S9V2-SF-2A-EN
49
Is external sta!c
pressure greater
than nameplate?
Correct applica!on or
duct issues.
Check opera!on
YES
Is heat rise
within
specifica!on?
See Nameplate
NO
Is return air
temperature
above 85 F?
NO
Return air
temperature is
above max limit
YES
Check for loose
insula!on or other
objects within
furnace air stream
NO
Is outlet
manifold gas
pressure within
specifica!ons?
YES
Correct gas pressure
Check opera!on
NO
Is temperature at
high limit above the
switch se#ng?
YES
Replace high limit
switch
NO
YES
Is temperature at
reverse flow switch
above
specifica!ons?
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
troubleshoo!ng.
Verify filters and blower wheels are
clean
Make sure any temperature
measuring devices
(thermocouples, dial
thermometers) used to
es!mate limit temperature are
within ¼ inch of limit disc
See next page for
addi!onal 04 faults
04 Fault Code
Does motor
con!nue to run
during the en!re
hea!ng cycle?
YES
Note:
Furnace will need to be checked in
both 1
st
& 2
nd
stage opera!on
Reverse flow switch
checkout
Are either of the
flame roll-out
switches tripped?
NO
YES
Is blower motor
running?
See troubleshoo!ng for
motor
NO
Is 24 VAC
measured across
main thermal
limit?
YL49-YL51
YES
NO
YES
