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Failed Smog Test

1995 BMW M3 Gross Polluter High HC & NO @ 15MPH

I tried to test my BMW and failed bad and am now classified as a gross polluter. I have had trouble in the pass passing smog but the previous time I got smogged I replaced the catalytic converter and 02 sensor through recommendation by the smog tech I was working with and passed fine. Now that the catalytic converter is 1 year old with less than 5k miles on it I failed as a gross polluter. At 15mph my HC measured 210 PPM and my NO measured 2729 PPM, at 25 MPH my HC measured 52 PPM while my NO measured 209 PPM. What should be the next step I take?


Having replaced the catalytic converter last time around may have not solved the underlying issue. A smog station should have conducted a thorough diagnosis prior to condemning the last CAT. It may have been that the CAT was replaced unnecessarily, and the real culprit was a problem with the engine's air/fuel mix ratio.

It's possible that something is causing your BMW to run "hot", resulting in the creation of high NOx. The fault may be due to a lean fuel condition, high compression due to carbon build-up on the piston heads and valves; pretty much any condition which would cause the engine to run hotter than normal.

We recommend starting with a wet/dry compression check, followed by a fuel feedback test. This will ensure your BMW is in proper fuel control. Conduct a vacuum leak test next. This will show whether any unmetered air is entering the intake system; making the air/fuel mixture too lean.

Above all this, make sure you're filling up with high octane fuel. Higher octane burns cooler.

posted by SmogTips Support 09-30-2021 03:39 PM

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