Re: Error Code P0303 Help
P0303 is cyl 3 misfire & raises the question of "which is cyl 3 ?" since Merc uses odd cyl numbering.
Once that is sorted there are three things that usually cause a 303 (which is basically that the engine slows down when #3 is supposed to fire):
1) mechanical - e.g. a burned valve - rarely heals
2) fuel - clogged injector (high engine speed & high load)
3) bad secondary ignition (low engine speed, moderate load) (coil, wires, spark plug)
I'd probably pull the spark plugs and take a look first.
ps if misfiring I use an IR temp gun to determine which - the bad cyl will have a cooler exhaust manifold.