York Gas Furnace: TM9T

Re: No Heat - Flashing Code 7

Code 7 = Indicates that the flame could not be established. This no-light condition occurred 3 times (2 retries) during the call for heat before locking out. 

► The furnace will lock out for one hour and then attempt a restart.

CHECKOUT:

1. Check Ignition Sequence

• Initiate a call for heat from the thermostat and monitor the ignition sequence.
  • The inducer motor should start with no difficulty.
  • The pressure switch(es) should close within a couple seconds of the inducer starting.
  • The glow plug should energize within 30 seconds of the pressure switch closing.
  • The gas valve should open within 20 seconds of the glow plug energizing.
    • No Glow from Glow Plug → Step 2. Check Glow Plug
• 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.
    • Any Delay or Burners Not Igniting → Step 3.  Check Gas Pressure / Valve
    • Burners Ignite Properly → Step 4.  Check Flame Sensor 

2. Check Glow Plug

• Look for any visual cracks.
• 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 + No Glow = Bad Glow Plug

3. Check Gas Pressure / Valve

• Check Gas Valve Power
• Check for 24VAC between the Yellow and White wires at the gas valve at the point of ignition.
  • No Power = Bad Ignition Control
  • Power + No Ignition ⇒ Check Gas Pressures
• Check the Inlet Gas Pressure
  • Minimum Pressure = 5" w.c. for Nat. gas, and 11" w.c. for propane.
    • Low Pressure = Bad Meter or Regulator
• 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 the Manifold Gas Pressure
• Activate a call for heat and monitor the manifold gas pressure during an ignition attempt.
  • Normal Pressure = 3 - 3.5" w.c. for Nat. Gas, 9 - 11" w.c. for LP
    • No Gas Pressure = Bad Gas Valve
    • Low Gas Pressure ⇒ Attempt to Increase Manifold Gas Pressure
    • Normal Gas Pressure + No (or Delayed) Ignition = Clogged Burners ⇒ Remove the burners, clean the spreaders, and recheck ignition.

4. Check Flame Sensor

• On the multi-meter:
  • Switch the test leads to "COM" and "mA".
  • Set the meter on DC 200ù.
  • Check the flame signal when the burners ignite.
    • Normal Reading = 2 to 6 mA
    • Minimum acceptable reading = 1 MicroAmp
      • Low Reading = Dirty Flame Sensor ⇒ Clean the Flame Sensor & Re-check
  • CAUTION! Move meter leads back to "COM" and "V" before proceeding!