Lennox Furnace Error Code LED-SLOW_FLASH_1ON_5OFF

This slow flash code, one flash on followed by five flashes off, is generated by the G776 ignition control on Lennox -1 and -2 model furnaces. It indicates the control failed to sense a flame during an ignition attempt and has entered a retry period. The furnace will attempt ignition again automatically before proceeding to a full lockout if the problem is not resolved.

During the retry period indicated by this code, the furnace is attempting to light normally and has not yet locked out. If the furnace successfully ignites, it will resume normal operation. If it fails and locks out, the gas valve will close and the unit will stop trying. You do not need to vacate your home for this code alone, but if you smell gas at any point, leave the building and contact your gas utility immediately.

Repair costs depend on the root cause. Flame sensor cleaning is often handled as part of a routine service call, which typically runs between $80 and $150. Replacing a hot surface igniter generally costs between $100 and $250 including parts and labor. If the G776 ignition control board itself needs replacement, expect costs in the range of $250 to $600 or more depending on parts availability and local labor rates.

Some causes of this code, such as a dirty flame sensor, are within reach of a knowledgeable DIYer with basic tools. Cleaning the flame sensor rod with fine emery cloth and verifying that the gas supply valve is fully open are reasonable first steps. However, testing gas manifold pressure, replacing an igniter, or diagnosing the G776 control board requires proper tools and gas system knowledge, so a licensed technician should handle those tasks.

A manual reset will clear the current fault state and allow the G776 control to begin the ignition sequence again. However, if the underlying problem, such as a dirty flame sensor or gas pressure issue, is not corrected, the furnace will fail to sense flame again and return to the same retry period and eventual lockout. Resetting without diagnosing the root cause only delays a repeat occurrence.