Furnace Troubleshooting Guide

Start with four safe checks

These four resolve a meaningful share of calls before anyone opens a toolbox. If heat returns, note what fixed it — a filter that clogs in weeks is telling you something worth following up on.

1. Thermostat: set to HEAT, fan on AUTO, setpoint at least five degrees above room temperature, fresh batteries.
2. Power: furnace service switch on, breaker not tripped, access panel fully closed against its door switch.
3. Filter: replace it if light will not pass through the media.
4. Airflow: open supply registers and clear obstructions from return grilles.

Match the symptom to the guide

• Nothing happens at all → furnace won't turn on
• Blower runs, air stays cold → furnace blowing cold air
• Runs but the house never warms → furnace not heating
• Starts and stops every few minutes → furnace short cycling
• Never shuts off → furnace keeps running
• Banging, squealing, grinding or rattling → furnace making noise
• Very little air from the registers → weak furnace airflow
• Tries to light and fails → furnace ignition problems
• Fan issues specifically → furnace blower problems
• Thermostat unresponsive or inaccurate → thermostat problems
• Burning, musty or electrical odors → furnace smells
• Costs climbing without a weather change → high heating bills

Read the furnace's own diagnostic code

Nearly every furnace built in the last twenty years stores fault codes. Look through the small window in the access panel for a blinking LED, count the pattern, and compare it to the legend printed inside the furnace door. Some newer models display a two-character code directly.

Writing that code down before you call is one of the single most useful things a homeowner can do. It points diagnosis at the right circuit immediately.

Understanding the sequence of operations

Knowing the order helps you describe the failure precisely: thermostat call, inducer starts, pressure switch proves draft, igniter heats, gas valve opens, burners light, flame sensor confirms flame, blower starts after a delay, limit switch monitors temperature throughout.

Telling a technician 'I hear the draft fan, then a click, then nothing' locates the fault far faster than 'it is broken.'

When to stop troubleshooting entirely

Gas odor, smoke, visible flame outside the burner area, a sounding carbon monoxide alarm, a grinding blower or a furnace that will not shut off all mean stop. Leave the home if the issue involves gas, smoke or CO, and call from a safe location. Furnace safety covers each scenario.

Deciding what to spend

Once you know roughly what is wrong, furnace repair cost explains what drives pricing, the cost calculator gives you a planning range, and repair vs. replacement helps if the furnace is older.