TM9X/TM9E - Flashing Code 7

Article ID: 381
Last updated: 25 Apr, 2024

York Gas Furnace: TM9X/TM9E

Re: Flashing Code 7

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

Qty Possible Causes:
1 Dirty Flame Sensor
1 Bad Glow Plug
1 Clogged Burners
1 No or Low Gas Pressure
1 Bad Gas Valve
Bad Ignition Control


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.

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.

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
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Article ID: 381
Last updated: 25 Apr, 2024
Revision: 16
Access: Public
Views: 173
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