My 1995 BMW M3 had failed a smog test and was classified as a gross polluter. I had the cat converters changed and that did make a big improvement. However it still did not pass the smog test. The engine had a lean misfire and my normal mechanic could not isolate the reason, no engine check light was on. The staff at Villa Automotive was able to identify the source of the problem, a sluggish O2 sensor, and the car passed the smog test after it was replaced. This was not an easy fault to diagnose because the O2 sensor was still working and did not produce a normal error code.