GUC/GUX - Locking-Out / Sometimes Doesn't Heat

Amana Gas Furnace: GUC/GUX

Re: Locking-Out / Sometime Doesn't Heat

The customer reports that the furnace sometimes does not heat, and that they are able to get the furnace to restart by turning the thermostat off and back on or by cycling power to the furnace.

These furnaces use a White-Rodgers 50A50 controller.

• There is no fault recall on this control, so once the fault is reset it cannot be retrieved.
• Try to catch the furnace in a lockout mode and look through the sight-glass on the blower door to see the  flashing lockout code.
• The only "Hard Lockout" on the 50A50 control board is a Code 1 - Failure to sense flame. (All other faults reset automatically once the condition returns to normal)
  • So, if you are unable to catch the furnace in a lockout, suspect flame sensing issues as the most likely cause.

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 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.
    • Observe the control board diagnostic indicator for any fault indications.
    • A Code 2 or Code 3 would indicate a Pressure Switch Fault.
  • The glow plug should energize within 30 seconds of the pressure switch closing.
    • Glow Plug Doesn't Glow or Sparks ⇒ Remove the Glow Plug and Inspect for cracks
  • The gas valve should open within 30 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 4.  Check Flame Sensor
    • Any delay or burners not igniting = Clogged Burners, Low Gas Pressure or Bad Gas Valve → Step 3.  Check Gas Pressure / Valve

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 = Gas Supply Problem
• 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 + No or Delayed Ignition = Clogged Burners ⇒ Remove burners and clean spreaders.

4. Check Flame Sensor

• DO NOT clean flame sensor before checking!
• 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!