DRC P0351

[Mark 1]

Hi guys

I have a 2004 Mk1 Focus 1.8 petrol. After a great deal of trouble , DTC code P0351 came up but replacing the coil , plugs and checking the connection to the coil and the HT leads has made no difference. The car still idles terribly with no power and seems like it's only firing on three cylinders whereas I'm sure it's actually firing on all 4 in reality.
My question is , if everything tested fine and new plugs and coil were fitted , what else could cause this particular fault code to display ?
Thanks in anticipation

[isetta]

have you tested it is firing on all 4 by removing plug lead one at a time.   if the engine behaves exactly the same with one particular lead off then you know that cylinder is not firing.  in the old days I would have pulled them off one at a time whilst the engine is actually running but doing it that way is not advisable these days