Every frost-free refrigerator melts the frost off its evaporator on a schedule, using a heater and a sensor that tells the control when the coil is clear. LG's refrigerators report a failure in that routine with dH, prefixed by F for the freezer section or r for the refrigerator section, and some models show it as Er dH. In LG's description the defrost sensor did not reach the expected temperature within the time allowed. The heater may not have come on, the sensor may be reading wrongly, or the frost may simply have been too thick to clear.
What You Will Notice Alongside It
dH is often the first sign of a problem that becomes an Er FF fan code or warm food weeks later. Look for frost on the freezer's rear wall, ice under the bottom drawer, a fan that has become louder, and a fresh-food section that runs warmer than its setting while the freezer seems fine. Sometimes there is nothing to see yet, and the code is genuinely early warning.
Two Owner Checks That Are Not About Defrost
A defrost system can be healthy and still overwhelmed. A door left ajar for hours, a gasket that no longer seals or a drawer pushed in over a bag that props the door open lets humid room air in continuously, and that air becomes frost faster than the heater can melt it. Check the gaskets by closing a strip of paper in the door at several points; it should resist being pulled out everywhere. Check that nothing blocks the vents between the compartments. Fix those first, because if they are the cause the code will not return.
The Reset and Its Limits
Disconnect the refrigerator from power for a few minutes and restore it. That clears the code and starts a fresh defrost timer. If dH returns, the reset has told you the fault is real. Do not try to force a defrost by heating the freezer with anything hotter than room air, and do not scrape ice off the rear panel: the evaporator behind it carries refrigerant in thin tubing that a knife or a screwdriver tip will pierce.
Food Safety While the Fridge Is Marginal
A refrigerator with a defrost fault usually keeps cooling for a while, so the question is how well. Put a thermometer in the fresh-food compartment and read it after an hour with the door closed. The FDA's threshold for perishable food is 40°F; if the compartment cannot hold that, move the perishables to a cooler with ice or another refrigerator. Judge by the thermometer, not by whether the milk feels cold.
The Repair
The components behind dH, the defrost heater, the defrost sensor and the control board that drives them, are checked with the refrigerator disconnected from power and the freezer rear panel removed. The heater is a resistance element clamped to the evaporator; the sensor is a small thermistor clipped to the coil. Which one has failed is a meter reading, not a guess, and replacing either means working against the evaporator. Book an LG refrigerator repair with the code as displayed, how quickly it returned after a reset, and what frost you found where.