From the manual
Diesel particulate filter
The diesel engine particulate filter eliminates
most of the soot from the exhaust gas system.
Under normal driving conditions, the filter
cleans itself. The diesel particulate filter is
cleaned automatically without need for indication
by the warning lamp . This may be
noticed because the engine idle speed increases
and an odour may be detected.
If automatic filter purification cannot be carried
out (because only short trips are taken,
for example), soot will accumulate on the filter
and the Diesel particulate filter warning
lamp will switch on.
Facilitate the automatic filter cleaning process
by driving in the following manner: drive
for approximately 15 minutes at a minimum
speed of 60 km/h (37 mph) in 4th or 5th gear
(automatic gearbox: gear S). Maintain the engine
speed at approximately 2,000 rpm. The
rise in temperature causes the soot on the filter
to burn. On completion of the cleaning
the warning lamp will switch off. If the warning
lamp does not switch off, go immediately
to a specialised workshop to rectify the problem.