In the end, it was the battery being low and a change solved the problem: „In Garmin Connect App and on my FR 255 the battery for HRM Pro was shown as „OK“. But during yesterdays run, an alert came up „Battery of HRM low“. I changed the battery today, also doing the „reverse polarity for 30 sec“ trick.Garmin sent me a new HRM Pro! I already had to change the battery after less than a year. The day before I was told "Battery OK", next day dead. Neither the watch nor the phone found the HRM. I changed the battery although the multimeter told me that it wasn't completely empty. But it worked again with a new one.
The pro plus battery door is good though. No worries about the contacts going out of alignment or having water seep in. And there's not really metal on the outside the way some straps have. XC ski power is reasonable. It's a lot more jumpy than running. But using 30s averaging it tracks fairly nicely with effort.
HRM strap issue - high readings. My cardiosensor began showing 220-250 bpm. I'm reading some forums but haven't found the solution. I replaced the battery in Garmin - no changes, then replaced the battery in the sensor - no changes. Most likely, the problem was in the strap, and it started after the last wash.I’ve changed the battery to my Suunto HRM many times. Usually I know the battery is dying by either not connecting at all or constant (long) drop outs during an activity. Other than that the Suunto HRM works without issue…when the battery is new. Sure the HRM pro loses its signal every once in a while, but it’s nothing crazy.
Press the “mode” button on the watch unit and check the display to see if the device responds. If the display appears frozen, attempt to reset the device by holding down all of the buttons for five to 10 seconds. If the watch unit fails to respond, install a replacement battery in the device. Press the watch unit's “mode” button untilnhFSxtn.