I recently took my car in for a 100,000 mile inspection. It felt as though the engine was “working hard”, the check engine light had come on and I was also preparing for a daily commute of 100 miles.
Everything came out okay in the inspection except for needing a new battery and the diagnosis of a blockage in the catalytic converter. I was quoted 773.00 for the part and 204.00 for the labor. I checked prices online, saw converters ranging from $85-300, and contacted the mechanic (who is reputable and has provided honest and fair service in the past). He explained he has run into countless problems with after market converters, which are not as specific to the vehicle and also the sensitive computer systems not responding properly- leading to check engine lights coming on again after service. He also noted they do not have the efficiency of the factory converters. I contacted the dealership and was quoted a similar price, and given the same explanation.
So- my questions are: Is this diagnosis, explanation and quote fair and reasonable? Is changing the converter fixing the symptom of an engine problem which caused it in the first place, which may lead to continued problems down the road. (no pun intended. 
On the cat converter, it is a perfectly reasonable thing for your mechanic to go OEM only since the story about problems with aftermarket converters is true. As far as I know it tends to be a problem only with certain makes/models. I don’t know what those are but I’d not be surprised at all if VWs were in the mix. You mechanic’s problem will be that if an aftermarket cat goes on and doesn’t work right then he’s on the hook for it. If there’s any question why take the risk?
You could ask to have an aftermarket converter put on anyway, but you’d have to agree to relieve the mechanic of responsibility. Then if things go wrong, you pay all over again.
So you have 2 quotes. You could also get a second opinion on diagnosis. No one here can help you with the diagnosis since you’d need to provide 1) a better description of what the car was doing; 2) the specific error codes that were stored along with that check engine light.
I have no idea what this means: “Is changing the converter fixing the symptom of an engine problem which caused it in the first place” - but if the question is whether a blocked cat and sluggish engine can & do go together, the answer is yes.
Yes, other engine problems can damage a cat converter.
Especially anything that allows unburned fuel to reach the converter, like a bad ignition coil.