Good morning,
I have a 2005 Mercury 40 2-stroke and the motor alarm just came on out of nowhere. The water flow is fine, oil seemed to be being pumped fine, it wasn't over heated as I have just started it and the alarm came on about one minute later. There were a few weeds in the area but the motor was clean and not plugged. Does anyone have any ideas what could be the problem? The only issue I can think of is my oil pump gave out. I can't find anywhere any information on these "alarms". To be honest, I didn't even know it had an alarm! Any help would be appreciated. Thanks.

There is a sensor on oil pump drive shaft and if shaft is not turning oil pump then will activate alarm. If any don't about oil injection system then 50:1 oil mix to gas tank until know everything is working properly.

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Regards
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Connect diagnostic software to see what the fault codes read. That's the quickest and easiest way to know exactly what it is.