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warning Miller
Error Code L04: Thermocouple #4 Open
Signal indicates thermocouple number 4 is open (not functioning properly).
? What Causes This
Broken, disconnected or faulty temperature sensor wiring.
✓ How to Fix It
Check and repair open connections; if damaged, replace with backup thermocouples as needed.
Source: page 47 of the official manual
From the manual
This error code appears in:
Miller ProHeat 35 Industrial Heater