GF - I 3
Main Limit Switch
Figure 5
If the limit switch re-closes during a four-code lock-
out period, the furnace control will shut down both
blower motors and retry another attempt at normal
furnace operation.
If a limit switch opens, the diagnostic fault code indi-
cated by the furnace control board diagnostic LED
will be a flash four-sequence (Figure 5.)
Figure 6
Step 1
Refer to the furnace data plate to determine maxi-
mum supply air temperature.
Step 2
Block off return air or disconnect the indoor blower
motor.
Step 3
Insert temperature probe into supply air plenum (Fig-
ure 6.)
Troubleshooting
Testing main air limit operation
Furnace Control Board
Diagnostic LED
Step 4
Call for heat. All stages if the furnace is a multi-stage
furnace.
Step 1
Call for heat.
Step 2
Set multimeter to read voltage.
Step 5
When the temperature of the air leaving the furnace
is close to the maximum supply air temperature indi-
cated on the furnace data plate, the furnace should
shut off the burners and display a four-code diag-
nostic error on the furnace control board LED.
Step 6
If the furnace does not shut down at or near the proper
temperature, replace the main limit switch.
Step 7
If the switch is determined satisfactory and the fur-
nace is cycling on and off on main limit, perform an
indoor air volume measurement test to determine if
the air volume is adequate (refer to measuring air vol-
ume.)
Electrical check of main limit switch
To Next Page
Loading...
