Our Euro 5 Golf has cooled EGR and a particulate trap. Remapping the engine tuning and EGR to make less NOx and 'leaning' on the DPF to mop up the extra particulate and relying on the consumer to not get too annoyed by the reduced performance and fuel economy, is nominally a viable strategy.
Euro 4 and earlier diesels in the main will not have cooled EGR or particulate traps. Tuned to produce minimum particulates, pushes up NOx. Tuned to reduce NOx pushes up particulates. These pollutants just go straight out the tailpipe. Hence there isn't much that can be done from a remapping viewpoint, so no point in recalling them. I hate to say it but these older diesel cars will become regarded as gross polluters of NOx and/or particulates and are likely to be banned from major cities in the not so distant future.
