Supplies the engine module and, on most vehicles, the sensors and actuators around it. The module holds this relay energised itself and releases it only after the ignition is off and it has finished writing to memory — which explains two otherwise unrelated symptoms: a relay opening too early corrupts the module's learned settings during shutdown, and one that never releases keeps everything powered and flattens the battery over a few days. Cheap, a common failure, and worth testing before anything else in a no-start.
Also known as: main relay, ecu relay, pcm power relay, system relay, engine main relay
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A relay is a switch operated by a magnet instead of a finger, and its whole purpose is to keep heavy current away from places heavy current should not go. The original puts that be...
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