Hi would appreciate some help here. We have in our shop 1999 ML320, first presented 12/7/07 with severe misfire, codes P0300,1,2,3,4&6 stored. Checked spark plugs obvious point of call, platinum tips worn right down. Spoke to last servicing dealer (whom has maintained vehicle from new) Advised they were not renewed in last 2 services. Renewed plugs and cleared codes. Roadtested vehicle, heard slight tinny rattle under harsh acceleration from RH cat, and misfire returned. Carried out live data check on cats which showed to be working OK. Found 4 suspect leads, renewed whole set as cheaper than buying 4 from dealer (Bremi parts) Misfire only now evident on 1 & 2. Tried substitue coil packs, still evident. P0301,P0302
Advised client suspect RH cat failure causing problem.

Spoke to local dealer who said our diagnosis sounds wrong and possible head gasket failure Vehicle has done 143,000klms Dealer is also now stating they replaced plugs 23,000klms ago, yet it would be the first time I have seen that ammount of wear in that short a time. I am very reluctant to spend anymore of customers money, NO engine warning lights, vehicle just fails to react when driven hard and misfire occurs. Thank you