Make sure its wet prior to starting, I just started seeing mine spike very high (had never wet it prior as I sweat a fair amount as well) Making sure it has good contacts to start may help. Outside of that, I am not sure, if its not displaying any numbers you may try to replace the battery.

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Jeff Vanis' Blog | Challenge Family USA - Use code JVTRI15 to save 10% off your entry

contact Garmin and ask for a replacement. Some people use gels to make the contact better but you shouldnt have to. Is it the soft strap? I have now been through 3.

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http://longwaytogo-ironman.blogspot.com/

I have the same problem - solutions have been to change the battery in the HR monitor and tighten the strap all the way after I moisten the contact points.

It is very annoying!

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Straightenin' the curves; Flattenin' the hills
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Coached by Mike Plumb @ TriPower MultiSports
https://www.strava.com/athletes/1149072 - https://www.instagram.com/thoswoods/

I've resorted to using the soft strap from my Polar HRM and plugging the Garmin head unit into that. It is much more reliable for me. Occassionally towards the beginning of a run it will spike a bit, but after that it's stable.

I think Polar just makes better HRM stuff while Garmin has better computing. so I've combined the two.

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Blogging

I purchased a polar hrm strap for this reason. My garmin hrm worked flawlessly for about 3 months then started giving strange readings, and never at the start but always in the middle of a workout. My hr would be over 200 on an easy ride, and if I stopped it would jump below 60 and if I started again it would go over 200 again. I ended up getting a replacement that worked flawlessly for about 3 months and then it did the same thing. I never had to wet it at the start - I could put it on and it would register a reliable hr reading while sitting in my lounge chair. The issues always started after half and hour or more of activity and stayed inconsistant for the rest of the workout.

I've sent my replacement one back under warranty as well now and am waiting for another one. The guy I spoke to at garmin 'said' they have a new model of the hrm (same form factor) that is spose to have solved the issue!

I have my polar strap for when this one goes south.