Code P0300, P0301, and P0304, without symptoms?

A local mechanic checked out the vehicle and saw that the engine timing system belt was cracked and old; he then ran the engine and quickly heard that during the startup it behaves a little rough, after which he decided that the engine timing system is due for replacement. This is to be done over the weekend, and we will see if this solves the codes.

Did these codes come up after the tune up, if so, which spark plugs did they install? They must be the OEM spark plugs or you will have this problem. Here are the spark plugs that must be installed and the gap must be correct as well.

Gap: 0.028-0.032 inches (usually preset but should be checked).
Type: Iridium or Platinum-tipped.

OEM Part Number: Volvo recommends using genuine parts or high-quality equivalents like NGK or Denso.

Also, did you clear the codes once you changed out the ignition coil, BTW I would change out all of the ignition coils with the tune up which is an industry standard.

P0300 - Random/Multiple Cylinder Misfire Detected:
P0301 - Cylinder 1 Misfire Detected:
P0304 - Cylinder 4 Misfire Detected

Please go over this guide as well:

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Check out the images (below). Please let us know what happens.

Just got my engine timing serviced. That was indeed the issue. Unfortunately, nothing related to your post, Ken, but thanks a lot for trying to help!

"engine timing serviced"? Do you mean timing chain?

This engine uses a belt. Got the belt replaced, the engine timing camshaft sprocket and gear adjuster, and one camshaft gasket cap.

When you swap the coil and spark plugs do you clear the codes after the repairs? You say the engine runs okay, no difference?