Gas Furnace - Burners Cycling Off

Gas Furnace

Re: Burners Cycling Off / Burners Won't Stay On

The furnace burners fire and run for several minutes, then the burners shut off and the furnace goes through another ignition sequence.

These furnaces use either a Robertshaw SP735 or White-Rodgers 50E47 hot-surface ignition controller. (A common replacement control is the IGN.02 Universal Hot Surface Ignition controller).

• Once 24VAC is received on "TH" and "TR (Common)", the control will start an ignition sequence by powering the on-board igniter relay (which breaks one leg of 120V to the glow plug).
• After 45 - 60 seconds of pre-heating the glow plug, the board powers the main gas valve "MV".
• The board will then have 6 seconds to receive a flame signal.
• The gas valve will stay energized until 24VAC is removed from "TH" or the flame signal is lost.

Diagnostic Light:

• Light On Continuous = Replace Controller
• Flashing = Ignition Lockout

CHECKOUT:

1. Check for Power at LTB

• Check for 24VAC between "R" and "C" on the terminal block.
  • No Power = Check Blower Door Switch and High Voltage Power to Furnace
  • High Voltage but no Low Voltage ⇒ Check transformer.
• Monitor 24VAC power between "W" and "C" on the low voltage terminal block.
  • No Power = No Heat Call from Thermostat ⇒ Recheck Stat
  • Power drops out when burners kick-off = Thermostat Cycling
  • Power present when burners kick-off → Step 2. Check for Power at Controller

2. Check for Power at Controller

• White-Rodgers - Check for 24VAC between "TH" and "TR".
• Robertshaw - Check for 24VAC between "24V" and "COM".
• IGN.02 - Check for 24VAC between "TH-W" and "24V (Gnd)".
  • Monitor voltage as the furnace operates.
    • Power drops out when burners kick-off = Safety Trip → Step 3. Check Safeties
    • Power present when burners kick-off = Loss of Flame Signal → Step 4. Check Flame Sensor

3. Check Safeties

• Pressure Switch
  • Monitor 24VAC between "NO" on the pressure switch(es) & "C" on the terminal block.
    • Power drops out when burners kick-off = Pressure Switch Trip
    • Power present when burners kick-off ⇒ Check Primary Limit
• Primary Limit
  • Monitor 24VAC between the "Brown" or "Violet" wire on the Primary Limit to "C" on the terminal block.
    • Power drops out when burners kick-off = Limit Switch Trip
    • Power present when burners kick-off → Step 4. Check Flame Sensor

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.
    • Minimum acceptable reading = 1 MicroAmp
      • Low Reading ⇒ Clean Flame Sensor & Re-check
      • Normal Reading (2 to 6 mA) but Shuts Off = Bad Ignition Control
  • MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!