HVAC Troubleshooting Guide

• Confirm the thermostat is set to COOL and calling (check settings/batteries)

• Check the air filter and indoor airflow first — the single most common cause

• Outdoor unit silent or humming? Suspect a failed capacitor, pitted contactor, or locked compressor

• Measure superheat & subcooling to check charge, a restricted metering device, or a dirty condenser

• Watch the full sequence: inducer, igniter glow, gas valve, then flame proof

• A dirty flame sensor is the single most common no-heat call — clean it first

• Inspect the hot-surface igniter for cracks or an open circuit

• Verify gas supply and confirm the rollout and high-limit switches are closed

• Shut the system off and let the coil fully thaw before testing

• Replace the air filter and open every supply and return register

• Check the blower motor and wheel for dirt, slipping, or failure

• Low charge freezes coils too — verify superheat/subcooling and check for restricted ductwork

• High-limit trips point to low airflow: dirty filter, weak blower, or restricted ducts

• Confirm the temperature rise falls within the nameplate range

• Pressure-switch faults: check the flue, inducer motor, and condensate trap

• Blocked or disconnected ductwork raises static pressure and trips safeties

• Light frost is normal; a solid ice block means the defrost cycle isn't working

• Test the defrost board, sensor, and timer for proper cycling

• A stuck reversing valve leaves the unit locked in the wrong mode

• Verify the outdoor fan runs and clear any restricted airflow around the coil

• Clear the condensate drain line — algae and debris are the usual clog

• Check the drain pan and confirm the float switch is closing the circuit

• Flush the trap and verify proper slope on the drain line

• Inspect low-voltage wiring, the transformer, and connections for related faults