Gas Furnace - Locking Out / Sometimes Doesn't Heat

Article ID: 175
Last updated: 20 Mar, 2017

Gas Furnace

Re: Lockout / Sometimes Doesn't Heat

Customer reports that the furnace sometimes stops working. They can get it to work again by turning the thermostat or power off and back on.

Small Control

Most of these gas furnaces, 80 & 90%, use a White-Rodgers or Robertshaw Hot-Surface Ignition Control.

  • The control's primary function is to power the ignitor, energize the gas valve and monitor for flame.
  • The only real lockout with these controls is a failure to sense flame.
  • All other functions (inducer fan, pressure switch, limits, etc) are controlled external of the ignition control.

A diagnostic light on the lower left-hand corner of the board indicates "Lockout" status.

► For Control Replacement, use IGN.02 Universal Hot Surface Ignition Control.

Qty Possible Causes:
2 Dirty Flame Sensor
Pressure Switch Fault
Clogged Burner(s)
Low Gas Pressure
Bad Inducer Motor/Capacitor
Bad Gas Valve
Faulty Control


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 observe the ignition sequence.
    • The inducer motor should start with no difficulty.
      • Difficulty starting --> Check inducer motor and capacitor.
    • The pressure switch should close within a couple seconds of the inducer starting.
      • Pressure Switch fails to close = Pressure Switch Fault --> Check Pressure Switch.
    • The glow plug should energize within 30 seconds of the pressure switch closing.
    • The gas valve should open within 45 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 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

4. 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!

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Article ID: 175
Last updated: 20 Mar, 2017
Revision: 19
Access: Public
Views: 54
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