I ran it decatted for years no issues.

The check engine light can be cleared by yes a tune most proper way. I have read something about buying the correct resistor and tricking the ecu thinking there is a working cat in the car.

I bought this small cat converter that screwed onto the o2 sensor bung and gives okay reading. Problem is every few months it got clogged with carbon build up had to clean it and the light went away.

PS from memory K series ran 2 O2 sensors primary (sits before cat) and secondary (sits after cat)

Its the primary one that measures correct AFR's etc etc to keep ur motor running.