Bad coil? how to check with voltmeter

2009 mercury 20 hp 4 stroke. can idle all day long, runs fine for about 15-20 minutes then goes to about half power. no missing, no sputtering, can limp back home. come back next day same thing. could this be a bad coil? i have one coil for two cylinders. could the coil be getting hot then the problem starts? how do i go about checking to see if its bad with a voltmeter? i tried to check the ohms with the positive lead in the spark plug cap and the negative on the coil ground wire. nothing sits on "open" "l", tried the positive coil wire same thing. for giggles i put the leads in both spark plug caps, i get .018 ohms. i would assume that i should be getting more than that. just not sure. the manual i have only goes out to year 2000. so i am trying what i can, i just dont want to buy a fuel pump and a coil and a CDI box. how would i check the CDI box? thanks again!

the ohm test isnt really that great of a test, it only tells you the resistance, not if you are getting fire or not... and .018 ohms sounds about right, less resistance more electricity.

Thanks! i see you are from maryville! i get all my KTM stuff from Dave Farris at Sportcycle KTM down there.

sweet, ive lived here my whole life, if you are ever down here stop by autozone lol thats where i work

Will do. Haven't been down there in a few years but I enjoyed the heck of the trail riding we did. Need to get back down thee

We have some absolutely beautiful country out here... miles and miles of beautiful trails

Oh I know. We got a private tour on the backside of coal creek, president of the riding club took on the enduro loop that gets run once a year. And then got to go to some huge farm that the guy le

Opens once a year, huge grass track, trails,
Had a blast

sounds like lots of fun!