Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
What year, model and engine vehicle are we working on?
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The general issue with a P0421 is that the catalyst for bank 1 has depleted and it is no longer removing HC content as efficiently as the computer requires.
This is determined by reading the fuel content before and after the catalyst to see if the content is changing as it passes through the catalyst; if it is too similar, the catalyst is deemed depleted and the trouble code is set.
On some vehicles (for example Bosch type systems used by older General Motors or VW/Audi etc) you can have an O2 sensor fail and cause this, however on the Mitsubishi platform this is extremely rare, and nearly impossible without also setting additional sensor faults too. Basically... if you see this on a Mitsubishi with no other trouble codes, unless there are other variables in play like a massive exhaust leak or incorrect (Bosch) sensors being used, it is a bad catalytic converter.Without knowing the model I can't tell you which it is with precision.... if you have a four cylinder model generally the exhaust manifold will incorporate your only monitored catalysts (the larger under-car unit is not monitored). On V6s if it is modern (post 2000 or so) bank 1 will be for the side of the engine with cylinder #1 and bank 2 for the side with cylinder #2 (whereas much older v6s will only have one monitored cat and call it bank 1 regardless). Depending on the application they may be built into the manifolds like most four cylinders (Endeavor, newer Eclipse and Galant) or they may be attached to the bottom of the manifolds (Montero, Diamante, older Eclipse/Galant).Additionally certain vehicles had warranty extensions on these or software recalls to improve the lifespan of the catalysts too.