Catalytic converters are a sort of devil's bargain for many car owners. They do a lot to clean the emissions of the vehicle when it's operating, but they are tricky, sealed, and heavily regulated units that, when they go bad, affect both the performance of the car and its ability to be legally licensed for road use.
When a catalytic converter does go bad, it usually requires full replacement and that can be expensive. While the unit itself is easy to remove and install, it can cost several hundred dollars to purchase.
So, are there ways to fix a catalytic converter and save that money? That, of course, depends on what's wrong with it and determining that may mean you'll need some mechanical skills. But yes, there are ways to repair a catalytic converter in some cases.
Note: the laws regulating catalytic converters and what you can and cannot do to them vary by location and are usually very strict. Be sure to know the applicable laws in your area before you proceed.
Note also that objects that are part of the engine exhaust system can be extremely hot! Make sure all are cool before working on them.
If your catalytic converter has a buildup of carbon or oil, it may be possible to get that off of there with simple fuel additives that help to burn that stuff away. Several options are available here, but they should be able to do more than just your injectors. Some additives are catalytic-specific and are likely your best bet, costing around $20 a bottle.
Another option, if the converter is really messy or even somewhat clogged, is to remove it from the vehicle and soak it in a bath of diluted citric acid. This leaves the platinum and other metals for the catalyst in place without harm and cleans most of the deposits that are likely clogging your converter. The soaking should be overnight or about 6-10 hours. Make sure this is legally allowed in your area - it is in most.
This is a fairly complicated process that requires some special (but cheap) tools. You'll need to remove the catalytic converter from the vehicle so you can have access to both sides of it (inlet and outlet).
Using a mechanic's mirror or scope, look for the loose pieces of substrate by shaking the unit as you look. Once identified, use a long-bit on a drill to add drywall screws to the loose pieces, screwing them into place.
This is a temporary fix that will only delay the ultimate breakdown of your catalytic converter's substrate in time, but it can delay your having to purchase a new converter for a few weeks.
Fixes and Repairs
These fixes for a problematic catalytic converter are only useful in a few circumstances. Most of the time, full replacement is probably going to be required. Because these units are required in most states, that is unavoidable.