Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
What is the reasoning behind changing the O2 sensor? The P0421 will not set as a relation to that sensor without either (1) there being other faults related to the sensor or (2) being an incorrect (like Bosch) sensor to begin with.Knowing that the catalyst is registering a fault, has the exhaust been flow checked to make sure it is not clogged as that would cause every one of your symptoms.
P0421 won't be related to the O2 sensor... that doesn't happen on these cars without either an additional fault (O2 no activity, for example) or there being incorrect sensors installed to begin with. Auto parts stores love selling those for this because they stock the sensors and not the cats, but on a Mitsubishi you will pretty much never see this code mean a sensor except in the two previously mentioned capacities.
How often does the throttle response issue occur? Can you match it to any specific circumstance (like only after driving a certain amount of time or distance, only at certain temperatures, only with a certain amount of fuel in the tank, etc)?
Whenever you have a scenario like that where you get the pedal to the floor and you are unable to increase speed you are really looking at either a fuel control issue or an exhaust flow issue.Nothing spark related is going to do that (well not without running absolutely awful and setting misfire faults etc); compression would, but it would not come and go so that is out. The throttle body/cable could, but likewise it would not come and go.How long did this condition last for? More than one tank of gas (and perhaps there was some contaminant in that tank full)?
Mitsubishi computers do not behave like that; when they fail they fail for good and don't suddenly start working again.
Luckily it is rare to see them fail anyway.