I am not so sure about the O2 sensor.
Usually, when they fail ( or ail ), you end up with an overly rich mixture, which in turn would lower exhaust gas temperatures, i.e. NOx.
Yes HC readings would help also.
The puzzle is, why is he passing the pre-test & failing the actual test ?
Is it different test procedures, or does somebody use inaccurate equipment?
2007 C 230 Sport.