This is an iOS bug. Please try to soft reset your phone following the instructions on this page.
It's essential for Pulse Oximeter to have access to the Camera. Please go to the iOS Settings App, scroll down and find "Oximeter", and allow Oximeter to access the Camera.
After the first successful measurement you'll get a pop-up which asks for permission to write to Health. Turn on sharing for both heart rate and oxygen saturation and you're good to go!
The slim design of the iPhones 6/6+ results in a protruding camera lens. When placing your finger on the camera and the flash, make sure to only touch—not press—your finger on the camera and flash. If the protruding camera makes an indention on your finger, the app won’t work properly.
You might be pressing too lightly/hard down on the camera lens or your phone might be running hot (an issue with the iPhone 4S). Make sure to wait for the calibration to finish. If you keep getting "--" warnings, remove your finger, tap "Stop", then "Start", and replace your finger on the flash and camera lens after ½-1 second. Also, the app doesn't measure values below 93%.
Press the "Home"-button to exit the app. Long-press the app icon, then tap the "X" to remove the app. Reinstall it from the App Store. This will erase all your data, so export first for safe keeping.
Be sure to hold the phone as shown in the App Instructions. Alternatively, try to restart the app.
The app can be susceptible to tremors or other movements. Try sitting and resting your hand against a table or some other solid structure.
If it is completely dark, camera-calibration will be slightly off. Even a single, minor light source in the room will result in better results. You might need to exit and re-enter the app for the calibration to reset though.
From update 1.3.2 and forward you no longer need to squeeze. Just cover the flash and camera lens evenly with your finger.
If you manage to get steady results while resting it is actually possible to get steady results during exercise as well. Try it out and let us know what you think.
We recommend that you remove the case as some cases interfere with the accuracy of the app.
You can try to fidget your finger a little around, apply more or less pressure or exit the app and try again. The data curve will change to show a better/worse signal as you do this.