Whirlpool Microwave Error Code MV-4903

Error code MV-4903 on a Whirlpool microwave indicates a failure within the MW (microwave) relay circuit, specifically that relay 4903 located on the Appliance Control Unit (ACU) has experienced a welded contact condition. A relay is an electromechanical switch that opens and closes to control high-voltage power delivery to the magnetron and related components. When a relay contact welds closed, it means the metal contacts inside the relay have fused together, preventing the relay from opening as commanded. This leaves the controlled circuit in a permanently energized state regardless of user input or control board commands.

A welded relay contact is typically caused by repeated high-current arcing across the contact surfaces over time, which gradually degrades and eventually fuses the metal. This can happen due to a faulty relay that was never rated for the actual load demands, a short circuit elsewhere in the microwave that caused excessive current to flow through the relay, or a failing magnetron drawing abnormal current spikes. Wiring issues such as loose or corroded connections at the relay terminals can also create resistance and localized heat that contributes to contact welding.

Because relay 4903 is integrated into or directly connected to the ACU, the diagnostic process requires disconnecting power to the unit before inspecting the wiring harness leading to the relay and physically confirming whether the relay contacts have welded. If the contacts are confirmed welded, the relay itself or the ACU assembly will need to be replaced. This is a safety-critical condition since a welded relay can result in the magnetron energizing unintentionally.