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.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"