Cause
Misfire dumping unburnt fuel into the converter
A misfiring cylinder sending its entire fuel charge into the exhaust unburnt, where it ignites in the converter. This is the fastest way to destroy a catalytic converter — far faster than gradual rich running — because a single dead cylinder delivers a large quantity of fuel to a component with no way to shed the heat.
It is why a flashing engine warning light means stop, and an illuminated one does not. The flash is the module reporting a misfire at a rate it calculates will damage the converter within minutes, and continuing to drive converts a coil or plug into a converter as well. That distinction is worth telling any driver who asks.
The order of repair follows from the mechanism. Fix the misfire first and establish that it is genuinely gone, then judge the converter — which may have survived if the misfire was caught quickly. A converter efficiency code stored alongside a misfire is usually a consequence, and clearing both after repairing the misfire will show within a few drive cycles whether the converter recovered or not.
Typically a moderate job.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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