Your repair reminded me of a similar experience that I had had a few years back on a late-90s Toyota truck with no EGR, no codes, and a significant ASM NOx problem. I was able to determine that the HO2S was too lazy to keep the catalyst happy, but not lazy enough to code.

A fresh HO2S made a HUGE difference in the NOx emissions, enough to surprise me __and__ the bureau representative who called me up after remotely viewing the passing report to question .. er .. congratulate me on the success of the repair. That was one of the few completed CAP repairs in which I've participated that came in under the spending limit with a solid pass.

Sincerely,

Michael Barry
Technician
Quick Stop Smog & More
Sacramento, California, USA