When my girlfriend goes for walks and tracks it in GPS mode the heart rate is accurate. When she forgets her phone and just uses cardio activity tracking, it's wildly inaccurate. Today it said her heart rate was between 167 and 185 the entire mile walk around the neighborhood. We checked her pulse with the 15 second rule and it was 90bpm. This issue only happens if she uses cardio activity. The other activities seem to show the correct hr.