Correct. As catalysts age, the temperature at which they will enable a given percentage of possible reactions goes up. One of the tests I posted a few years ago was catalyst temperature shot with an IR gun after driving, and then after five minutes of idling. The temperature reduction was significant.

I go to a drive through emission test station and hit them mid-morning when it's usually slow, so I don't have to wait too long. I turn on the A/C and open the windows and sunroof to keep some load on the engine and maintain 1500 revs until the tech is ready to drive it into the test bay. By the end of the test the aux. electric fans on on full, but that's good!

In all my recent tests with the R16/1 shorted and the vacuum line to the EZL plugged, my measured O2 was 0.0% at both 15 and 25 MPH, which means the cat was functioning at peak efficiency, and it showed with emission readings of only 10-50 percent of the cutpoints. My car only has 82K miles, so the cat is still in good shape, but M103s have always been marginal in California's ASM test, particulary HC at 15 MPH.

A lot of guys replace the OE cat with an aftermarket replacement, but often they don't do much better and degrade faster. (If CO is low then the O2 sensor is probably okay.) Retarding the ignition map as I discussed will considerably heat up the cat during the test in addition to lowering engine out NOx, so a car that fails marginally with the OE ignition map should pass comfortably with the retarded map, and if there is no detectable O2, the cat is functioning at peak efficiency and enabling all possible reactions. Once all the O2 is consumed, the cat can't do any more.

BTW, about 15 years ago I had high emissions and noticed that idle smoothness degraded. This was before California adopted the ASM test, so the test was the "two-speed, no load" test at 2500 RPM and idle. The idle HC was barely under the cutpoint. I think one year I only passed by 1 ppm. I finally traced it to the flaky distributor cap. The terminals should measure about 2K ohms, but one was "flaky" and it would measure open circuit intermittently, especially if I applied some force to the terminal. I bit the bullet and bought a new Bosch cap, which was EXPENSIVE, but it smoothed out the idle and lowered HC emissions, so as started previously, you should check your ignition system. All wires should measure about 2K ohms as should the cap terminals. Twist and bend the wires while measuring and the readings should be constant.

Also, make sure you have non-resistor spark plugs installed. Resistor plugs alter the voltage wave form and reduce spark energy, which can increase idle misfires, and this will raise emissions, especially in two speed no load tests.

Duke