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Troubleshooting
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 4:1 Modulation 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 or 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
Continues on next page...
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
4:1 Modulating Furnace Will Not Light
®
Model IG / IGX Make-Up Air