What does the warning depollution system fault mean on a peugeot 407 mean?

had this warning message on the computer screen. car running rough, down of power. Checked by dealer who said there were large number of misfires on cylinder 3 (V6 Model) and that replacing the ignition coil was the solution - recommended I replace all 6 ignition coils at the same time to save on labor costs, and because in his experience once one goes it is usual that others may fail - replaced 6 coils at a cost of $120AUD each and labour 1.5 hrs - running fine now