I had a similiar problem with a C5 2.2 Hdi engine. Difference was that it
was flashing "ESP/ASR not functioning" and subsequently "Antipollution
fault". Warnings were disappearing after few starts of engine and suddenly
appeareing again and again with no reasonable cause.
Proxia test showed no disorders.
Try to drive at lower gears, keeping engine about 3000-3500 rpm for few
hundreds kilometers. That should clean up the catalysator and the FAP, if
there is one in your car.
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