York Gas Furnace: TM9X/TM9E

Re: Flashing Code 7

Code 7 = Failure to Sense Flame (3 Failed Ignition Attempts)

CHECKOUT:

1. Check Ignition Sequence

• Initiate a call for heat from the thermostat and monitor the ignition sequence.
  • 1. The inducer motor should start with no difficulty.
  • 2. The pressure switch should close within a couple seconds of the inducer starting.
  • 3. The glow plug should energize within 30 seconds of the pressure switch closing.
  • 4. The gas valve should open within 20 seconds of the glow plug energizing.
    • No Glow from Glow Plug → Step 2. Check Glow Plug
    • Glow Plug Glows → Step 3. Check Ignition

2. Check Glow Plug

• Turn off power to the furnace.
• Unplug the 2-wire connector to the glow plug.
• Use a meter to test resistance of the glow plug.
  • No Resistance = Bad Glow Plug
• Start an ignition sequence.
• Check for 120VAC power on the 2-wire plug feeding the glow plug during an ignition sequence.
  • No Power = Bad Ignition Control
  • Power Present but No Glow = Bad Glow Plug

3. Check Ignition

• Visually inspect the burners during ignition.
  • The burners should light smoothly with little or no delay from the glow plug to the flame sensor.
  • Make sure the flame sensor comes in good contact with the flame.
    • Delayed Ignition or Not All Burners Igniting = Clogged Burners, Low Gas Pressure or Bad Gas Valve → Step 4.  Check Gas Pressure / Valve
    • All Burners Ignite Properly → Step 5.  Check Flame Sensor

4. Check Gas Pressure / Valve

• Check Inlet Gas Pressure
  • Minimum Pressure = 5" w.c. for Nat. gas, and 10" w.c. for LP.
    • Low Pressure = Gas Supply Issue
  • Re-apply power to the furnace and activate a call for heat.
  • Make sure the inlet gas pressure stays above minimum when the gas valve energizes.
    • Pressure Drops Below Minimum = Gas Supply Issue
• Check Manifold Gas Pressure
  • Normal Pressure = 3 to 3.5" w.c. for Nat. Gas, 9 to 11" w.c. for LP.
  • Activate a call for heat and monitor the manifold gas pressure during an ignition attempt.
    • No Gas Pressure = Bad Gas Valve
    • Low Gas Pressure ⇒ Attempt to Increase Manifold Gas Pressure
      • Won't Increase = Bad Gas Valve
    • Normal Gas Pressure + No or Delayed Ignition = Clogged Burners ⇒ Remove burners and clean spreaders

5. Check Flame Sensor

• Check the flame sensor reading when the burners ignite.
• The factory control board has a "Flame Current" test point (+ and -) built onto the board. 
• Use the 20 VDC scale to check the flame signal. (1 DC volt = 1 mA)
  • Normal Reading = 2 to 6 mA
  • Minimum acceptable reading = 1 MicroAmp
    • Low Reading = Dirty Flame Sensor ⇒ Clean the Flame Sensor & Re-check