"I have calibrated the oxygen sensor heating controls for several applications, including both open loop and closed loop control types. This included all the option selections for whether to use certain specialized functions. Of course the main thing is setting the heater voltage as a function of engine operating conditions and service environment. In the end, oxygen sensor heater control is all about controlling the heater within a small range of desired temperature. That's easiest when you have direct information about the sensor's temperature based on heater resistance or sensor impedance. But you may not have that direct temperature feedback, depending on the application, and the control must be done in open loop based on the operating conditions. That can be a bit of an inexact science involving a lot of empirically determined coefficients. Getting all those coefficients to their best values can be challenging, partly because they can end up interfering with each other.
Most oxygen sensors in current use are also sensitive to thermal shock, especially when the engine is first turned on, and you have to find the right balance between that risk and the risk of increased exhaust emissions due to not getting into closed loop fuel control soon enough. It can be challenging calibrating legacy-type controls, which tend to be more empirical in nature, rather than physics model-based. Of course, those controls are constantly being improved, which brings its own challenges - you just get something working, and the next software revision has a new function or a new way of doing the old function and you have to start over. It's worth it though, to know that you took the control idea that was in one person's mind and married it up to the actual hardware and made it all work just like it was supposed to."
