Injecteur monté sans saisie de son code de calibrage

An injector was replaced and its calibration code never programmed into the module, so the module continues applying the previous injector's correction — or none.

This is the commonest reason a diesel runs badly immediately after injector work, and the reason it costs money is that it looks exactly like a faulty new part. Uneven idle, a harder knocking note from the affected cylinder, smoke and poor economy are precisely what a worn injector produces, so the natural conclusion is that the replacement is defective. Fitting a second one changes nothing, because it will not be coded either.

Nothing is broken. A step in the job was not done. Coding takes a scan tool and a few minutes, and nothing about fitting the injector accomplishes it automatically.

A related trap catches reconditioned injectors specifically: a remanufactured injector is flow-tested again after rebuild and issued a new code, supplied on the reconditioner's paperwork. The code stamped on its body is the original one and is no longer correct — entering it produces the same fault as entering nothing.

Beyond the running quality, there is a reason not to live with it. An uncorrected injector may be over-delivering, putting more fuel into one cylinder than the module believes on every mile driven, at higher local temperature and pressure. That is a slow risk to the piston rather than an immediate one, but it is real.