Mercury - Malfunction Light Harness 20-Foot Single-Helm Cable 84-8M0048055. Overview. Catalyst-equipped MerCruiser engines rely on a clear dash-mounted indicator to alert you when the catalyst system needs attention. The Mercury 84-8M0048055 Malfunction Light Harness gives you a 20-foot run-ideal for single-helm boats with longer distances between the engine compartment and the dash. Once connected, it provides a clean, reliable path for the OBDM catalyst warning signal so the light can turn on