LG GEN2 Series Installation And Operation Manual page 82

Troubleshooting - 4:1 Modulating Furnace
4:1 Modulating Furnace with Stand-Alone Controller Will Not Light
In all cases, verify correct terminals as shown on the unit-specific wiring diagram.
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Line voltage at primary side of
furnace transformer (TR5)?
Yes
24 VAC across 20 and 101?
Yes
24 VAC across C1 and G0 on
furnace controller (SC1)?
Yes
Blank screen
on furnace controller?
No
Furnace Controller Alarm:
'Furn Alarm - Check Log'?
No
Furnace Controller Alarm:
'Supply Sensor (U2) Err'?
No
Furnace Controller Alarm:
'Inlet Sensor (U1) Err'?
No
Inlet air temperature above
heating lockout setting?
No
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No
No
Discharge temperature was above high limit control (HLC)
No
High limit control (HLC) does not close when at room temperature
Yes
Furnace controller defective or not powered
Yes
New furnace alarm(s) have been logged
Press the down arrow to scroll to the end of the alarms menu.
Press enter to go to the data logger and review alarms.
Discharge air sensor (U2) not wired or defective
Yes
Wire or replace discharge air sensor (U2)
Yes
Inlet air sensor (U1) not wired or defective
Wire or replace outdoor temperature sensor (U1)
Yes
Increase heating lockout setting to above inlet air temperature
(Be sure to return this setting to the proper temperature
Fuse (FU7) Blown
Test and replace blown fuse(s)
Fuse (FU9) Blown
Test and replace blown fuse
Furnace Transformer (TR5) fault
Replace Heating Transformer
Wait for high limit to cool and reset
Replace high limit control
Replace or wire furnace controller
after testing furnace)