Error Code Slow Green Flash
Info

York TM9V080B12MP11 Error Code Slow Green Flash: Normal Operation - No Thermostat Calls

TL;DR
Your furnace is powered on and in normal standby mode with no call for heat.
Disclaimer
This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code Slow Green Flash Mean?

A slow-flashing green LED on the York TM9V080B12MP11 control board indicates the furnace is operating normally in standby. The control board has run its self-diagnostic checks and found no faults. No heating demand is being received from the thermostat. The LED flashes at a rate of two seconds on and two seconds off.

If you expect the furnace to be heating but see this standby indication, check your thermostat. Confirm it is set to "Heat" mode and the target temperature exceeds the current room temperature. A programmable thermostat may have reduced the setpoint according to its schedule, and dead batteries can cause loss of thermostat settings.

This LED state is completely normal and requires no action or service.

Contact a licensed HVAC technician if any of these apply:

Sources

  1. Installation Manual - Residential Gas Furnace Models TM9V*MP (96% AFUE Two Stage Variable Speed ECM Multi-position)

✓ Verified against manufacturer service manual — March 2026