Having an issue, wanted to run it by the group to see if anyone has any ideas. I can't make any repairs but I am trying a new mechanic and I want to be careful before I go any deeper. I can't tell if this issue is minor or major.

Last week, my 2016 S60 T6 (4 Cylinder, 85K miles) got the CEL with a single error code: P0303

Aside from an intermittent slight knocking noise that has developed on the right side towards the back a month ago (occuring at idle so not movement-related), there are no noticable performance issues. Car drives pretty well and doesn't struggle to start.

As background, I had the dealership put a new set of spark plugs in about a year and a half ago.

So, this w/e I had a backyard-type mechanic change the ingnition coil on cylinder three only. I don't think he performed any real diagnosis, probably just replaced the coil (with OEM though) and that's it.

The CEL stays off for about a day but comes back on, with the same P0303 code.

So I go into a different mechanic, a proper brick and morter repair shop that had decent online reviews. The shop owner gives me a call after an hour or two and tells me, since I recently replaced the spark plugs and that particular coil, I should replace the PCV Valve/Oil Trap as he noticed there is negative air flow and it gets better flow when they take off the oil cap.

He is pretty certain a new oil trap will stop the misfires and fix the issue. He did not see the value in doing more diagnostics like a pressure test at this time, and I don't necessarily think he did anything like switching the spark plugs or ignition coils to look further.

I google it a bit and see nothing that would necessarily negate or support this, but I see some folks advocating for a PCV Valve replacement after 80K.

So, I've gone ahead with it with at a bit less than $700.

My question is, what would you suggest I do next if this doesn't fix the problem? Request a compression test? Bite the bullet and get all new spark plugs? Look at the fuel injectors?

Hoping it's not anything terribly major. I've spent about $1000 so far and want be stop short of throwing good money after bad...

What do you guys think? I've read a lot of the similar threads but nothing seems exactly like my situation. Keep in mind, I am trying this new mechanic for the first time, and haven't established a real trust relationship yet. Any input would help. Thanks.