Koelvloeistoftemperatuursensor defect

The sensor telling the module how warm the engine is, drifted or failed. It drives more decisions than almost any other single input — cold-start enrichment, when closed-loop fuelling begins, ignition timing, cooling fan operation, the cold-start catalyst warm-up strategy, and on a diesel whether particulate filter regeneration is permitted at all.

Which is why its faults present as something else entirely. A sensor reading warm on a cold engine silently disables the whole warm-up strategy: the module believes the engine is up to temperature, so it skips the enrichment and the retarded ignition that light the catalyst off. Nothing feels wrong and the emissions are far worse. A sensor reading cold does the opposite — permanent enrichment, poor economy, and a catalyst fed a rich mixture.

Many vehicles carry two sensors, one for the module and one for the gauge, so a normal-looking temperature gauge does not rule this out.

The cold soak is the decisive test: after standing overnight it must read close to ambient and close to every other temperature sensor. Check the wiring too — added resistance shifts the reading consistently and looks exactly like a drifted sensor.