Engine remapped or aftermarket software installed

Modified control software in place of the manufacturer's calibration — a remap, a tuning box's altered parameters, or emissions equipment disabled in software. It is a legitimate cause to record because it genuinely produces these codes, and because a diagnosis that does not know the software has changed is working from the wrong assumptions entirely.

Modules increasingly detect it themselves. A calibration verification failure or an unauthorised software code is the module comparing what it is running against a signed reference and finding a mismatch. Those codes mean the software is not what the manufacturer shipped and say nothing about whether it works.

The practical consequences are what matter. Modified fuelling or boost outside the standard's expectations trips monitors that were calibrated for the original map. Software written to ignore an emissions component leaves the rest of the system reporting its absence. And a manufacturer update will overwrite the modification, sometimes producing a vehicle that runs badly because hardware was changed to suit the map. Ask what has been done to the vehicle before pursuing these codes — it is a question owners often do not volunteer, and in many jurisdictions disabling emissions equipment makes the vehicle illegal for road use.