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    hvac troubleshooting

    Furnace no heat

    Common symptoms: no heat; cool air heat mode; furnace not heating; heat not working

    Stop and call a pro if:

    • gas smell
    • smoke
    • CO alarm
    • sparks

    Step-by-step diagnostic flow

    1. Step 1

      Any gas smell, smoke, CO alarm, or electrical hazard?

    2. Step 2

      Is thermostat set to Heat with a setpoint above room temperature?

    3. Step 3

      Are nearby electrical devices powered normally and other gas appliances (if any) operating?

    4. Step 4

      Through any view window (if present), do you see a status light blinking or a code on the furnace?

      Do not remove panels.

    Possible outcomes

    Stop and seek professional help immediately

    high confidence

    Safety hazard detected.

    Thermostat settings may be the cause

    medium confidence

    Incorrect mode or setpoint.

    Safe next steps
    • Document mode, setpoint, schedule/hold
    What to document for a pro
    • Thermostat brand/model
    • Photos of settings

    Possible electrical power issue

    low confidence

    Nearby devices without power.

    Safe next steps
    • Document which rooms/devices lack power
    What to document for a pro
    • Breaker panel labels
    • Time of power loss

    Possible gas supply issue

    low confidence

    Other gas appliances not operating.

    Safe next steps
    • Note which gas appliances are affected
    • Do not attempt to relight equipment
    What to document for a pro
    • Gas appliance list
    • Time of issue onset

    Ignition lockout or fault indicated

    medium confidence

    Blinking/status code visible.

    Safe next steps
    • Photograph the label showing blink code legend
    • Record blink count/pattern
    What to document for a pro
    • Furnace model
    • Blink code pattern
    • Photos of status indicators

    Further professional diagnosis recommended

    low confidence

    Non-invasive checks inconclusive.

    Safe next steps
    • Note sounds and sequence (blower, click, flame attempt)
    What to document for a pro
    • Furnace model
    • Recent maintenance history
    Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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