Some say to run a couple of tanks of Chevron Techron, BG-44K, or something similar, then change plugs. Run premium fuel. The day you go in for the test, take car for an extended drive - get it good and warm and hope to have a tech examine it before it starts cooling down.
You may search the archives here. Folks who actually have to deal with this likely have more advise than what I've offered.
I personally would not spend $$$ on a high $$$ item like a cat converter until I tried the simple, less costly things.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.