Mechanics 101

A fault light has appeared in our car. We called Mercedes and they said we shouldn't drive the car but have it transported to them (2 hours away) for attention. $$$.
Our local mechanic said the repair could cost $5,000 at Mercedes but he gave us his computer gadget with instructions to drive around for half an hour at a maximum of 50kph and 2,000 - 4,000 revs to diagnose the fault and hopefully burn off the offending debris clogging the filter. Fortunately that can be done in the valley. You would have no hope in traffic or with stop signs etc.
It took a while for us to get it working with Mr isbi driving and me working the computer but eventually we got past the pesky error message and completed the test.
We are not certain yet, but the mechanic is hopeful it may just be a faulty sensor which would be much cheaper and he can fix it.
In any case it was worth a try and he said he will give me a Cert 111 in diesel diagnostics :-)