I'd been having a difficult time getting my '86 740GLE through the Colorado emissions test... The car had been running fine, so I didn't expect much problem; put in new NGK plugs, changed oil & filter, and installed a new air filter - car failed the test. I muttered some unkind words, then went through the engine compartment, replacing vacuum hoses, checking plug & coil wires, and all the usual suspects... The car ran better, smoother idle, etc., so I took it back for the retest - the results were worse than before! Having reached the end of my abilities with the tools & facilities I have, I took it into the mechanic's.

Hooking it up to the exhaust gas analyzer & doing some basic tests, the process of elimination pointed to the O2 sensor; I had initially ruled it out because it was only a year old, with less than 20k miles on it. When the sensor was removed, it was obviously the cause of the problem... The metal "hood" protecting the sensor element was completely gone (probably lodged in the catalytic converter by now) and when the sensor was set down on the bench, the ceramic fell apart! (The sensor was a Walker brand, sourced from my local NAPA parts store.) Installing a Bosch sensor & tweaking the AMM to get the exhaust gas numbers to spec was only a 15 minute job; took the car back for testing, and it passed handily.

Lessons learned:
1 - Just because a part is relatively recent doesn't mean it hasn't failed.
2 - I'm using Bosch O2 sensors from now on.

(Also, many thanks to Dan at Swedish Motors in Boulder, Colorado for his patience and help.)
--
Tom Harper ('86 740GLE; NØMKC; Denver, Colorado)