Despite my confidence in having a (relatively) new catalytic converter and a new O2 sensor, my lovely 1994 BMW 325i Convertible got flagged as a "Gross Polluter." The test results are summarized below:

CO2   O2    HC    CO    NO
13.6  0.1   333   2.00  ~80

Essentially, HC and CO are far too high while NO is fairly low. Also, CO2 is low, indicating lean conditions.

What are some possible causes, and some ways to investigate them, that could cause the test results?

My current thoughts are:

  • Compression Loss
  • Fouled Plugs
  • Faulty MAF
  • Excessive Oil from PVC

3 Answers 3


The supplied gas values suggest a rich run condition and possibly a non-functional catalyst. Using a mixture calculator for the supplied gas values shows that the mixture is 6.4% rich.

CO elevated is the key indicator as it is not present in lean combustion in significant amounts.

NOx is produced in significant amounts in lean combustion and requires high gas temperatures usually on seen under loaded conditions. Little is produced at rich mixtures.

CO2 is maximized at stoiciometric mixture. So low CO2 can mean rich or lean.

FWIW in my vehicle emission testing region (Seattle WA) this engine would pass a tailpipe emission test.

Refer to the graph below for with help with gas value interpretation. The left side of the chart is rich and the right is lean.

Gas analysis graph

Of the list of possible failures the MAF is a possible cause of a rich condition. Fouled plugs would result in much higher HC due to misfire. Low compression would likely be corrected for by a properly functioning fuel control system including an accurate MAF reading. Oil in the combustion chamber does not greatly affect combustion gas values until it gets to excessive amounts. Failures of the PCV system do usually present as lean run problems.

  • 1
    This is remarkably thorough! Thanks @FredWilson! The chart is especially useful for interpreting the results. Since the cat is relatively new, I suspect the problem is a rich burn condition. I've also noticed a sharp drop in my fuel economy, which corroborates a rich burn. I might try to read my O2 sensor to validate this. Aside from the MAF, are there any other usual suspects? Also, as an aside, I'm surprised it would pass in your region, but CA is very strict.
    – Hari
    Mar 3, 2017 at 5:25

The most likely culprits are clogged fuel injectors, carbonized intake & exhaust valves. The best solution is 1 oz. Marvel Mystery Oil (MMO) for every gallon in the gas tank, and a fresh oil change with 20% of the oil substituted by MMO.

"Rich" and "lean" become too confusing because both can happen at the same time. When the injectors are dirty, they do not atomize fuel effectively. Micro-globs of gas pass through the combustion chambers unburned -- effectively creating a lean condition in the chambers -- so that the O2 sensors add more fuel to the mix, which in turn produces a rich mixture in the exhaust and uneven flame in the chambers.

The cars EEC attempts to compensate by vacillating between more and less fuel every few seconds, which will also confuse the values on a scanner.

  • From what I understand, O2 sensors are supposed to fluctuate normally. The reading should be periodic as the car oscillates slightly around a stoichiometric ratio. Also, I've never been particularly sold on additives. Test results vary wildly across brands and even within brands, indicating that there is no definitive proof they either work or don't work.
    – Hari
    Mar 3, 2017 at 16:29
  • I'm skeptical about additives, too, but I've actually seen the results for myself on pistons and cats with MMO. Just be sure to take the engine up to 3,500 RPM for 15 minutes every quarter tank of gas. You can combine MMO with Berryman's B12 (acetone) to accelerate the process.
    – Carguy
    Mar 3, 2017 at 23:09

Hope this helps anyone:

First thing to do was to look at the LTFT(Long Time Fuel trim); you´d need a scanner for that,but unfortunately since it's before 1996(if in the US),it does not have OBD2; if it did you could use a 5 dollar scanner with bluetooth,and your cell phone;

In this case,your scanner is gonna be: (wait for it...) the spark plugs: each will tell,by its color,rich or lean; well,sort of.

ANYWAY, for an ICE, high CO and RICH are "synonyms",meaning that a rich burn ALWAYS produces high(more than 0.5%) CO.(do not confuse w/CO2!). Now,all you have to do is make your mixture lean: check fuel pressure,to see if the regulator is ok or not,and go from there; the MAF and the temp sensor control that too.

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