my car, 89 300e failed emission test yesterday
with high amounts of HydroCarbon. my independent
mechanic insists my cat needs to be replaced.

since it is not cheap, is there a real way to test
if it is toast or not? thanks of any help.

Just to let you know, when I changed the cat in mine I went to a muffler shop and had them install a generic cat which cost me $150 including installation. It flows more than the stock cat and it costs only a fraction of the price. There is no need to buy an MB cat and spend a fortune.

Aside from that, there is no real test for cat that I am aware of.

The Cat should be the last resort in solving a high HC Level.
If your mechanic tested everything else, he or she is probably right.
More likely culprits are components that affect:

1)Mixture
2)Timing
3)Spark

The Cat is only there to deal with residual gases from incomplete combustion. Reduce these residual gases, and the Cat has less 'work' to do.
Make sure your car is completely warmed up before testing to give your Cat it's best chance of keeping up.
With some work, the exhaust going into the Cat could be compared to the exhaust coming out to show whether or not it is providing an adequate reduction in HC.

This is a good link on Cats:
http://www.howstuffworks.com/question66.htm

__________________
1986 300E 5-Speed 240k mi.

a quick test can be done with a digital thermometor like those used in meat packing. In a split second it can be determined if the cat is working if the outlet side is hotter than the inlet side. I have seen it done.

__________________
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T

thanks for all the replies and a good link.