Is external stac
pressure greater
than nameplate?
Correct applicaon or
duct issues.
Check operaon
YES
Is heat rise
within
specificaon?
See Nameplate
NO
Is return air
temperature
above 85 F?
NO
Return air
temperature is
above max limit
YES
Check for loose
insulaon or other
objects within
furnace air stream
NO
Is outlet
manifold gas
pressure within
specificaons?
YES
Correct gas pressure
Check operaon
NO
Is temperature at
high limit above the
switch seng?
YES
Replace high limit
switch
NO
YES
Is temperature at
reverse flow switch
above
specificaons?
Replace reverse flow
switch
NO
Increase blower off delay
YES
DEFINITION
Limit switches are safety devices
that will open when an abnormal
high temperature has been sensed.
REMOVE ALL JUMPER WIRING TO
SWITCHES!
Under no circumstances, shall these
switches be le jumpered when not
troubleshoong.
Verify filters and blower wheels are
clean
Make sure any temperature
measuring devices
(thermocouples, dial
thermometers) used to
esmate limit temperature are
within ¼ inch of limit disc
See next page for
addional 04 faults
04 Fault Code
Does motor
connue to run
during the enre
heang cycle?
YES
Note:
Furnace will need to be checked in
both 1
st
& 2
nd
stage operaon
Reverse flow switch
checkout
Are either of the
flame roll-out
switches tripped?
NO
YES
Is blower motor
running?
See troubleshoong for
motor
NO
Is 24 VAC
measured across
main thermal
limit?
YL49-YL51
YES
NO
YES