Changing plug heat range has no effect on changing engine or cyl temp. Plug heat ranges are designed for matching engine and induction types to a given plug.
A hotter plug will glaze in a leaner engine a cooler will carbon foul.

BG44k is the best bet if everything else tests OK.

O2 sensor can have an effect on NOx, but most times it would have to be so bad it would effect other areas, like very poor MPG.

NOx is the most missdiagnosed etest failures.

The only way it is measured is under load, making it tough to duplicate in a shop.

SD