Definitely make sure your exhaust is tight. I use solid copper gaskets everywhere and double nut the collectors. Even a slight air leak will affect the readings.
I also let the 02 sensor warm up before starting. Mine goes to 22.4 when ready when the motor is cold.
Once in a great while mine will lock at 15.1. I just stop and shut the motor off and restart. Usually this happens if I don't let it warm up before the first start of the day.