I am looking at a 1999 Volvo S70 that the Mechanic
Where I'm doubting your mechanic, I still have put my input below since this is a common topic that comes up a lot. Unless the catalyst is known failed (such as being hit by a large rock or rusted), the below items are things to consider. MAF sensor failure generally does not cause cat failure if it's caught in time. Even still MAF sensor failure is quite rare nowadays though they do require cleaning using MAF cleaner (not brake cleaner!) and careful cleaning of the electrical connections.Keep in mind that detection of a truly-failed catalytic convertor failure is difficult, given other variables, such as engine tune and o2 sensor problems. The below article illustrates some of these items. Unless the catalyst is mechanically damaged (rusted, hit by a rock or otherwise damaged it will be difficult to say if the cat is truly bad. Even emissions readings may not accurately detect it - in fact high emissions on any vehicle are usually due to the need for a tune up or malfunction of some other emission control system, such as the EVAP system. High CO content does not necessarily mean the catalyst has failed. Consider the condition of the engine, air filter, plugs, efficacy of the ignition coils (the 1999 models have coil over plug type ignition systems), o2 sensor condition and electrical connectivity. I had a 1998 s70 that was suspect here (with many misfire codes) all being set by poor electrical connections in many of the connectors. So always take these into consideration before replacing a good cat with another good cat. Usually, unless the cat is obviously broken or otherwise obviously damaged, the only real way to check one is to physically remove it from the vehicle and test it from there. However, if you read on, there are some clever ways that manufacturers have attempted to detect catalytic convertor failure.Catalytic convertor failure in the field is rare. It is usually caused by large quantities of unburned fuel entering the substrate and overheating it. If there are ODBII misfire codes (P0300-P0305), this is a possibility, especially if the engine is truly misfiring. Misfiring is a cat killer, and will quench the substrate rapidly if the condition isn't corrected. If the catalyst substrate becomes completely blocked, the engine will not run. Detection of convertor failure in the field is usually accomplished by having two oxygen sensors installed, one ahead of it (the upstream sensor) and one behind it (the downstream sensor). The theory behind the extra sensor is that it should be showing relatively fewer transitions from low o2 content to high o2 content relative to the upstream one which cycles rapidly between those states. If the two sensors start to mimic each other in activity, one could reasonably assert that the catalyst is beginning to break down to the point of failure.The MAF fitted on these models can be ruled out quite easily by powering down the vehicle, cleaning it with MAF cleaner (not brake cleaner!) and then powering up the vehicle. Drive the vehicle in accordance with two OBDII relearn trips and see if a code sets for the MAF or either o2 sensor; rule these components out first before condemning the catalyst. Generally, when a catalytic convertor has completely failed, the engine stops running. This is due to rapid overheating of the catalyst and subsequent blockage of the exhaust by the substrate having melted, forming a large blockage in the exhaust.Bottom line - rule out everything else before paying a large sum of money on an item that isn't defective.
