Hello! Looking for a bit of help on this; details below:

I bought my new (to me) Volvo back in April, it currently has just over 150,000 miles on it.
About two weeks after purchasing it, it started flashing the P0420 error code (Catalyst System Efficiency Below Threshold Bank 1). Admittedly (and possibly ignorantly), I had assumed this was an O2 sensor issue and had yet to fix it. I'm lazy, that is a problem.

Just tonight, I started her & she had a low rumble & the headlights were gently flickering. Upon reading the codes, she flashed the following:

Confirmed:

• P0204 Cylinder 4 Injector "A" Circuit
• P0304 Cylinder 4 Misfire Detected
• P0420 Catalyst System Efficiency Below Threshold Bank 1

Pending:

• P0204 Cylinder 4 Injector "A" Circuit
• P0304 Cylinder 4 Misfire Detected
• P0420 Catalyst System Efficiency Below Threshold Bank 1

Permanent:

• P0420 Catalyst System Efficiency Below Threshold Bank 1
• P0896 Shift Time Too Long

Now, I'm trying to determine whether this is just a spark plug/ignition coil issue or a symptom of something greater. I don't know if I'll be able to get her to a mechanic tomorrow before I'm back to work on Thursday, so I'm hoping to give an at-home fix a college try. The previous owner didn't have any information on when the spark plugs were last replaced, so it'll be a good idea to replace them anyway. I've figured that the best way to diagnose if it's the ignition coil is to swap them around and see if the code changes; I don't know if I'll get that far tonight since it's getting late, but I'll update this thread as soon as I do. I am a bit more nervous about the P0204 flash, but at the moment, I'm assuming that would connect with it being a spark plug issue from what I've read so far.

I guess the guidance I'm looking for is whether this combination of errors could be a sign of greater issues. I am an eager-to-learn layman, so any help or opinions would be greatly appreciated.

EDIT 08/13:
She's still running a bit hot, so I'm going to take a run at the coils in the morning. In terms of the P0420 code, she was running well, and I didn't have any notable mileage issues, which is why I assumed (hoped) that it was just an issue with the sensor rather than the Cat itself. I typically get ~580 km out of a tank. I've been filling with 91 octane, but from my understanding, the previous owner was using 87 for the extent of the time he had her.

Tonight, after the misfire, she struggled to get to highway speeds promptly on my way home. I feel like it's just the spark plug, but again - I might just be stupid. Thanks in advance, folks <3

UPDATE 08/14:
Alrighty, so the trail continues. I swapped around the ignition coils between Cylinders 4 & 5, and the misfire was still on 4, so that was good - one issue down.
I replaced the spark plug for cylinder 4; nothing too crazy. Started her up & she was running fine until I brought her up to speed, and then the misfire was back. Damn It.
I had hoped this would be an easy fix, but alas. I'm looking at booking an appointment with my mechanic for a general checkup, but in the meantime, I'm going to keep working at this.

Any help/insight would be appreciated - I will continue updating this post as I go in hopes that I can save someone else some time in case they're in a similar situation.

UPDATE 08/15: My best guess is that it's a fuel injector issue, so now it's just a matter of figuring out whether it's the part itself or electrical. I hope it's the part since that's an easier replacement, but we'll see. I have a buddy with a multimeter, so hopefully, I can recruit his charity tomorrow. The mechanic is booked in for Monday, so worst case scenario, I just wait it out and pay someone more knowledgeable to finish up the legwork on this.

List of work done so far:

• Cylinder 4 & 5 Ignition Coils Swapped (No Change)
• Cylinder 4 Spark Plug Replaced (No Change)

Numbers that I am a layman to that might help (I believe these numbers are averages of 2 trips, both ~15 minutes long with little highway driving)

• Intake Pressure is marked as 62.6 kPa (Infocar Engine Diagnosis)
• RE. is 55.8rpm/% (Infocar Engine Diagnosis)
• Short-Term Fuel Trim: 0.23%
• Long-Term Fuel Trim: -1.18%
• Mass Air Flow: 10.4g/s
• Average Engine Load: 25.3%