Cause
Incorrect catalytic converter fitted
A converter of the wrong specification for the vehicle. It is distinct from an aftermarket one below original specification — this is the wrong part rather than a cheaper equivalent, and it can be the wrong catalyst chemistry entirely.
Fitting a three-way petrol converter where a diesel oxidation catalyst belongs, or an SCR catalyst without the coating the vehicle's strategy expects, produces conversion figures the module cannot reconcile. The codes name efficiency and the part is new, which defeats the obvious repair. Check the part number against the vehicle identification number, not against pipe diameter. A converter that also lacks the correct approval marking will fail an emissions test regardless of what the module reports, which is worth establishing before it is fitted.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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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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