Display shows E4 and the unit may stop cooling or cycle off prematurely; heavy frost may be present on the evaporator.

Safety: Unplug the cooler before removing interior panels. Melted frost can create slippery surfaces—use towels.

E4 is commonly associated with a defrost system problem.

• Check for frost buildup: Remove the interior rear panel and inspect the evaporator coil. A solid block of ice indicates defrost failure.
• Inspect the defrost sensor: The sensor is usually clipped to the evaporator tubing. Ensure it is firmly attached and the wires are intact.
• Test the defrost heater (if equipped): With a multimeter, measure resistance across the heater terminals. An open circuit indicates a failed heater that must be replaced with a Luma evaporator defrost heater.
• Test/replace defrost sensor: Check the sensor resistance at room temperature and compare to specifications. Replace a sensor that reads open or shorted with a Luma defrost sensor.
• Manual defrost: While waiting for parts, fully defrost the unit by leaving the door open (unplugged) until all ice melts. Do not chip ice with sharp tools.

If heater and sensor test good, the main control board’s defrost relay may be faulty and the board may need replacement.