کاتالیزور غیراصلی با مشخصات پایین‌تر از اصل

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.