Q: My engine is about to kill itself.

If the vehicle transmission has entered limp mode and the engine is really misfiring, these faults have to be diagnosed separately. The engine misfiring should be dealt with first. Typically, the engine is scoped to look at secondary firing patterns in each of the 6 cylinders and that diagnostic will identify which cylinders are faulty. Typical causes of misfiring are faulty spark plugs or wires, faulty ignition coil, compression problems (broken piston rings, burnt or bent valves), broken valve springs, worn camshaft, and fuel injector faults. YourMechanic does offer a misfiring diagnostic that you can request and it will be performed right at your location.

With regard to the Diagnostic Trouble Code P0420, that code could have set as a direct consequence of engine misfiring. Once the misfiring is resolved, if the P0420 Code is erased and yet returns, the following causes would be looked at (omits O2 sensors themselves as you have replaced those): Damaged muffler or leaks in the muffler; Damaged exhaust manifold or leaks in the exhaust manifold; Damaged exhaust pipe or exhaust pipe leaks; Oil contamination in catalytic converter; Faulty catalytic converter; Faulty engine coolant temperature sensor; Damaged oxygen sensor wiring; Oxygen sensor wiring that is not properly connected; Damaged oxygen sensor connectors; a fuel injector that is leaking; Fuel pressure that is high; and use of the wrong kind of fuel (using leaded fuel instead of unleaded fuel). Once the fault causing the Code P0420 to set is resolved, the recommended service to evaluate transmission shift failures (limp mode) is a shift diagnostic.