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A common rail injector holds back fuel at fifteen hundred bar or more with a needle and seat measured in microns. As that seat wears it fails in both directions at once — delivering more than commanded while open, and continuing to pass fuel after it closes — and the engine module sees the sum as excess fuelling on that cylinder.

The leak-back test is the measurement that settles it, and it works by comparison rather than against a specification: with the return lines instrumented, a worn injector returns visibly more fuel than its neighbours. That comparison is also what answers the question that actually decides the repair — whether one injector has failed or the whole set is wearing together. On an engine where the labour to reach them dominates the bill, doing a drifting set in one visit is far cheaper than doing them one at a time.

Check the oil. A leaking injector puts diesel down the cylinder wall into the sump, and an oil level that has risen above maximum on a diesel is a hazard rather than a curiosity — in the worst case the engine runs on its own oil and cannot be switched off.