I am having a problem with "rich" exhaust and a rattling catalytic converter. I assume the internals of the cat are shot due to the internal noise. My question is concerning the o2 sensors and the rich exhust fumes: Do I need to replace the sensors or will a new cat solve this problem. As soon as the cat started to rattle the rich fumes began to be noticiable. I don't have access to a diag. code tool. Any suggestions? I plan to replace the cat myself. s70 w/130,000

The sensor in front of the CC helps control the fuel mixture and the one behind "checks" to see how well the CC did at cleaning up the exhaust. A bad "Pre cat" sensor can give a rich mixture (just been through this with ours) which isn't good for the CC and will ruin it over time. The Cat is passive in the process and doesn't cause the rich condition. If you install a new one w/o tracking down the cause of the rich condition, you'll ruin that one as well. The o2 sensor may or may not be faulty and instructions for checking them are in the 700-900FAQ. A vacuum leak will "fool" the sensor into enriching the mixture and a bad ECT sensor will fool the ECU into thinking the engine is stone cold all the time. Do you have a Check engine light on?

joey

the check engine light did come on when I began noticing the exhaust problem.

Check Bay 13 http://volvospeed.com/bay13.htm He will run you through some (free) troubleshooting tips such as where to look for vacuum leaks etc. I'd try to eliminate the simple, no cost options first before condeming the O2 sensors. As the sensors fail, they usually set a CE light code.

bl