I agree completely with Dnkldorf. I always inform customers we only come back to service trouble on the motion sensors we purchase from the parts house. If they want hardware store or box store sensors they are the customers problem after we leave. It works. Very few, if any call backs over twenty years using the high end sensors.
Regarding warranty work I find I have to treat every situation differently. /some are completely free, some labor is charged parts are replaced. Do not get much in the way of warranty work on our installations so it isn't a big headache.
Ron