Check Engine Soon (P0430) after self replacement of radiator fan

You don't have to disconnect the battery to replace the radiator fan as long as the ignition switch is off. In fact, on newer cars a lot of the insane engineers are getting carried away with things that need to be reprogrammed or unlocked by the dealer after doing so.

It's pretty likely the two things are not related. You'll need a full scanner, not a simple code reader, to see what the rear oxygen sensor is showing. Normally the front O2 sensor will switch between rich and lean about two times per second. When the catalytic converter is working properly, the rear O2 sensor will switch perhaps once per minute indicating the converter is cleaning up the exhaust.

When the converter loses its efficiency, as this fault code indicates, no change is taking place in the composition of the emissions. As a result, the rear O2 sensor starts switching between rich and lean faster and faster until it approaches the same switching rate as the front sensor. Both sensors are seeing the same thing so the switching rates are the same. That's what the computer looks at to know when to set this code.

Most likely the catalytic converter will need to be replaced.

Now that you know the fault code, you can try to erase it. You'll need a regular scanner though to view live data, meaning sensor readings, and what the computer is doing.

Those of us answering questions here are in all different cities so we aren't familiar with shops and which ones are especially good. Your best bet there is to listen to friends and coworkers for recommendations. You can also inquire if a shop specializes in certain things. Don't ask if they specialize in emissions or engine performance because some people may just say they're good at "whatever you need". Instead, ask if they specialize in anything before you let on what kind of problem you need diagnosed. In my city, we have one really good electrical shop, and a few that are good at alignments and tire wear diagnosis. I know of a shop in IL that only works on the cars all the other shops have given up on.

You might look at the exhaust and muffler shops, although I'd be wary of the large chain franchise shops. Their mechanics are often paid on commission, so the more stuff they sell you, the more money they make. One of those by me is well-known for repair estimates that are two to three times higher than other shops, including the new-car dealership where I used to work.

I just posted this for another person so as long as it's in memory, I'll share it with you too in case it will help your understanding:

There's well over 2000 potential codes, and only about half of them turn the Check Engine light on. Those are the codes that refer to something the could adversely affect emissions.

You can get an idea of the severity of a code by how the check Engine light acts. If the light turns on for a minor problem, it could turn off while you're driving if it's an intermittent problem and it goes away for a while. (The fault code stays in memory). If it's more serious but the problem goes away while you're driving, the light will stay on until you turn the ignition switch off and restart the engine. Then it won't turn on again until the problem occurs again. For even more serious problems that go away, the light will stay on any time the engine is running. The most serious is when the light is flashing. That means stop the engine right away because too much raw fuel is going into the exhaust where it will overheat and damage the catalytic converter.

Regardless of how the light acts, when a fault code is set, it stays in memory for a specific number of engine starts, or until it is erased with a scanner or by disconnecting the battery. The most important thing with intermittent problems is to read and record any codes before doing anything that will erase them because you don't want to lose that valuable information.