My Garmin 945 always shows significantly lower heart rate than it really is. Occasionally, it would work accurately, but only for a few minutes in a training session (if at all). How do I fix this?
Shouldn’t this be a question for a Garmin forum?
Also asked there
Here are some tips:
- Tighten it up a notch when exercising.
- Shave your wrist if you’re one of the more insulated people.
- Don’t use it for HR when cycling. Use a strap instead.
I know many people who have issues w/the optical while running and usually tips #1 and #2 above solve it. However, cycling is a different problem. Something to do with the angle of the wrist causes issues both indoors and outdoors, but especially outdoors.
I recommend you wear a strap for cycling. It’s not going to move around and chafe like it might when running. If you don’t like the chest strap, there are alternatives like the Scosche (armband) which come highly rated.
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buy Polar instead 
How much lower is it reporting?
Are you using a second device at the same time to compare?
It is reporting just mildly over resting heart rate throughout the workout. My resting is about 38, and it reports about 75 all along. I do make sure it is tight on my wrist. However, when I ride outside, I don’t tighten anything and it reports the heart rate more accurately (in the 130-150 range, doing a similar effort as on Zwift).
Are you recording an activity on the watch at the time or just broadcasting your HR?