PHU - Burners Ignite & Shut-Off

Article ID: 414
Last updated: 22 Mar, 2018

York Gas Furnace: PHU

Re: Burners Ignite & Shut Off

Code 7 = Failure to Sense Flame

Qty Possible Causes:
2 Dirty Flame Sensor
Clogged Burners
No or Low Gas Pressure
Pressure Switch Trip
Bad Control Board
Thermostat Cycling


1. Check Diagnostic Indicator

  • DO NOT cycle power to the furnace.
  • The furnace control board has an on-board LED that will flash to show any active diagnostics. (Killing power clears the diagnostic.)
  • Look through the blower door sight glass to see if the LED is flashing.

2. Check Ignition

  • Initiate a call for heat from the thermostat.
  • 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 Controls

  • Monitor 24VAC from "W" to "C" while the furnace cycles. (It may be necessary to manually close the door switch.)
    • Power Drops-Out when Burners Cycle = Thermostat Short-Cycling Check Thermostat & Wiring
    • Power Consistent ⇒ Continue
  • Monitor the diagnostic indicator when the burners shut-off.

4. Check Gas Pressure

  • Inlet Gas Pressure
  • Check the 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
  • Manifold Gas Pressure
  • Check the manifold gas pressure.
    • Normal Pressure = 3 - 3.5" w.c. for Nat. Gas, 9 - 11" w.c. for LP
    • 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

  • DO NOT clean the flame sensor before testing.
  • On the multi-meter:
    • Switch the test leads to "COM" and "mA".
    • Set the meter on DC 200ù.
    • Check the flame 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!
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Article ID: 414
Last updated: 22 Mar, 2018
Revision: 6
Access: Public
Views: 45
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