Did it fail for Nox? I'm assuming so since it passed a TSI (Two Speed Idle) test. If the head was surfaced, it may have raised the compression ratio just enough to cause high Nox. Also, was the O2 a Bosch?
Yes it was the NOX test I kept failing, as the head being surfaced. I don't know if it had been done before I bought it. When we did the valve job it did not appear to have been surfaced. I cant imagine how much would have to be removed to increase the compression ratio enough to make a difference. The O2 sensor is not a Bosch.