2013 Mercury 150 Pro XS:

Recently took boat to river to fish and take a ride. Engine cranked easy and ran to first spot about 5 minutes from ramp. After an hour, I ran 15-20 mins to next spot. After 2 hours of fishing, I decided to head back to the ramp. Shortly after getting on plane, the Mercury alarm horn started and blew constant for about 6 seconds.

I shut off engine and found no cooling issues, good water pressure while running, no plastic or obvious obstruction on water intake, motor was peeing good stream also. I cranked engine and got on plane running about 3500-4000 rpms when the horn sounded again. This time the Guardian system was activated and reduced rpms on engine. I came off plane and stopped engine. After looking in the Mercury manual I located the alarms and what they indicate.

*CONSTANT BEEP
1-Cooling System
2-Oil tank critical low level-----False Alarm(Bad Float)
3-Oil Pump Failure
4-Engine Overspeed
5-Sensor out of Range

UPDATE: I bypassed engine mounted oil tank level sensor due to bad level float inside tank(Critical low oil level alarm cleared). All is good now! I will replace small oil tank in Spring!

Thanks, Mike

Well, I bypassed the level sensor/alarm on the engine mounted oil tank and the alarm went away. Evidently the float went bad inside the oil tank. I will replace the tank at some point, but for now, all seems fine.

I have the same problem on my Optimax. The previous owner just cut the horn out rather than just disconnect it. I installed a new one and it is constant. I did some research and it appears that you have to replace the whole tank. For now I just left it disconnected. I did replace on temp sensor but did not want to continue chasing the problem with parts.

It will save you a lot of time and money if you will get a scanner for your Merc. like this https://www.rinda.com/techmatepro/

Good job.

Fairly common problem and fix on your Opti.

Thanks Mr. Purdy!

Thank you T Bird, I was kinda nervous on the water when this happened because this engine has been so good to me sine new. But thanks to this site and Youtube, I was able to research issues and find the problem. One positive thing from this, I took the time to check my engine compression, inspect all fuel and oil lines, inspect prop shaft and replace prop sleeve, etc. while sitting in garage waiting on parts delivery.

Ran engine on river yesterday and it ran great! Problem was a bad level float in engine mounted oil tank that put engine in Guardian mode and only allows engine to run 3000 rpm max. I bypassed level sensor and that stopped the alarm. This reset Guardian mode engine protection and allows me to operate engine normal. Permanent fix is to replace entire engine mounted oil tank.