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The module concluding that emissions equipment has been removed, disabled or defeated. It is a finding rather than a fault, and it is recorded here because the codes are real and a diagnosis that does not know the vehicle has been modified will chase components that are no longer fitted.

Modern systems detect it several ways — a sensor reporting values impossible for a working component, a filter showing no pressure difference at all, a reductant system consuming nothing, or a calibration that does not match its signed reference. A differential pressure of near zero across a filter at all loads means there is no filter there, which is a stronger conclusion than any single code.

The practical position is worth stating plainly. In most jurisdictions removing or defeating emissions equipment makes a vehicle illegal for road use and will fail its test, and reinstating it is the only repair. Establish what has been done before quoting anything — the hardware, the software or both may have been changed, and a vehicle running a map written for absent equipment will run badly if the hardware is restored without the calibration.