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I have a 2015 Merc 150 hp FourStroke on my 18' Dauntless, with 360 engine hours. Performance has been good. However, recently when I start up cold, I get a very threatening message on the SmartCraft tach:
System Fault 80 - Critical
Sensor Power Supply Voltage is Low
Return to Port Immediately
On first start at the dock, the engine runs very rough and dies. However, when I re-start, all is good.

The manual says the sensor power wires need +5/-5 volts. Is the Main Power Relay responsible for providing this? Could it be intermittent and need replacing? Or just work better when the engine has started already? Or is there another likely culprit? I have 2 new batteries in the boat, the charging system appears to be working fine, and the SmartCraft shows ~ 14.3 volts. Thanks in advance for any advice!