I had a few issues with mine a while ago, asked on here and the general consensus was that extended periods of high readings (200 bpm+) was an indicator of a worn out/damaged strap. Is there any way it could have been damaged in transit?

There are a lot of reports of it with the Garmin soft straps (especially the older versions), have a Google and see if the reports match what you're seeing. I'd be surprised if it's an issue with the watch, but I suppose anything is possible with Garmin. If it's dead then you can still reuse the clip-on module, so you're not looking at the (quite frankly ridiculous) $100USD MSRP for a whole new one.

It is the strap, well known problem.
The little clips where the centre clips on gets bad contact with the sensor strips, you can measure this with a multimeter.

Get a different brand strap to use, as long as it is ant + it will work, the Garmin ones are simlpy crap, nice and comfortable, but crap in the reliability department.
Pretty much most Garmin straps won't make it to the first battery change before the reading becomes unreliable.

Thanks guys. Thought that might be the problem. I think I have a Suunto ANT+ one laying around and will switch back to that instead of wasting time trying to fix this.

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Had some issues with mine as well. Went with a Wahoo strap and haven't had a problem since. It was recommended at the store, since they knew about the Garmin strap problems. Mine was a soft strap by the way...

Really?? I never knew this. I can use any HR strap that uses ant+? How'd I miss this? That's great news.

I've been using my Garmin hard strap for 3 years

jaretj

Polar makes an excellent ant+ soft strap that is available for under $20 on Amazon. You do have to do a small hack to get it to fit a Garmin unit. The Polar has a plastic/rubber surround around the silver snaps that prevents the Garmin from making a solid click lock. Simply use a razor knife to "circumcise" the top half of the surround and it works perfectly. (You may have to gently "saw" through the plastic until you feel the metal of the snap). Pictures from electrohero .com

After trimming