I'm new to the board - second posting.

I just picked up a 95 E420 with 86,000 miles on it. Beautiful shape, well looked after with complete service records - every service is stamped from PDI to today.

Of course it had problems with the wire harness, now replaced, and a problem with the ETA and engine thermostat, both now replaced as well.

Here in BC we have "AirCare" emission testing, and once the repairs were done I took it in for an overdue inspection. It failed in a spectacular way:

HC - measured 1.4 vs. limit of 0.5 (grams per km) - way over! FAIL!
CO - measured 7.7 vs. limit of 9.3 (grams per km) - passed, but only just
NOx - measured 0.8 vs. limit of 1.2 (grams per km) - passed, some margin

The test is a "rolling road" dyno test, ranging from stopped to 55MPH, with speeding up and slowing down through a complex test cycle that runs for 4 minutes. The faster it went, the higher the HC and CO, and even NOx was spiking during acceleration (if it flunks, they print a scan of readings vs. time including speed). The tests are done by Government certified labs that don't do anything else, no mechanics, no advice - just testing.

Since the car had been sitting on the dealer's lot for a couple of months, and then had been through the shop with a bunch of troubleshooting and new parts, I figured a "good run in the country" might help.

The next day, I took it for a run on the highway, and drove for about 150 miles or so, and then took it in hot for a retest. The tech ran it for 1 minute or so at 40MPH on the dyno to heat up the cat (very thoughtful of him) and then did the cycle. It failed again, but not by much:

HC - measured 0.6 vs. limit of 0.5 (grams per km) - still over - still FAIL
CO - measured 1.9 vs. limit of 9.3 (grams per km) - passed, much lower!
NOx - measured 0.4 vs. limit of 1.2 (grams per km) - passed, much lower

Armed with the new readings, I went back on the highway and notched it into third gear and drove another 100 miles at 70 MPH or so.

The last test results were a PASS:

HC - measured 0.2 vs. limit of 0.5 (grams per km) - Clean PASS!
CO - measured 0.9 vs. limit of 9.3 (grams per km) - a big drop!
NOx - measured 0.3 vs. limit of 1.2 (grams per km) - another big drop!

Can any of the experts here help me understand what was happening?

I did the fast driving because of an intuitive belief that it was the right thing to do. Ok, and it was. But why?

Was this the LH module "learning" the new components? Or was it fuel saturation in the whole system (the LH sensor was inoperative from the wiring harness and it was running super rich for the last few weeks, if not months).

The improvement in emissions was startling - HC down by 84%, CO down by 88% and Nox down by 65% from the initial readings. I went from expecting to spend big $$ on fixing emissions to getting it all done for the price of a tank of gas and a day cruising the freeway and listening to a great stereo.

Any insight from the experts would be welcome!
Thanks
JJ