I keep getting a 'service engine soon' indicator.

They should honor the warranty if the converter is indeed the cause of the problem. The problem with your assumption is diagnostic fault codes never point to parts or say to replace them. They only indicate the circuit or system that needs further diagnosis.

The other problem is catalytic converters rarely fail on their own. The most common thing to happen is there's an engine misfire that sends lots of unburned gas into the exhaust system where it burns in the converter and overheats it. That can melt part of the catalyst which reduces the surface area the gases come in contact with. That reduces its efficiency and can set a "converter efficiency" code.

Another common problem is many universal one-size-fits-all converters don't meet the requirements of all the applications they're designed for. Even when they do work, their operating characteristics might put them right at the borderline of passing the Engine Computer's self-tests, and they may no longer pass those tests after being in use for just a little while. That seems to be the issue more with import vehicles but to be safe, the dealer has the correct converter for your car.

The first question is what is the exact fault code number? The next thing is to be sure the downstream oxygen sensor is working properly.

The last thing has to do with the shop that put the replacement converter on and whether they identified the cause of the failure. If they did not, they only addressed the result of the problem, not the cause, and that is their fault. If they did find the cause and you chose to just have the converter replaced, there is no way the manufacturer of the converter can be held responsible because there was no defect in manufacturing. He has no control of what happens if there's an underlying problem in the car. If you were told of an additional problem and you ignored it, that is your fault.

The last question has to do with the reason the shop won't honor the warranty. If the converter is doing its job as it was designed to do, the shop is right to not replace it, but to determine that they would have to perform a series of tests. They can't diagnose anything by just standing next to the car and looking at it. What reason did they give for not replacing the converter?