Every sensor anyone has built, or will ever build, to read any physical property, does not read 'perfectly' with no errors in accuracy or precision. They almost all, regardless of what they measure or how they do it, as based on a scientific response models that connect, for example, how resistance changes occur with varying temperature. It ain't perfect. They are designed to deliver a specified accuracy and repeatability. We as consumers do not see this data when we buy a gauge.
They are however, calibrated to internationally accepted standards (which also have errors, but much smaller ones) that everyone who builds this type of sensor is required to calibrate their sensors to, so that they can be responsible and accountable engineering businesses. Some comply, some don't. Companies that buy these sensors tend to buy from trusted sources. You can't as GM or Nissan, afford to buy whatever sensors you like from the cheapest vendor when they lay at the heart of how the engine runs.
Nothing you build at home will even remotely approach the precision, accuracy and repeatability or response time of what commecial O2 sensors and their related electronics now provide. It is a wonderful experience to try to do better, you will learn a great deal while you do it.