PHU - Flashing Code 7

Article ID: 398
Last updated: 20 Dec, 2023

York Gas Furnace: PHU

Re: No Heat - Code 7

Code 7 = Failure to Sense Flame

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


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.
    • 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.

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 (Part# S1-02532625000)
    • Resistance ⇒ Continue
  • Start an ignition sequence.
  • Check for 120VAC power on the 2-wire plug feeding the glow plug during an ignition sequence.
    • No Power = Bad Control Board
    • Power Present + No Glow = Bad Glow Plug (Part# S1-02532625000)

4. Check Gas Pressure / Valve

  • At the point of ignition, check for 24VAC between the two wires at the gas valve.
    • No Power = Bad Control Board
    • Power Present ⇒ Check Gas Pressures (below)
  • Check Inlet Gas Pressure
    • Minimum Pressure = 5" w.c. for Nat. gas, and 11" w.c. for propane.
      • 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 = Bad Meter/Regulator or Restricted Gas Line
  • Check Manifold Gas Pressure
    • Normal Pressure = 3 - 3.5" w.c. for Nat. Gas, 9 - 11" w.c. for LP
    • Check the gas valve outlet pressure when the gas valve energizes.
      • No Gas Pressure = Bad Gas Valve
      • Low Gas Pressure ⇒ Increase Manifold Gas Pressure
      • Normal Gas Pressure + No or Delayed Ignition = Clogged Burners Remove burners and clean spreaders

5. Check Flame Sensor

  • DO NOT clean the flame sensor before checking.
  • 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!
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Article ID: 398
Last updated: 20 Dec, 2023
Revision: 17
Access: Public
Views: 136
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