The pressure switch is a normally open switch that monitors
combustion airflow. Inadequate airflow resulting from excessive
venting system restriction or a failed combustion blower will
cause the switch to remain open.
The rollout switch(es) are normally closed switch(es) that open
when abnormal temperatures exist in the burner area. This can
be caused by a restricted heat exchanger causing main burner
flame to "roll out" into the vestibule area or burner box.
The rollout switch(es) must be manually reset by pushing the
button on top to restore furnace operation.
Primary Limit Control
This is a normally closed control that opens if abnormally high
circulating air temperatures occur. It is an automatic reset control.
Auxiliary Limit Control (on some models)
This is a normally closed control, located on the circulating air
blower housing, that opens under abnormal "reverse airflow"
conditions that could occur in a counterflow or horizontal
installation if the circulating air blower fails. It is an automatic
Blower Control Board
The blower control board operates the circulating air blower, the
combustion blower, the compressor contactor and any
accessories connected to it.
Gas Control Valve
The gas control valve regulates the manifold gas pressure and
turns the gas flow on and off.
Furnace Fails to Operate Properly
Review "Sequence of Operation" and visually inspect the
following before troubleshooting:
Is the power to the furnace on?
Is the manual gas shutoff valve to the furnace open?
The furnace has a built-in, self-diagnostic capability. The furnace
control continuously monitors its own operation and the
operation of the system. If a system problem occurs, the LED on
the Furnace Control Board flashes a failure code. See the Failure
Codes chart for a description of the LED flash codes.
1. Start the system by setting the thermostat above the room
2. Remove the burner access panel.
3. Observe the LED response through the LED viewing holes.
4. See the Failure Codes chart for a description of the LED flash
5. Replace the burner access panel.
1. LED viewing holes
Failure Codes Chart
Call for heat
Gas Valve lockout
Pressure Switch stuck closed or open
High Limit Switch open
Flame sense with gas valve off