Lennox Furnace Slow heartbeat error code
The LED pulses slowly — this is the normal 'heartbeat' showing the board is powered and idle with no call for heat.
Also shown as: slow pulse, slow blinking, heartbeat, normal idle
Last updated: September 30, 2026
What causes the Slow heartbeat error?
- Not applicable — this is the normal standby indication
How to fix Lennox Slow heartbeat — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- No action needed
If the house is warm and the thermostat is satisfied, a slow heartbeat is exactly what you want to see. - If the house is cold
Check that the thermostat is set to Heat, the setpoint is above room temperature, and its batteries are fresh.
When to call a technician
- The house is cold, the thermostat is calling, but the heartbeat never speeds up
Frequently asked questions
How do I tell slow from fast heartbeat?
Slow is a relaxed pulse about once per second; fast is a rapid flicker during an active heating call.
Does the heartbeat ever stop?
It should pulse continuously while powered. If it goes dark, see the LED-off entry.
Sources
Every meaning and fix above was checked against official manufacturer documentation:
FixCode provides general troubleshooting information only — not professional repair advice. If you are unsure about any step, stop and call a qualified technician. Repairs done incorrectly can be dangerous and may void your warranty.