a narrowband o2 can't really target 15.8:1, so if your trims are truly landing you at 15.8:1:

- messed something up in your bin regarding o2 configuration
- o2(s) is(are) bad
- o2 wiring or ground or something is bad
- the 15.8-16.1:1 has a measurement error.

my hunch says its the last one