There's nothing functionally wrong with the emission control system. The high HC at 15 MPH is most likely due to catalyst aging and is typical for Mercs of this vintage and many other older cars. As catalysts age they must be at ever higher temperature to catalyze the same amount of reaction as when they were new or near new. Also, O2 sensor response time degrades with age, which can also contribute to increasing emissions on successive tests, but your test results are a classic case of catalyst aging.

Note that the O2 reading at 15 MPH is 0.22%, and it is only 0.04% at 25 MPH. The load from the 15 MPH test steadily increased the catalyst bed temperature, so during the 25 MPH test more of the available O2 was consumed in oxidation reactions, which significantly lowered HC emissions. For every 0.1% reduction in O2, the HC count will drop by about 30-40 PPM, and if the catalyst is operating at peak efficiency (assuming the Lamda system is correctly adjusted and operating properly), the O2 should be less than 0.1%.

The O2 content is a key diagnostic tool. If O2 is high (above 0.1%) at 15 MPH and drops at 25 MPH, along with HC, the catalyst was "cool" at the start of the test and not operating near 100 percent conversion efficiency.

Also note that your NOx is commedably low. Does your engine have EGR?

I have written a TON of emission posts that are in the archives including how to "manage" your emission test, how to understand the gas analysis report and use it as a diagnostic tool, and how to reduce emissions with an aging catalyst that passes by a slim margin (or marginally fails) by altering the spark advance map - including various test results on my '88 190E 2.6 (77K miles, original catalyst and O2 sensor, lamda duty cycle 50-55/40-45 percent @ idle/2000, which means the basic mechanical mixture is a bit rich at idle - better cold starts and cold driveoff prior to going into closed loop mode - but within, barely, the OE spec of 10 percent max variation between idle and 2000.) My last emission test in February gave similar results to those that I reported from early 2005.

Just search suitable terms like "emissions" and my screen handle.

Now I have a couple of additional questions for you.

1. What is your car's mileage, and on what date was it tested?

2. Do you know the history of your O2 sensor - is it original or has it been replaced? If replaced, how many mile does it have, if you know?

2. What is the software version number on your test report? It is printed in the "Smog Check Inspection Station Information" block just above the signature block.

The reason I ask is that I had my '91 MR2 tested last week. The software version is 0703 (March 2007) and O2 is listed only in tenths, not hundreths of a percent. Your car must have been tested with a newer software version, which is now listing O2 to the nearest hundreth of a percent, which is good.

Duke
Retired Emission Control Research Engineer