HVAC Troubleshooting Guide: Diagnose Common AC and Furnace Problems

The Diagnostic Mindset

Good troubleshooting is not about memorizing every possible failure. It is about following a logical process that eliminates possibilities. Before you open a panel or connect gauges, start with the basics:

1. Talk to the customer. When did it stop working? Did it happen gradually or suddenly? Any unusual noises or smells? Was any other work done recently? These questions save you time.
2. Check the thermostat. Verify it is set correctly, has power, and is calling for the right mode. You would be surprised how often this is the entire problem.
3. Check power. Verify the disconnect is on, breakers are not tripped, and the unit has voltage.
4. Check the filter. A clogged filter causes a shocking number of problems that look like equipment failure.

Common AC Problems and What to Check

AC Running but Not Cooling

This is the most common summer call. The system runs, the fan blows, but the air is not cold (or not cold enough). Work through this sequence:

• Check airflow first. Dirty filter, collapsed duct, closed dampers, or a failing blower motor can all cause poor cooling with the refrigerant system working fine. Measure supply and return temperatures. You should see a 16 to 22°F temperature split across the evaporator coil.
• Inspect the condenser. A dirty condenser coil raises head pressure and reduces capacity. Check for debris, cottonwood, or dirt buildup. Also verify the condenser fan is running and spinning in the correct direction.
• Connect gauges and check pressures. Low suction pressure with low subcooling typically indicates low charge. High head pressure with normal subcooling points to a condenser airflow issue. Both pressures low may indicate a restriction.
• Check superheat and subcooling. These tell you the real story. See our complete superheat and subcooling guide for normal ranges and what abnormal readings mean.

Frozen Evaporator Coil

Ice on the evaporator is always a symptom, not the root cause. The two most common causes:

• Airflow restriction. Dirty filter, dirty evaporator coil, failed blower motor, or collapsed return duct. When airflow drops, the coil temperature drops below freezing and moisture in the air freezes on the coil surface.
• Low refrigerant charge. A leak reduces system pressure, which lowers the evaporator saturation temperature below 32°F. The coil freezes even with adequate airflow.

In either case, turn the system to fan only and let the ice melt completely before taking any diagnostic readings. Pressures and temperatures measured on a frozen coil are meaningless.

Bad Run Capacitor

Capacitor failure is extremely common, especially in hot climates. Symptoms include:

• Compressor hums but does not start (single-phase systems)
• Compressor runs but draws high amps
• Condenser fan does not spin (if using a dual-run capacitor)
• System trips on overload or breaker

Test with a capacitance meter. A capacitor rated at 45 microfarads should read within 5 to 10% of that value. If it reads significantly low or open, replace it. Always replace with the same microfarad and voltage rating (or higher voltage, never lower).

Compressor Not Starting

If the outdoor unit has power but the compressor will not start:

1. Check voltage at the contactor. You should have 24V on the coil side (from the thermostat) and line voltage across the load side when energized.
2. Check the capacitor (see above).
3. Check for a locked rotor. Measure amps on the common wire. If the compressor is drawing locked rotor amps (LRA), it may be mechanically seized.
4. Check compressor windings with an ohmmeter. Measure resistance between C-R, C-S, and R-S terminals. Also check for a ground fault (winding to shell).

Common Furnace Problems and What to Check

Furnace Not Heating

When a gas furnace will not fire, work through the ignition sequence:

1. Check for a call for heat. Verify the thermostat is calling and you have 24V at the W terminal on the furnace control board.
2. Check the pressure switch. The inducer motor should start first. Once it proves draft, the pressure switch closes. If the inducer runs but the pressure switch does not close, check the drain trap, condensate line, and flue for blockages. Verify the pressure switch hose is connected and not cracked.
3. Check ignition. On an HSI (hot surface igniter) system, the igniter should glow orange/white. If it does not glow, check for voltage to the igniter (120V on most units). If it has voltage but does not glow, the igniter is cracked or failed. On spark ignition systems, verify you see/hear a spark.
4. Check the flame sensor. If the igniter fires but the burners light and immediately go out (within 3 to 7 seconds), the flame sensor is likely dirty or failing. Pull it out and clean it with fine emery cloth or steel wool. Measure flame sense current; most boards need 1 to 6 microamps.
5. Check the control board LED codes. Most furnace boards have a diagnostic LED that flashes error codes. The code chart is usually on a sticker inside the furnace panel. This is often the fastest path to the problem.

Furnace Short Cycling

Short cycling means the furnace starts, runs briefly, shuts off, and then repeats. Common causes:

• High limit tripping. The furnace overheats and the high limit switch opens. Usually caused by poor airflow: dirty filter, closed registers, failed blower motor, or undersized ductwork. The furnace protects itself by shutting down, cools off, then tries again.
• Flame rollout. The flame rollout switch trips when burner flames roll out of the combustion chamber. This indicates a heat exchanger or venting problem. Do not bypass rollout switches; diagnose the cause.
• Pressure switch issues. Intermittent draft problems can cause the pressure switch to open during operation, shutting down the burners.

Furnace Ignition Failure

If the furnace attempts ignition three times and fails (lockout), the board will typically stop trying and flash a lockout code. Most boards reset after one to three hours, or you can cycle power. But do not just reset it and walk away. Find out why ignition is failing:

• No gas: check the gas valve, manual shutoff, and verify gas pressure with a manometer
• Weak igniter: an HSI igniter that glows dimly (orange instead of bright white) is failing and will not reliably light gas
• Gas valve not opening: verify 24V to the gas valve when the board commands it open

Reading Pressure Gauges: Quick Interpretation

For quick field diagnostics on a standard R-410A cooling system (assuming 95°F outdoor ambient and correct airflow):

These are rough guidelines. Always combine pressure readings with superheat, subcooling, and temperature measurements for an accurate diagnosis.