleshooting steps to accurately identify the root cause of a non-starting compressor, ensuring efficient and effective repairs. By following these procedures, technicians can minimize misdiagnoses, reduce callbacks, and extend the lifespan of HVAC systems.

General Safety First

Before beginning any diagnostic work, always prioritize safety. Failure to follow proper safety procedures can result in serious injury or death.

• Lockout/Tagout (LOTO): Always de-energize and lock out all electrical power sources to the equipment before servicing when you don’t have full control of the point of disconnect. Verify with your multimeter that the circuit is dead before proceeding.
• Personal Protective Equipment (PPE): Always wear appropriate PPE, including safety glasses and gloves. When handling refrigerants, ensure you are using refrigerant-rated gloves.
• Refrigerant Handling: Be aware of the dangers of refrigerant, including frostbite and asphyxiation. Always work in a well-ventilated area and follow all regulations for the safe handling, recovery, and storage of refrigerants.

Confirming Electrical Failure

This section will guide you through a systematic electrical diagnosis to verify the health of the compressor and its related components.

1. Performing an Initial Assessment at the Panel

Your first check starts where the power does. This assessment helps you understand the initial state of the system.

Has a breaker tripped, or has a fuse blown?

• YES: This can indicate a short or a component drawing excessively high current. In some cases, it can even be a damaged breaker. Proceed to Step 2: The Short-to-Ground Test.
• NO: The issue is likely not a direct short to ground. Proceed to Step 3: Checking for Thermal Overload.

2. Performing the Short-to-Ground Test

If the breaker tripped, you must find the short.

1. SHUT OFF ALL POWER at the disconnect and verify that the circuit has no power.
2. Disconnect the compressor wires.
3. Set your multimeter to measure Ohms (Ω).
4. Check the compressor terminals for a short to ground. Measure the resistance from each compressor terminal (Common, Start, Run) to a clean ground point on the compressor chassis or copper pipe.

Any continuity or low resistance reading to ground?

The compressor is shorted to ground and has failed. You have confirmed the diagnosis. Per Copeland, some compressors could read as low as 0.5 MΩ, depending on the compressor and meter combination.

No reading to ground (OL) or a high mega ohm reading?

The compressor windings are not shorted to ground. The term OL stands for “open line” or “over limit,” indicating infinite resistance. The short likely exists elsewhere. Check other components:

• Condenser fan motor
• Crankcase heater wiring
• Any other wiring in the high-voltage circuit

Do not use a megohmmeter (megger) to condemn a modern scroll compressor. According to Copeland's AE-1294 bulletin, a scroll can read as low as 0.5 MΩ and still be perfectly fine. Factors like refrigerant in the oil and winding temperature, along with moisture or acids, can give false “bad” readings on a megger. Many good compressors have been condemned due to improper tools or tool usage. A meter that says “bad” does not mean that the compressor is bad.

A standard ohmmeter test to ground is the correct method.

As a final step to rule out other potential causes for a blown fuse or tripping breaker, you must isolate the compressor. Disconnect its wiring completely, then re-energize the unit. If the rest of the A/C system powers up and operates normally, you have definitively confirmed that the electrical short is internal to the compressor and not elsewhere in the system's wiring or components.

3. Compressor Won't Start: Checking for Thermal Overload

If the breaker isn’t tripped but the compressor won’t start, one of the most common scenarios is that the compressor’s internal thermal overload protector has tripped. This safety device shuts the compressor off when it gets too hot to prevent motor damage. You must identify and address this before proceeding to more invasive electrical tests.

How to Identify Thermal Overload:

• Feel the compressor: Carefully touch the top of the compressor. If it is extremely hot to the touch, it has likely tripped on thermal overload.
• Listen for a hum and click: When the thermostat calls for cooling, you might hear the compressor hum for a few seconds and then a distinct “click.” This is the sound of the motor trying to start and the overload protector tripping.

Cool-Down Procedures:

The internal overload will not reset until the compressor cools down sufficiently. This can take a significant amount of time naturally. Even if the compressor shell cools down, the heavy internal components retain a lot of heat, so it's possible for the internal thermal overload to remain tripped. To speed up the process:

1. SHUT OFF ALL POWER at the disconnect.
2. Use water: Gently spray cool water over the top and sides of the compressor shell. Use extreme caution to avoid spraying any electrical components, terminal connections, or the fan motor. Be aware that you are working with electricity in the presence of water, and if it's an RTU (Rooftop Unit), be cautious of water getting inside the building.
3. Use air: Place a fan to blow air directly onto the compressor to help dissipate heat.

What to Do After Cooling:

Once the compressor is cool to the touch, you can proceed.

• If the compressor starts and runs: The overload was likely caused by an underlying system problem, not a failed compressor. You must now investigate why it overheated. Common causes include:
  • Dirty condenser coil
  • Failed condenser fan motor or capacitor
  • Refrigerant charge issue (over- or undercharged)
  • Low starting voltage
• If the compressor still won't start (hums and clicks again): The issue is more severe. It could be a failed run capacitor, a wiring issue, or an internally seized compressor. Now is the time to proceed with Step 4: Verifying Voltage & Components and Step 5: The Winding Resistance Test.

4. Verifying Voltage & Components

If the breaker was not tripped and the compressor is not coming on, we need to confirm that the compressor is receiving the correct power and that its supporting components are functional.

1. SHUT OFF ALL POWER and verify that the circuit has no power.
2. Inspect the “Big 3” electrical components:
   • Capacitor: Test the run capacitor. Is it within its rated MFD tolerance (+/- 5-6%)? A weak or failed capacitor will prevent the compressor from starting.
   • Contactor: Are the points pitted, burnt, or welded? Manually press the contactor; does it move freely? Ants or even a small beetle can prevent the points from making contact. Check the terminals for discoloration, a sign of overheating.
   • Wiring & Plug: Carefully inspect the wiring and confirm that it matches the manufacturer's diagram perfectly. There can be instances where someone before you may have miswired the equipment. Look for burnt or loose connections, especially at the compressor plug and contactor lugs but also at the disconnect. A bad connection is a common point of failure.

WARNING: COMPRESSOR TERMINAL INSPECTION BLOWOUT HAZARD: Exercise extreme caution when inspecting compressor terminals. The Fusite™ holding these terminals can become brittle and rupture under high compressor pressure, leading to a blowout. SAFETY PRECAUTIONS: Adhere to all safety protocols. Keep your face clear of any potential projectiles during inspection.

Inspect the wiring thoroughly. Look for nicks, cuts, or rubouts in the protective outer lining, especially where the wire enters the cabinet (knockouts can be rough). Also, check for contact points with refrigerant tubing; system vibrations can cause holes in the wire lining, exposing copper. Discoloration or stiff spots in the wire may indicate overheating.

A loose connection in the main electrical panel can sometimes cause problems. Depending on local regulations, working in the main panel might require an electrician's license, so always be aware of all applicable local codes.

If the components above are good, proceed to Step 5: The Winding Resistance Test.

5. Performing a Winding Resistance Test

This test checks for shorts or open circuits within the compressor's motor windings.

1. Ensure power is off and the compressor wires are disconnected.
2. Set your multimeter to measure ohms (Ω) on manual range.
3. Measure and record the resistance between the three terminals:
   • Common to Start = _____ Ω
   • Common to Run = _____ Ω
   • Run to Start = _____ Ω
4. Analyze the readings:
   • Check the formula: (Common to Start) + (Common to Run) = (Run to Start). Do your readings add up? Note: this does not apply to three-phase or inverter compressors.
   • If YES: The windings are likely okay, but compare with the manufacturer's specifications to be sure.
   • If NO: You likely have a turn-to-turn short inside the compressor. The compressor has failed.
   • Check for an open winding: Do you get an OL (Over Limit) reading between any two terminals? This means the winding is broken internally. The compressor has failed.

Best Practice: Don’t just rely on the formula. Use the Copeland Mobile app or manufacturer data to look up the exact winding resistance specifications for the specific compressor model you are working on. This is the most accurate way to verify your readings.

Investigating the Cause of Failure (“The Why”)

Now that you’ve confirmed the compressor is bad, you must find out why it failed. Installing a new compressor into a faulty system will only lead to another failure.

6. Check for Signs of Overheating

Overheating is a slow death for a compressor, often caused by a low refrigerant charge, which leads to poor cooling and high head pressure.

• Visual Inspection: Look for discolored or burnt paint on the compressor shell, especially near the top. This is a tell-tale sign of excessive operating temperatures.
• System Check: Review the system’s charge and operating conditions. Was the system running with a low charge? Was the condenser coil dirty or the fan failing, causing high head pressure?

7. Check for Liquid Refrigerant Damage

Liquid refrigerant will destroy a compressor, which is designed to compress vapor only. This happens in two main ways: floodback and flooded starts.

• Check for Floodback (Liquid in a Running Compressor): This is often caused by low or zero superheat. Investigate the metering device (TXV/piston) and evaporator airflow. Is the TXV bulb mounted correctly and insulated? Is the evaporator coil or filter clean?
• Check for Flooded Starts (Liquid in an Off-Cycle Compressor): This happens when liquid refrigerant settles in the crankcase during shutdown and violently boils at startup, washing oil away from critical parts.
• Inspect the Crankcase Heater: Is it working? Is it wired correctly? A common mistake is miswiring a heater after replacing a single-pole contactor with a two-pole, rendering the heater useless.
• Evaluate System Design: Are there long refrigerant lines without proper traps? Is the system overcharged? These factors increase the risk of flooded starts.

8. Check for Loss of Lubrication

Oil is the lifeblood of the compressor. Insufficient oil return leads to friction, overheating, and mechanical failure.

• Inspect Piping Practices: Are the line sets properly sized? Improper piping is a primary cause of poor oil return.
• Connect to Other Failures: Remember that flooded starts (Step 6) also cause oil loss as the foaming refrigerant carries oil out of the compressor.

9. Check for System Contamination

Contaminants act like poison in a refrigerant circuit, leading to chemical reactions and physical damage.

• Perform an Oil and Acid Test: When you remove the compressor, take an oil sample. Is it dark or sludgy, or does it have a burnt smell? Use an acid test kit. If acid is present, a thorough system flush and the installation of a temporary suction line filter-drier are mandatory to protect the new compressor.
• Identify the Contaminant:
  • Moisture: The most common contaminant, leading to acid formation and copper plating.
  • Non-Condensables (Air, Nitrogen): Increase head pressure and system temperatures.
  • Solid Debris: Debris from a previous failure or installation can cause blockages and wear. Check the old filter-drier and screen at the metering device.

Troubleshooting Flowchart/Checklist

The following table summarizes the diagnostic steps for a compressor not starting:

Step	Action	Possible Outcome / Indication	Next Action
1	Initial Assessment at Panel	Breaker tripped/Fuse blown?	YES: Proceed to Short-to-Ground Test NO: Proceed to Thermal Overload Check
2	Short-to-Ground Test	Continuity/Low Resistance to Ground?	YES: Compressor shorted to ground (Failed) NO: Check other high-voltage components
3	Thermal Overload Check	Compressor hot, hums & clicks?	YES: Cool down compressor, then retest. If still fails, proceed to Verify Voltage & Components NO: Proceed to Verify Voltage & Components
4	Verify Voltage & Components	Capacitor, Contactor, Wiring OK?	YES: Proceed to Winding Resistance Test NO: Replace faulty component
5	Winding Resistance Test	Readings match formula/specs, no open windings?	YES: Windings likely OK (Investigate root cause) NO: Compressor winding failure (Failed)
6	Check for Overheating	Discolored paint, high head pressure?	Address refrigerant charge, dirty coils, fan issues
7	Check for Liquid Refrigerant Damage	Signs of floodback or flooded starts?	Inspect metering device, airflow, crankcase heater, system design
8	Check for Loss of Lubrication	Improper piping, oil loss?	Correct piping, address causes of oil loss
9	Check for System Contamination	Dark/sludgy oil, acid present, moisture, non-condensables, debris?	Perform oil/acid test, system flush, install filter-drier

Frequently Asked Questions (FAQ)

What are the initial steps to diagnose a compressor that is not starting?

Begin by checking the thermostat settings and power supply. Inspect the air filter for dirt buildup and examine the condenser unit for any obvious issues. Verify if a breaker has tripped or a fuse has blown at the electrical panel.

How can I check for an electrical short to ground in the compressor?

First, shut off all power to the unit. Disconnect the compressor wires and use a multimeter set to ohms (manual range) to measure resistance from each compressor terminal (Common, Start, Run) to a clean ground point on the compressor chassis. Any continuity or low resistance indicates a short to ground.

What does it mean if the compressor hums and clicks but doesn't start?

A hum and click often indicate that the compressor's internal thermal overload protector has tripped due to overheating. The compressor attempts to start, but the safety device shuts it down. Allow the compressor to cool down and investigate potential causes of overheating like dirty coils or fan motor issues.

Why is it important to identify the root cause of compressor failure?

Identifying the root cause is crucial because installing a new compressor without addressing the underlying problem will likely lead to another premature failure. Common root causes include overheating, liquid refrigerant damage, loss of lubrication, and system contamination.

What are the 'Big 3' electrical components to check if a compressor isn't starting?

The 'Big 3' electrical components are the run capacitor, the contactor, and the wiring/plug. Test the capacitor for correct MFD tolerance, inspect the contactor for pitted or burnt points, and check all wiring for burnt, loose, or damaged connections.

References

[1] HVACR School. (2025, September 16). Compressor Failure: Diagnosis and Replacement Checklist for Residential and Light Commercial Equipment. http://www.hvacrschool.com/compressor-failure-diagnosis-and-replacement-checklist-for-residential-and-light-commercial-equipment/

[2] ACprosite. (2025, November 7). Compressor Failure Diagnostic Checklist. https://acprosite.com/compressor-failure-diagnostic-checklist/