Gas Furnace

Re: No Heat - Code 7

(Assuming original control board)

Code 7 = Failure to Sense Flame

CHECKOUT:

1. Check Ignition Sequence

• If the furnace is running, turn the thermostat off and allow the furnace to cool and the blower to stop operating.
• 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.
    • No Glow from Glow Plug → Step 3. Check Glow Plug
  • 4. The gas valve should open within 20 seconds of the glow plug energizing.

2. 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.
    • Burners Ignite Properly → Step 5.  Check Flame Sensor
    • Any Delay or Burners Not Igniting = Clogged Burners, Low Gas Pressure or Bad Gas Valve → Step 4.  Check Gas Pressure / Valve

3. Check Glow Plug

• Look for any visual cracks.
• 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

4. Check Gas Pressure / Valve

• Inlet Gas Pressure
• Turn off power to the furnace and close the gas safety shut-off valve.
• Hook-up a gas pressure gauge to the inlet test port of the gas valve.
• Open the gas safety shut-off and check 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 = Bad Meter/Regulator or Restricted Gas Line
• Manifold Gas Pressure
• Turn off power to the furnace and close the gas safety shut-off.
• Move the gas pressure gauge to the manifold (leaving) test port on the gas valve.
  • CAUTION! Be sure to put the plug back in the inlet test port before proceeding!
• Open the gas safety shut-off valve and re-apply furnace power.
• 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 ⇒ Increase Manifold Gas Pressure
    • Normal Gas Pressure but No or Delayed Ignition = Clogged Burners ⇒ Remove burners and clean spreaders

5. Check Flame Sensor

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