TG9S/GY9S - No Light on Control Board

York Gas Furnace: TG9S/GY9S

Re: No Light on Control Board

The LED light on the furnace control board indicates the status of the furnace.

In this condition, the light is constantly off.

CHECKOUT:

1. Check Low Voltage

• Check for 24VAC between "R" and "C" on the control board LTB.
  • Power Present → Step 3. Check Control Board
  • No Power ⇒ Continue
• Check for 24VAC across the output of the transformer.
  • No Power Present → Step 2. Check Line Voltage
  • Power on Transformer + No Power on "R" ⇒ Check Low Voltage Fuse 
    • Fuse OK = Bad Control Board
    • Fuse Blown ⇒ Test for Low Voltage Short

2. Check Line Voltage

• Check for 120VAC on the inlet side of the transformer.
  • Inlet Voltage Present + No Low Voltage = Bad Transformer
  • No Inlet Voltage ⇒ Make sure the blower door switch is closed.
    • Check for 120VAC on the line voltage wires entering the furnace.
      • No Voltage Present ⇒ Check the following:
        • ​​​​​​​SSU Switch
        • SSU Fuse
        • Breaker

3. Check Control Board

• Place a jumper between "R" and "G" ⇒ ​​​​​​​Check if the fan motor starts.
• ​​​​​​​Place a jumper between "R" and "W" ⇒ ​​​​​​​Check if an ignition sequence starts.
  • No Operation = Bad Control Board
  • ​​​​​​​Fan and Ignition Starts = Bad LED on Control Board