the best way to test the operation is if you can get hold of an oscilloscope, or lots of later Mot gas analysers have a lambda test function
measure the signal and it should look like mine, alternating from rich to weak around the stoich. point

measuring lambda and CO are not ideal as an exhaust blow will raise lambda and drop CO
which sun analyser was it , dga 1800 or 2500? was the reading you're talking about at the top or bottom of the screen?

cheers
Stu