Symptoms
- Small red or green LED visible through a viewing window blinks a repeating pattern
- Furnace may be running, short cycling, or completely off
- No display screen, just a blinking light
Likely causes
- CommonIgnition failure codeA common code indicates the burners failed to light within the allowed attempts.
- CommonPressure switch fault codeOften means a blocked exhaust vent, condensate blockage, or a failing pressure switch.
- PossibleLimit switch or high-temperature codeUsually tied to airflow restriction like a dirty filter.
- PossibleFlame sensor fault codePoints to a dirty or failing flame sensor cutting ignition short.
- Less commonControl board fault codeIndicates an internal board problem rather than a simple mechanical cause.
What to do
- Count the number of blinks in the repeating pattern, noting any pause between groups.
- Open the furnace door panel; a code chart is often printed on a sticker inside.
- Look up the exact blink count in the manual or on the manufacturer's website if not printed inside.
- For airflow-related codes, check and replace the filter and make sure vents are open.
- For exhaust or condensate codes, check that the outdoor exhaust pipe isn't blocked by snow or debris.
- If the code points to ignition, flame sensor, or control board issues, this needs a technician rather than DIY repair.
Tools and parts
- Flashlight
When to call a pro
Call an HVAC technician with the exact blink code; most codes beyond airflow issues (ignition, flame sensor, control board) need professional repair.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.