I have a 1991 300CE with about 150m and I've put up with a rattling cat over the past several years. At each successive smog test, I'd make sure the engine was good and hot and kept my fingers crossed. Since the rattling started, I think I passed 2 tests (every 2 years) but now finally failed one and here are the results:

-----------CO2-----O2------HC max/meas-----CO max/meas-------NO max/meas
15mph----14.50----.07---------106/18----------.70/.04-----------762/1082 failed
25mph---- 14.50---.08----------81/14-----------.57/.02-----------701/1044 failed

From other threads, it looks like could be the EGR valve or most likely a new cat is needed, both expensive.

Any help in narrowing this down?

Thanks,

glenmore
1991 300CE
2000 C280
1990 LS400