2009 Mini Base 218,000 miles

Threw a P0420 OBD code and it keeps returning once cleared.

It appears as if I need a new catalytic converter.

Recently, my belt broke, causing some (not too serious) overheating. I started getting a P2414 code as well as a few others. My AFR was reading 29.38. O2 sensor 1 was reading 2.0. Replaced the upstream O2 sensor and the code went away and the AFR is normal. The overheating also took out the thermostat which I replaced.

So any or all of the above may have pushed the already high mileage Cat over the edge. The downstream O2 sensor reads .8 give or take.

Questions:

1) Might it not be the Cat?

2) Do I need to put the car in service mode (extend the frame) to change the cat?

3) What's the cheapest Cat that I can get that will work?

4) Am I damaging anything while I run with a potentially bad Cat?

Thanks!!

1. Ok. The Gen 2 MINI N series engines uses wide-band upstream O2 sensors (as opposed to the Gen 1 using narrow band sensors in both locations). The downstream sensor is a narrow band. Using your scan tool, start the engine and let it reach operating temperature. Select OBDII (instead of MINI) from the scan tool menu, and go to live data. Look for your short-term and long-term fuel trims. Make sure those are OK before you change the cat. Also Pressure check the cooling system to make sure you aren't poising the CAT with coolant.

2. Yes you have to put the car in the service mode to change the CAT/Exhaust manifold.

3. I had the same problem with my '09 MINI (N12); I used the Eastern CAT from Rock Auto; it is working well, and is less than 1/2 the price of one from the dealer.

4. It depends; if the CAT is plugged, that can cause problems, but a plugged CAT will usually mean poor engine acceleration and the car won't get out of its own way.

When doing the OBD scan that mkov608 recommends, look at the voltage output of the downstream sensor. It should remain steady with the engine idling. If it oscillates that's an indication of a bad cat.
Before spending money on replacing it, try an "Italian tune-up."

Italian Tune Up?

Thanks for the helpful response!

The car is out of my hands for a few days while a rusted section of the sub frame is welded.

I'm using Torque to read codes. It doesn't seem to allow me to read fuel trim. I do have:
O2 sensor 1 wide range current
O2 sensor 1 Equivalence ratio
O2 sensor 2 voltage
Air Fuel ratio

I'd love the problem to be the downstream O2 sensor. mrbean wrote above: It (the downstream sensor) should remain steady with the engine idling. If it oscillates that's an indication of a bad cat. Any other readings that I should look for.

The car runs well with good MPG. It will skip or hiccup a bit when I give it throttle while in high gear going uphill (when I should likely be in a lower gear).

Basically, once up to temperature, drive it hard for several miles, running through the gears shifting at/near redline several times.
https://www.roadandtrack.com/car-cul...-up-explained/