Drive with caution — fix soon.
This code means the catalytic converter on bank 2 has dropped below the efficiency the computer expects, so it isn't cleaning the exhaust properly. It's commonly caused by an aging converter, but a faulty oxygen sensor, an exhaust leak, or an underlying misfire can also be to blame. Diagnosis usually checks the sensors, exhaust, and engine condition before assuming the converter itself needs replacing.
$200 – $2200
Varies by vehicle and root cause.
Usually yes for the short term, as the car typically drives normally. But the converter isn't doing its job, so emissions rise and you may fail a smog test. Get it diagnosed within a week or two, especially if a misfire is involved.
A bad oxygen sensor or exhaust leak often runs $200 to $500 to fix. If the bank 2 catalytic converter needs replacing, expect roughly $900 to $2,200 depending on the vehicle.
It's a moderate concern. It won't typically strand you, but a weak catalyst means more pollution and can worsen if an underlying misfire or rich mixture isn't addressed. Fix it before it leads to a costly converter replacement.
Both are catalyst efficiency codes, but P0420 refers to bank 1 and P0431 refers to bank 2, the engine's other cylinder bank. The causes and fixes are essentially the same, just on the opposite side.