Glow Plug is a device, just like a spark plug, used to aid starting diesel engines. The Mazda BT-50 glow plug relay has a feedback line to the PCM to facilitate failure detection. The PCM operates the glow plug relay for max. 120 s at an engine coolant temperature below 32 °C. The following schematic illustrates the Mazda BT-50 Glow Plug Relay Wiring Diagram.