My 2001 (154k) S40 started throwing P0301, P0304 codes after the oil leak, go figure...
I fixed the leak, pulled all the plugs, wires and coils, replaced them with new parts. I know it's kind of overkill, but I was planing on having no issues for awhile.
I used new parts but off ebay not genuine Volvo, maybe that is my problem, but the P0301 is still there. It actually showing one MIL and one stored, but the same code non the less.
Any ideas?, suggestions?
Thanks in advance,
Oh, the engine starts and runs smoothly, it may have a very small hesitation from 1200 to 1250 rpm, but it also could be a figment of my imagination.

 

Aftermarket coils are a gamble sometimes. Maybe swap the places of the coils and see if the CEL code moves with it?

What type of plugs did you get? I've read that these S40's can be picky... I've had good luck with NGK's, both platinum and plain coppers. Also did you check the gap before installing the new plugs? The platinums I bought we gapped at about .04 from the store. I always reset the plugs to .03 like the owners manual says; although alot of stores say to use .028, and I read that alot on forums too. I doubt I could gap a plug that precisely anyway, so .03 is easier to shoot for with the gauge.

 

Thank you Livens,
that was my next move, to try my verified good coil in the "bad" spot and see... I did use NGKs and did not gap them, but kind of eyed them up to be a least the same...
I need to pass the smog and dislike driving under the CEL influence anyway