My car is running rough, but not too bad really. Yet my mechanic is telling me I have a failing CAT which I don't quite belive. I have a ECM code280A for the front o2 sensor. Could this be a vaccuum leak I am experiencing and not the sensors or the CAT. I have replaced the rear o2 sensor 2x. Would it be worth reeplacing all the vaccuum elbows and lines before the CAT? Is it resonable to expect the CAT to fail at 100k mi. And is it resonable to expect the vaccuum lines to crack with age tripping the o2 sensors and possibly the CAT?

I'd suggest if you didn't do it yet to reset the system and see which codes reappear - just to be sure the set of codes you have represents what is really going on.

If the cat is not working to clean the unburned fuel this is detected in the OBDII system by a comparison of rear vs front oxgyen sensor signals - if the system's internal criterion is not met one or more codes will set. These may not tell you directly what is defective - they only mean the cat isn't doing its job. If you clear the codes for this they may reappear after some days as they are set based on a set of driving cycles.

Usually if one of the o2 sensors is bad a more specific code will be set - such as for slow response, heater circuit not operating, etc. Evidently from various posts on this board the sensors do get tired after some years of use.

The cat may not work right if the engine is running too rich or too lean - the fuel is then too dirty for the cat to cope with. If that's the case then fixing up vacuum lines, ignition, fuel pressure, MAF sensor, etc would be the way to go. These things do loosen up over the years. The cat may not be defective but will go bad if it is fed with very dirty exhaust for a long time and becomes saturated with crap. With vacuum leaks, etc., sometimes the code you get (if any) is also not exactly related to a particular part. For instance with a big vacuum leak, you could get a MAF sensor code because the sensor can't cope with the leak, even though the sensor is still ok.

Another possibility is that your cat or something else in your exhaust system is clogged up and restricting flow - this can happen as they age and fall apart internally. I had one car that did this and it would stop running right after a half hour or so on the highway once the cat got really hot.

I did have the mechanic reset the system. No lights have come on yet. Although I am waiting. Can I ask my mechanic to replace the vaccum lines and clean the MAF. Would that be a good place to start? And is this something that should eventually be done anyhow? I believe at 102k mi. I am due for a new fuel filter. Will that make any difference? Neither mechanic has mentioned that. I just read that in my manual. Why wouldn't they do this in the first place? I am working with two indy volvo mechanics here. All they work on is Volvos. I am a stay at home mom and I have to say I feel like an ass making such requests because I don't know a thing about cars. And they know it. I wouldn't mind at all getting my hands dirty and learning to do things for myself. Thanks for the input all.

If there are problems in the cat or o2 sensors it may take some days for codes to reappear. Several different fuel related codes could come up as I said without it necessarily being those parts. If you get some, maybe post what they are.

When you say the engine runs rough can you describe this more? If it is rough at idle but otherwise ok then the throttle body may be dirty. On the older 850s this is a purely mechanical part which seems to need cleaning every 2 or 3 years. I don't know what is on your car but if similar you could ask your tech to clean it.

It would be a good idea to inspect vac lines for clogs, looseness, cracks. The ones to the idle air motor (if your car has one) would be suspect, as well as the crankcase vent (PCV or rebreathing lines). Also the bellows that connects the air filter box to the throttle bore. Sometimes one or more of these will separate, crack, or fill up with carbonized gunk. Is your air filter clean enough to let light thru?

If something is wrong in the fuel system the car could be hard to start, emit smoke from exhaust (black smoke) or lack power at highway speeds (could also have rough idle). Fuel filters are often replaced after about 100K miles but this would depend on what the filter experienced. If no dirty gas was ever in the tank the filter might last for ever. If lots of dirt then the filter could clog soon. A good test for the fuel system is to check the fuel pressure with a gauge. If the pressure is too low or fluctuating or too high, then pump could be weak, filter clogged, regulator sticking or injectors or lines leaking. Checking is not a big job.

If the MAF sensor is not right the engine may have many different symptoms. There should usually be a code and it would probably come up right away. The MAF sensor on Volvos I've had is self-cleaning and doesn't require attention. It is a delicate part so shouldn't be opened up or disturbed without reason. If it quits it will usually burn out suddenly and then the car will try to start but won't run at all. The electrical connector on the MAF can get loose or dirty and cause erratic operation of the engine. On some other cars (read Ford) the MAF sensor does get dirty and needs to be cleaned.

good luck