What country? What's the year and mileage reading? Is the emission control system OE or added on? If the car has a Lambda system it should have an O2 sensor and three way catalyst. Does it?

I assume the test was done at idle? Does your car require emission testing to be registered, or were the emission numbers taken purely for diagnostic use?

It appears that the basic mixture was leaned out, but the Lambda system should correct it back to stoiciometric as long as it has sufficient control authority (and assuming it's funtioning properly), and there should be no noticeable change in perceived power.

The fact that CO went down substantially and HC went up doesn't make much sense. If both HC and O2 went up I would suspect misfires, but the O2 reading was about the same. A properly functioning Lambda system (including O2 sensor) with TWC should show less than 0.5 percent CO at idle and close to zero O2.

Duke

P.S. There is virtually no NOx at idle. NOx can only be detected when the engine is under moderate load such as California's Acceleration Simulation Mode (ASM) test on a chassis dyno at 15 and 25 MPH, or the IM240 test.