In which case I would suggest that you have an injector problem, i.e the injectors are not passing the amount of fuel the ECU believes they should be. You will say that the car drives fine and it probably does BUT it ain't quite optimum for some reason. If everything is spot on the trip computer is very accurate, certainly better than 3 mpg. (This is C5 can't speak for 406 but doubt it is little different)Citroenmad wrote:Are you going by the on board trip computer for the 406 and C5? If so they are not always accurate. I know our eldest underreads its MPG by 3-4.
cachaciero