Hi all I am not sure where to post this so I will give it a shot here. I will try to explain how things progressed and what I have done along the way. I hope it helps.

After I bought the car used the check engine light was off and it came on a few days later and went off again for a few weeks.

I checked the code and it was P0301, (Cylinder 1 Misfire Detected). The CEL would turn on and off occasionally so I changed the Wires, Plugs and coil pack and went ahead and did the 2 - O2 Sensors.

Now over time I have earned 2 additional codes for my efforts. I have all three, (P0301, (Cylinder 1 Misfire Detected), P0303, (Cylinder 3 Misfire Detected), & P0420, (Catalyst System Efficiency Below Threshold)). I am not sure where to look or spend my money on the next possibility.

The car runs great, doesn't miss or run rough at all. Starts without hesitation and moves down the road like there are no issues at all. When I bought it the Head Gasket, Water Pump and timing belt were all done.

I wonder if I have an exhaust manifold leak that I cannot hear. If I have the outside vent open and am sitting idle at a light or a stop sign, Basically just sitting idling), I can smell exhaust. When driving I do not, and when I have the re-circulation, (Vents closed), I have no issues.

Could I have an exhaust leak at the manifold and not hear the leak? Would this explain the codes and the smell of exhaust? I have read that an exhaust leak can throw these codes. I do not know what else to think since the car is running great, with exception of what I have mentioned.

Any real input based on direct knowledge or experience with what I am experiencing is greatly appreciated. I have searched and went through forums for help. If you have direct experience and knowledge then please give a guy a hand with this. 
Thank you all in advance.

Thank you all for your help.

year? miles? general condition?

 

what brand plugs/wires? Subarus are picky in this area.

year? miles? general condition?

 

what brand plugs/wires? Subarus are picky in this area.

2001 Forester "S"

220K miles

Vehicles runs great without issues and gets its' regular service. It has a new head gasket, timing belt and water pump within 1 year

 

NGK Plugs

Coil Pack is new, but is aftermarket

O2 sensors are aftermarket but direct fit

I had the p0420. 01 forester. SMALL hole in cat pipe was the cause. Didn't make noticeable noise. A bracket had broken, and the end rubbed a hole through the pipe.

I had the p0420. 01 forester. SMALL hole in cat pipe was the cause. Didn't make noticeable noise. A bracket had broken, and the end rubbed a hole through the pipe.

Thank you. I will check it out. Having no noise to chase has me stumped. LOL. I suppose if I start falling asleep under the car I may know I am in the correct area. The heat shields make it hard to check everything  . Have a great day.

I'd chase the misfires before chasing the P0420. Misfires cause extra O2 in the exhaust, which can trigger an 0420.

The 0420 is almost never a bad cat.

plug wires?

plug wires?

Sorry left that out. They are aftermarket and new.

I'd chase the misfires before chasing the P0420. Misfires cause extra O2 in the exhaust, which can trigger an 0420.

The 0420 is almost never a bad cat.

Yup. That is why I was leaning towards an exhaust leak since I can smell it, but I have never had a CAT go out on me so I really do not want to take a run at that one right away. I did the CATs' on my son's Imprezza and that was not the issue. I am just at sort of a loss with this one. I have received bad O2 sensors before on other vehicles and do not want to think I have that problem, but I can revisit that if all else fails. I suppose I should remove the heat shields and see what I can see if anything.

Always start with an running issues first before dealing with emissions related codes.

As CNY_Dave said. It could easily throw a P0420 code due to a misfire