It have the premium strap and I'm getting erratic readings: lower than normal HR then spikes to way over my Max HR. Even mid-range readings seem low. Do you find it's the strap or sender that's at fault? I have a new battery in with the same results which leads me to think strap. Strap is 4 years old, washed as they instruct. Thanks.

Almost certainly the strap. Buy a Polar strap, the Garmin head unit fits after you trim the plastic around the snaps. It has longer contact patches. Garmin brand strap never worked for me, same symptoms you are experiencing.

It's definitely the strap. I've never had a Garmin soft strap last more than a year, but keep original little plastic transmitter as a backup too, it's nice to have just in case.

Good point about a spare sender--thought I'd lost mine a few times over the years. I can get the strap solo but will buy the combo.

I had all kinds of problems with my Garmin strap, including the ones that you describe - at times I thought that I must have tachycardia, but it was the strap. I figured that the strap was getting old, so I replaced it with another Garmin - mistake. Problems persisted. I bought a Wahoo Tickr. No more weird readings. And (helpfully) it does both ANT and bluetooth.

I too had issues with my Garmin strap that is about two years old. The HR would go much higher than what was accurate and I would change the battery and then after a month it would do the same thing. Then I decided to wash the strap separate from the load of my kit after a ride. I let is soak with warm water and a small amount of detergent overnight. That seems to solve my problem. I did this 6 months ago and everything is perfect. I think that the salt from natural perspiration causes the issues all of you have seen. I also short out the contacts of the internal battery contacts in the heart monitor.

Tensive Conductive Adhesive Gel on the pads before you put it on will reduce the errors. I've been using it with HR straps for a couple of years and have had steady readings since.

For once, glad I sweat like a horse-- I've never had so much as a single HR spike with my soft strap Garmin.

But if I get four years out of a $40 HRM, I'll be ecstatic. I went through 3 wrist-based monitors in less than 3 years.