Gas Furnace - Burners Cycling Off

Article ID: 136
Last updated: 17 Dec, 2020

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

Qty Possible Causes:
4 Pressure Switch
Limit Switch
Losing power from the thermostat
Dirty flame sensor
Low gas pressure
Clogged burners


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)".

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.

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

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Article ID: 136
Last updated: 17 Dec, 2020
Revision: 32
Access: Public
Views: 64
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