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CONDITION POSSIBLE CAUSES
Display Blank, Fan off, LWCO lights off
• No 120VAC Power at boiler. Check breaker and wiring
between breaker panel and boiler
Display Panel Blank, Fan running
• Loose 120VAC connection wiring between boiler J-Box and
MCBA
• Blown “F1” fuse in MCBA (see Figure 13.2 for location).
Replace with 5A fuse provided
Display reads “U.125” continuously, Fan running
• Defective AT250 transformer
• Blown “F3” fuse in MCBA (see Figure 13.2 for location).
Replace with 4A slow-blow fuse provided
Boiler not responding to call for heat, Status code on display
=”0” (see Figure 11.1)
• Boiler is not seeing call for heat. Check thermostat or
zone wiring for loose connection, miswiring, or defective
thermostat/zone control.
Boiler res, but display panel is blank
• Loose ribbon cable
• Defective display
WARNING
WIRING.
Figure 13.2: MBCA Fuse Location