Here is a guide to checking electrical on your HVAC system!

⚠️ First: Safety

Before touching anything:

• Turn off power at the furnace/air handler switch and the breaker.

• Use a non-contact voltage tester to make sure nothing is live.

• Never touch exposed wires with bare hands.

1. Inspect Wiring & Connections (Visual Check)

Look for:

• Burn marks or melted insulation

• Loose or disconnected wires

• Corrosion on terminals

• Swollen or leaking capacitors
  If you find any of these, the part is likely bad.

2. Check Fuses

Most HVAC units have:

• Blade fuses (at the air handler/furnace)

• Disconnect fuses outside by the condenser

Use a multimeter on continuity mode:

• Remove the fuse

• Touch probes to both ends

• If it beeps → fuse is good

• If no beep → fuse is blown

3. Test the Capacitors (One of the Most Common Failures)

You’ll find:

• Start capacitor

• Run capacitor

To test:

1. Power OFF and discharge the capacitor (bridge terminals with a screwdriver with insulated handle).

2. Set multimeter to capacitance (µF).

3. Remove wires and touch probes to terminals.

4. Compare reading to the labeled value (within ±5–6%).
   

   • If reading is far off → it’s bad.

4. Check Contactors

The contactor sends power to the compressor/fan outside.

Steps:

1. Look for pitted, burnt, or stuck contacts.

2. Turn power ON and have thermostat call for cooling:
   

   • The contactor should pull in with an audible click.

3. With power OFF, use multimeter on ohms:
   

   • Coil terminals should show resistance (typically 10–40 ohms).

   • Infinite resistance = bad coil.

5. Test Relays & Transformers

Low-voltage transformer (24V)

• Set multimeter to AC volts.

• Probes on secondary side (usually R and C).

• Should read 24–28 volts.

• No reading? → bad transformer or open safety switch.

Relays

• Use continuity or voltage tests similar to contactors.

6. Check the Thermostat Wiring

• Remove the thermostat cover.

• Ensure wires are tight and not corroded.

• Use multimeter:
  

  • R to C → should show ~24V

  • R to Y (cooling call) → 24V when calling for cooling

  • R to W (heating call) → 24V when calling for heat

If you get 0V on a call circuit, something is open in that safety loop.

7. Check Safety Switches

Common switches:

• Float switch

• Rollout switch

• High-pressure switch

• Low-pressure switch

Most are normally closed.

Use multimeter on continuity:

• Beep = good

• No beep = tripped or failed

8. Check Motors

You can test:

• Continuity: motor windings shouldn’t read “open.”

• Amperage draw: use a clamp meter and compare to the rated FLA on the nameplate.

• Voltage supply: confirm the motor is getting full rated voltage.