Ruud U802VA050317MSA Error Code A126_F: Flame not Sensed with Gas Valve On - UL

What Does Code A126_F Mean?

Fault code A126_F on the Ruud U802VA is a UL (Underwriters Laboratories) safety fault that occurs when the IFC has confirmed the gas valve is open (energized) but the flame sensor is not detecting any flame. This means the control board has successfully opened the gas valve, but no flame signal is coming back from the flame sensor — raising the possibility that gas is flowing into the furnace without being ignited.

The U802VA's Bluetooth communicating IFC monitors both the gas valve relay status and the flame sensor signal simultaneously. A126_F is triggered when these two readings are inconsistent: valve shows open, flame shows absent. This is specifically designated as a UL fault because it directly relates to the furnace's safety certification — an open gas valve without flame is a condition that UL standards require furnaces to detect and respond to immediately.

Possible causes include a hot surface igniter that has failed and cannot reach ignition temperature, a completely failed flame sensor that cannot detect actual flame even when combustion is occurring, or a gas valve that opens electrically but provides inadequate gas flow due to internal failure. In any case, the risk of unburned gas accumulation must be addressed before the furnace is restarted.

Common Causes

When to Call a Professional

Contact a licensed HVAC technician if:

• Do not restart the furnace — call a technician immediately to diagnose this UL safety fault
• A technician must determine whether gas was actually flowing without ignition or whether the flame sensor failed to detect actual combustion
• The igniter, flame sensor, and gas valve all require professional testing before the furnace can safely be restarted

✓ Verified against manufacturer service manual — March 2026