Unfortunately, the user manual of the Trane XR90 furnace offers no explanation of  what the blinking light means that can be seen behind a small hole in the bottom front panel of this high-efficiency gas furnace.

The codes are explained on a wiring diagram that is glued to the back of the panel. 

Trane XR90 Diagnostic Codes

A continuously blinking light indicates normal operation and is no reason for concern. Here is the complete list:

• Flashing slow: Normal - no call for heat
• Flashing fast: Normal - call for heat
• Continuous on: Replace IFC (circuit board)
• Continuous off: Check power
• 2 Flashes: External lockout (retries or recycles exceeded)
• 3 Flashes: Pressure switch error
• 4 Flashes: Open limit device
• 5 Flashes: Flame sensed when no flame should be present
• 6 Flashes: 115 VAC power reversed polarity or poor grounding
• 7 Flashes: Gas valve circuit error
• 8 Flashes: Low flame sense signal
• 9 Flashes: Check igniter

Dr. Heet does not provide support or repair advice for this type of furnace. Please contact a licensed HVAC professional if your furnace does not function properly.