Multiple codes, CEL Flashing, don't know where to start

[splitshot37]

My girlfriend recently bought a 2000 GMC Yukon with 270k miles on it. From the start it had some hesitation problems, specifically when accelerating after stopping at a red light. It would hesitate for a second, then return to driving like normal. It also is leaking oil at a decent rate, about 1qt per week. I can see a lot of oil around the oil pan and surrounding areas, but can't seem to find the source. The other day the CEL started flashing on her way to work. I put my code scanner on it and it returned multiple codes:

P0420 (Current) Catalyst System Efficiency Below Threshold (Bank 1)
P0430 (Current) Catalyst System Efficiency Below Threshold (Bank 2)
P0303 (Pending) Cylinder 3 Misfire Detected
P0440 (Pending) Evaporative Emission Control System
P0449 (Pending) Evaporative Emission Control System Vent Valve/Solenoid Circuit

I then topped it off with oil, reset the codes and took it for a quick drive (10 min)
right before I got back home, CEL comes back on flashing again. This time only 2 codes:

P0303 (Current) Cylinder 3 Misfire Detected
p0449 (Pending) Evaporative Emission Control System Vent Valve/Solenoid Circuit

I have researched these codes individually and tried to piece together what may be happening, but I haven't found exactly what I was looking for. Does anyone have any advice on where I should begin? Should I replace the solenoid vent valve first and go from there? Should I attempt to trace down the oil leak first? Any advice at all would be greatly appreciated, we are trying to make this vehicle last as long as possible while we can save up for something newer. Cheers.

 

[iamdub]

I'd focus on the misfire issue, and start with the "free" tests. My first thought would be to swap the left- and right-side coil packs around, reset the codes, then see if the misfire moves to cylinder #6. If no change, you can leave the coil packs where they are and swap the #3 injector with another one to see if the misfire moves with that. The EVAP codes are non-critical and the catalyst efficiency codes (indicating below threshold) are probably not critical, either.

 

[Kenny D]

The CEL flashes when the vehicle is misfiring while driving. If it misfires once then the light is constant. You have something keeping that cylinder from firing. Along with iamdub's suggestions, I would also investigate that spark plug too.

 

[HiHoeSilver]

I would prob just start with pulling /swapping the plug and see

 

[splitshot37]

Thanks for the replies everyone. After seeing your replies, I decided to check the spark plugs to check their condition. While removing the spark plug wires, 3 of them were either broken or broke when I removed them. Spark plug wire in cylinder 3 was the worst, which makes sense given the codes. I replace all of the spark plug wires and the truck is driving great again, and the CEL for misfire has disappeared. I plan on replacing all of the spark plugs as well, this is her daily driver and we are on a very low budget so I opted for the immediate fix for now. Next I will try and tackle the EVAP system problem, followed by the ABS and parking lights always being on.

 

[iamdub]

Lol

I have a tendency to skip the super easy stuff, assuming it's already been ruled out. Glad it was a simple and cheap fix.