Error Code Slow Flash
Info

Trane TUD1B080A9H31B Error Code Slow Flash: Normal - No Call for Heat

TL;DR
Your furnace is in standby mode with no thermostat call for heat — this is normal operation.
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 Flash Mean?

A slow-flashing LED on the Trane TUD1B080A9H31B control board indicates the furnace is powered on and in standby mode. The White-Rodgers 50A65 Integrated Furnace Control has completed its self-diagnostics and found no problems. The thermostat is not currently requesting heat, so the furnace is simply waiting.

If you expect the furnace to be heating but see this slow flash, check your thermostat first. Make sure it is set to "Heat" mode (not "Cool" or "Off") and that the target temperature is set above the current room temperature. If you recently changed your thermostat batteries, verify the settings were not reset to defaults.

This is a completely normal status and requires no action or service.

Contact a licensed HVAC technician if any of these apply:

Sources

  1. Installer's Guide - High Efficiency Single Stage Upflow/Horizontal and Downflow/Horizontal Gas-Fired Furnaces

✓ Verified against manufacturer service manual — March 2026