Thank you for your question, my name is Steve and I am a fully qualified vehicle technician with over thirty years in the trade, hopefully I can help.
If you do normally use your Nissan for quite short runs, the soot can build up, in this case the advice you have heard should clear the exhaust and lower the reading. This would preferably to take the vehicle for a run of half an hour or so keeping the revs high, so possibly keeping it in a lower gear than you normally would. This should clear out the system.
If this does not work then the problem may be with a leaky injector or other fuelling problem, but if the engine appears to be running ok, with no problems during normal use, then giving it ia run should clear the systems.
It would also be a good idea before taking it for the run to add a fuel additive like this one (click the link)
I have found This has proved very effective.
I hope this answers your question, but do please feel free to ask any further details by replying to this thread
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