Error Code Slow Green Flash
Info

York TM9T100C16MP11 Error Code Slow Green Flash: Normal Operation - No Thermostat Calls

TL;DR
Your furnace is in normal standby mode with no heating demand from the thermostat.
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 TM9T100C16MP11 control board means the furnace is powered on and operating normally in standby. The control board has completed its diagnostics and found no fault conditions. The thermostat is not currently calling for heat, so the furnace is idle. The slow flash rate is two seconds on and two seconds off.

If you expected the furnace to be heating, verify your thermostat settings. Make sure it is in "Heat" mode with the temperature set above the current room reading. Programmable thermostats may have a schedule that reduced the setpoint, and thermostats with dead batteries can lose their programming.

No service or maintenance is needed for this LED indication. It confirms the furnace electronics are functioning correctly.

Contact a licensed HVAC technician if any of these apply:

Sources

  1. Installation Manual - Residential Gas Furnace Models TM9T*MP (96% AFUE Two Stage Multi-position)

✓ Verified against manufacturer service manual — March 2026