Cause
Aftermarket converter below original specification
A replacement catalytic converter carrying less precious metal or less substrate area than the original. It converts, just not well enough or not for long enough — which is exactly the condition the efficiency monitor is designed to detect.
The result is a converter efficiency code on a converter that is physically new, often within months of fitting, and it defeats the obvious repair completely. A vehicle on its second efficiency code with a recently fitted converter is this, not a recurring engine fault.
The distinction to make is between a universal part and a type-approved one. A converter approved for the specific vehicle is built to satisfy its monitor; a generic one sized by pipe diameter is not, whatever it costs. Ask what was fitted and check its approval marking — and note that in most jurisdictions a non-approved converter will also fail an emissions test regardless of what the module reports.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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