Sorry to hear that it's past the warranty mileage; my luck tends to run that way, too.
Read the "Plugging" section below; it sounds as though you may have a restriction on the passenger side front catalyst due to two misfiring cylinders pumping unburned fuel into the exhaust. The "Diagnosis" section should help to determine if that is truly the problem.
One option is to try to find a used catalyst/Y-pipe and "gut" the converters. Muffler shops are usually hesitant to actually remove converters from a vehicle, but if you bring them the part, they should be able to do the welding and fitting. Someone around here might have that part cluttering up their garage. Of course, if Massachusetts uses the "sniff" test at vehicle inspections, running with no catalysts may pose a problem.
DIAGNOSIS
How do you know if a converter needs to be replaced? In many instances, the only clue that the converter may need to be replaced is an emissions failure or a performance complaint (lack of power, poor fuel economy, stalling after starting). In any event, the first thing you should do is visually inspect the converter and air pump plumbing for obvious problems such as rust, physical damage or leaky connections. Loose heat shields can create annoying rattles and vibrations. Severe discoloration on the converter shell may be an indication of overheating and possible internal damage, but you can’t always count on it.
In the case of a failed emissions test, higher than normal HC and CO tailpipe readings from a properly tuned engine, combined with higher than normal O2 readings and lower than normal CO2 readings, would tell you the converter isn’t doing much and probably needs to be replaced.
On 1996 and newer vehicles with OBDII, the on-board diagnostics will usually catch a bad converter. The OBDII system monitors converter efficiency with a second oxygen sensor mounted behind the converter. The system compares oxygen levels ahead of and behind the converter to see if there’s a difference. Downstream oxygen levels should be lower if the converter is working properly. Little or no change in the downstream O2 readings means nothing is happening inside the converter. This will usually cause the OBDII system to illuminate the malfunction indicator lamp (MIL) or the Check-Engine light and set a fault code. The generic OBDII code for low converter efficiency is P0420.
On pre-OBDII vehicles, converter efficiency can be checked with a four-gas exhaust analyzer – which is fine if you have one, but rules out this method if you don’t. One way to check converter performance is to read the tailpipe emission levels, then disable the air pump or aspirator and read the emission levels again. If the converter is working, HC and CO should both be lower, and CO2 should be higher. O2 should also be higher when the air pump is working. No change in HC, CO and CO2 readings would tell you the converter has reached the end of the road.
Another way to see if the converter is working is to look for an increase in temperature as the exhaust passes through it. This can be done by checking the exhaust temperature ahead and aft of the converter. We’ve heard of some people drilling small test ports into the pipes and inserting a temperature probe to measure exhaust temperatures. This technique works, but requires welding or plugging the holes afterwards. A faster and easier way to check exhaust temperature is to use a non-contact infrared thermometer. These devices are relatively inexpensive and easy to use. Just point the gun at the pipe and read the temperature.
With the engine at normal temperature and idling, the outlet temperature should be higher than the inlet temperature if the converter is functioning. On 1980 and older vehicles with two-way converters, the difference should be at least 100 F. But on 1981 and newer vehicles with three-way converters, the difference may only be 20 to 30 degrees. No difference in temperature indicates a defective converter or no air from the air pump. In this case, the air pump diverter valve and plumbing will need to be checked. An increase of 500 degrees or more indicates converter overheating because of a rich fuel condition (check the fuel system), misfiring spark plugs or compression leaks.
PLUGGING
Converting plugging is another common problem you’re apt to encounter. A plugged converter will create excessive backpressure in the exhaust, reducing performance and mileage. If a vehicle has only one converter and it becomes completely blocked, the engine will usually die after it has started and run for a minute or two. But on a dual-cat system, only one side will be blocked. The engine will still run but not very well because of the blockage on one side. In the case of a piggyback dual-cat setup, the front converter will usually be the one that fails first if unburned fuel or contaminants find their way into the exhaust system.
Converter failures of this type usually occur because of ignition misfire or a leaky exhaust valve that has been passing unburned fuel to enter the exhaust system. When the fuel hits the converter, it ignites, sending the converter’s operating temperature soaring. This breaks down and melts the honeycomb that supports the catalyst, creating a partial or complete blockage. Replacing a plugged converter will temporarily restore free breathing, but unless the underlying cause of the converter failure is also diagnosed and corrected, chances are the replacement converter will suffer the same fate.
If you suspect a plugged converter, check the engine’s intake manifold vacuum at idle. Most engines should have about 18 or more inches of vacuum at idle. A lower than normal reading is a classic symptom of excessive backpressure in the exhaust, especially if the reading continues to drop (indicating a backup of pressure).
Backpressure can be measured directly several ways. One is to measure it at the air pump check valve by removing the check valve and installing a pressure gauge. The check valve must connect to the exhaust system ahead of the converter, however if the plumbing hooks up at the converter, this technique won’t give you reliable results. Backpressure readings should generally be less than 1.5 psi (though some do allow as much as 2.75 pounds at idle). Rev the engine to 2000 rpm and note how much the reading increases. If it is higher than 3 psi – or keeps climbing – you’ve identified a restriction problem.
Backpressure can also be measured by removing the oxygen sensor from the exhaust manifold and installing an 18 mm adapter to hook up a pressure gauge. But this will only tell you whether there’s too much backpressure in the exhaust. It won’t tell you where.
