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8:1 Staged Furnace Will Not Light
Troubleshooting
With the combustion blower
(CM) off, is the N.O.
airflow switch (PS2) closed?
Continues on next page...
Airflow switch (PS2) defective
Replace defective airflow switch (PS2)
Yes
No
24 VAC across terminals
S and T?
Discharge temperature was above high limit control (HLC1)
Wait for high limit to cool and reset
Inlet air sensor contact open
Adjust inlet air sensor setting (refer to 8:1 Staged Start-Up section)
Furnace controller (SC1) defective
Replace furnace controller (SC1)
24 VAC across 22 and 21?
Heat transformer (TR2) fault
Replace heating transformer
No
No
Yes
Yes
N.O. contact on heat relay (RH)
closed?
Heat relay (RH) is not energized
Check for loose connections. Repair of replace heat relay (RH)
No
Yes
24 VAC across W1 and X?
Heat switch (S4) open or not wired
Close or replace heat switch (S4)
No
Yes
24 VAC across R and X?
Main disconnect (DS1) open or defective
Close, repair or replace
Main transformer (TR1) fault
Replace main transformer
No
Yes
DT1 displayed on furnace
controller (SC1)?
Discharge air sensor (TS2) not wired or defective
Wire or replace discharge air sensor (TS2)
Yes
No
OT1 displayed on furnace
controller (SC1)?
Outdoor air sensor (TS1) not wired or defective
Wire or replace outdoor temperature sensor (TS1)
Yes
No
Blank screen on furnace
controller (SC1)?
Furnace controller (SC1) defective or not powered
Replace or wire furnace controller (SC1)
Yes
No
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Model IG / IGX Make-Up Air