vanderwiel

Hey everyone, I just bought my first subaru a few weeks ago. Its a 99 legacy sedan L sedan. So far I like the car, I think I lucked out getting a 99 since its the base model but I still have the sunroof/spoiler/alloy wheels combo. Its a 5 speed standard too which is fun since I spent the winter in an automatic jeep cherokee. I paid $800 for the car which has 155,000 miles on it. When I bought the car the man who owned it told me the check engine light was on for a front 02 sensor. When I had the codes checked it turned out it had codes for front 02 as well as a knock sensor. I replaced the knock sensor and it seems to be running more smoothly already. My main question and motivation for posting this thread is this: When I jacked the car up to change the oxygen sensor, the source of my exhaust leak became obvious. I have a fairly significant leak coming from where the header meets the manifold. It looks like the flange is pretty well rusted out. Is it possible that this leak before the 02 sensor could be causing the code? I was thinking of replacing the sensor to see if it would clear the light, but I feel like that might be a waste of time since the leak might be the reason for the code. I was thinking of ordering a replacement header, which runs about 140 bucks from advance auto in town, but they'd have to order it anyway so I'm looking around a bit for better prices. Should I replace the header & o2? or just one or the other?