Car runs great but K is off?

[Fugge]

Ive been fiddling with my K-constant all spring but i just cant get it right.

I have turned off O2 and adjusted it so it runs good at low load just around lambda 1.0.
When i turn on O2 STFT immediately goes to -15% - -25%, still running good att lambda 1.0.

Overall it runs great throughout the whole map.
Could it be a faulty O2-sensor even though it runs good?

Setup:
DW 800cc inj
HPX Z32 MAF
DW fuelpump holding fuelpressure.

On a side note, when switching to E85 in the future. Do i use K-constant for adding more base-fuel in the map?

[Matt]

If you have feature pack loaded in your board then use Total injection Multiplier instead

If you are holding around 14.7:1 on your wideband then it should not trim against this when you enable O2 sensing

So try taking the vehicle out on cruise and watch the AFRs and then once you have those about 14.7:1 then enable o2 sensing (restart vehicle) and then it should not need to adjust

O2 sensors need replacing about every 50,000 or so kms