443 Low water cut-off (LWCO) activated

What the control is telling you

The low water cut-off circuit has opened. The boiler uses a built-in pressure-sensor type LWCO (not serviceable or adjustable); an optional external LWCO can be wired to the PC Board.

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi and Solo)

Likely causes

• Low water level / pressure in the boiler
• Faulty internal or external LWCO device
• Missing LWCO jumper when using the built-in device
• Optional LWCO wired incorrectly
• No 24 VAC from transformer
• Failed transformer or PC Board

Test and repair steps

1. Check local codes to confirm whether the built-in LWCO is acceptable; any optional LWCO probe must sit above the minimum safe water level and be wired to the proper PC Board terminal.
2. Confirm the internal or external LWCO device is operating correctly.
3. If relying on the built-in device, make sure the LWCO jumper is in place across its terminals.
4. If using an optional LWCO, remove the jumper and connect the device to the three LWCO terminals.
5. Check for 24 VAC transformer output at PC Board connector CN202 between red pin #2 and red pin #4.
6. If 24 VAC is missing, check for 120 VAC at CN202 between white pin #1 and gray pin #3: if 120 VAC is present replace the transformer; if not, replace the PC Board.

Models that show this code

8 models in I-Series Plus 150K and smaller, I-Series Plus 175K and higher

Other codes on the same units

• 380 Exhaust thermistor fault
• 393 Outdoor temperature sensor fault
• 400 Water pressure sensor communication fault
• 430 High or low system water pressure
• 520 Gas solenoid valve circuit – flame after valve close
• 540 High exhaust (flue gas) temperature
• 610 Combustion fan motor speed fault
• 631 DHW recirculation pump – insufficient flow