Wearable devices are increasingly being used for monitoring health signals and for fitness purposes with typical uses being calorie tracker, workout assistant, and step counter. Even though these wearables can measure many health signals (e.g. heart rate), they are still not perceived as highly accurate, relative to clinical monitoring devices. In this paper, we investigate the accuracy of heart monitor as included in two popular wearables Motorola Moto 360 and the Apple Watch. We analyze the accuracy from a hardware and a software perspective and show the effects of body motion on the heart rate monitors based on the use of photoplethysmography (PPG) signals used in Android wearables. We then do a software reliability study of the Android Wear OS, on which many wearables are based, using fuzz testing.