exhaust leak caused a CEL?

So a few months ago the CEL on my 2004 Outback came on. I took it to a part s store, had them run the codes, and it said my primary O2 sensor was bad.
The 300 price tag for the part at the store was more than I could stomach, so I passed intending to find one online. I finally got around to buying on e at Rockauto for about 110 bucks about a month ago.
I went to install it, and realized I had two primary O2 sensors. the code d oesn't distinguish. I figured I'd try one, and if it didn't solve it, it sw itch the new one to the other side and put the first-removed O2 sensor back in.
The new part had the wrong plug. It was the standard 4 wire plug vice the 6 wire plug that my car uses. So I figured I'd just solder it together. I go t out the tools, clipped the wires and started stripping the insulation. Uh oh. That looks like either aluminum or stainless. Both of which are a pain to solder.
A little flux and I had some questionable solder joints. Felt solid, but th ey didn't look like the solder penetrated very far. I put on the car and it ran a lot worse than it had before. It had surging at medium throttle open ings when cold-that's indicative of a non-working O2 sensor.
I had to leave for two weeks on business during which time my wife drove th e car daily. She said it got worse to the point that the idle would fluctua te like it wanted to die, but never did. When I got back I could hear an ex haust leak.
So I suspected I had a bad solder joint, and knew I had an exhaust leak. I checked the solder joints, then crimped the wires any way. No luck. Still s urged and had horrible mileage. So I put the old part back in, which involv ed crimping it's original connector back on. The car ran better, but still a lit CEL.
So at this point I'm back to square one parts wise but also have an exhaust leak.
I got under the car and found it to be one of the bolts on the flange betwe en the muffler and catalytic converter had rusted to failure. So I cut off the flange on both sides of the joint, got a 2 inch ID joint from the parts store, clamped it on, and started it up.
This was the amazing point. The car was noticeably quieter than I remember it ever being. When I put my head out the door and rev the engine all I hea rd was sound from the front of the car. Nothing from the exhaust. I suppose this is normal, but apparently that flange had had a mild leak for a while that steadily got worse until it suddenly got a lot worse.
I drove it, and at the fourth startup it had no CEL!!! Still on the origina l O2 sensor.
So I'm drawing the conclusion that the original code wasn't due to a bad O2 sensor, but was because of an exhaust leak between the CC and the muffler.
My theory for the mechanism here is that the ECU determines primary O2 sens or performance based on what it expects to see in certain scenarios like th rottle lift or certain throttle settings, and the exhaust leak was reducing back pressure enough that it was creating a different environment where th e happenings in the exhaust manifold were different enough from expected th at the ECU didn't think the O2 sensor was up to snuff. So fixing the leak g ot the performance of the gasses in the exhaust back to where they should b e, so the ECU no longer thought the sensor was sending bum info.
Just a theory.
The car has better throttle response now. I hadn't thought it had any hesit ation on tip-in, as I thought it was just the torque converter's laziness, but now it's so immediate that I think it did have a little hesitation.
Incidentally, the flange bolts and flange were unsalvageable, but the pipe was fine. Still good wall thickness. The 12 bucks and two hours I spent wer e far better than what a shop would likely have charged me since they'd pro bably have replaced both the cat and muffler for tens of times more.
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On Tue, 7 Oct 2014 08:39:50 -0700 (PDT), weelliott

The air getting into the exhaust system makes the computer think the O2 sensor doesn't see what is happening and turns the light on. The computer alternates the engine from just rich to just lean of Stoich, and the O2 sensor is supposed to change voltage in response - if it doesn't the computer says "bad O2 sensor" If there is an air leak, the O2 sensor says the system is lean and is not responding to enrichment - so it condemns the sensor.
As for the repair - I know MANY mechanics, at MANY garages, who would have done exactly what you did to repair it, and others who would have welded in a replacement flange. Not all mechanics are mindless parts changers out to steal your money.
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The interesting thing is that the stoichiometry isn't changing from extra a ir getting in upstream of the sensor. The sensor is inches from the engine, but the leak was by the rear axle. I think it was the lack of back pressur e affecting scavenging and how the exhaust gasses were leaving the cylinder , and actually affecting the O2 content of the exhaust gasses.

I know that not all mechanics are just parts changers out to steal money, b ut I think that there are garages that are more apt to swap out the two par ts than to fix the two parts. I only know of one garage that has tried that maneuver on me when I had an exhaust leak at the front of my cat once.
I wanted them to weld the cat's pipe to the incoming pipe, and they said th at they wouldn't touch a cat other than to replace it because of emissions regulations. They could have been honest, or could have been blowing smoke. I don't know. I told them I'd just do it myself, and did. The only reason I had taken it there was because it was winter and I didn't feel safe or wa rm working on it in front of my house parked in the street. In the end I de cided to make the 45 minute drive to a house with a heated garage.
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weelliott wrote:

sounds more like you spent several weeks on the problem, not two hours. A good shop would have repaired the exhaust, reset the Oxygen Sensor Monitors, and told you what they did, and also advised you that since the car is ten years old, replacing the Oxygen sensor now, would save you possible headaches in the future.
And they would have bought the right part, not butchered up the wiring harness like your dumb-ass did.
GW
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Boy, what got you crossthreaded?
I spent two hours on fixing the exhaust leak, not the O2 sensors. That incl uded the trip to the parts store to buy the parts. But I guess you're into manipulating or interpreting facts for your own agenda, in this case launch ing insults. Reminds me of another person that could be called G dubya.
Although the time I took the new sensor out, cut the connector off, crimped it back onto the old sensor, and reinstalled took less than 20 minutes. So the wiring aspect probably is about two hours total.
As for the O2 sensors, I don't see where you get off calling me a dumb-ass. Normally I'd just let your childish antics go, but this used to be a fairl y civil list, and perhaps you just need to be told that your behavior is un acceptable. The point where you could call me dumb would be what, not knowi ng that O2 sensor wire is stainless? If that makes me a dumb ass in your ey es, I guess I am, but your standards for dumbassedness must make you really fun to be around.
The crimp connectors are totally fine too. In fact because of wire length i ssues, those crimp connectors are still on the car along with another set. They work fine. It wasn't my crimping job that was goobered up. But if you want to find something to insult others on, you can make assumptions.
The part I bought is listed as the correct part for the car. I was sent a d ifferent one. I did not feel like going through the pain of exchanging the part, so I attempted the swap. It is not uncommon to buy O2 sensors that ha ve no plug at all. I've done it several times and used supplied crimp conne ctors to attach it to the original plug with 100% success rate. I chose to solder this time because I didn't have any crimp connectors for that gauge of wire.
I do not resort to your mystical mechanic's philosophy of swapping O2 senso rs based on mileage or age. I've drank that kool-aid and done that before. I replaced three or four on cars nearing 120K and never saw a bit of differ ence in mileage or performance. Not one bit. So I firmly believe in waiting until the CEL tells you to replace them. As for saving me headache later, it hasn't happened yet. If you were paying attention, the O2 sensor was not the culprit. Yes a bad O2 sensor will kill mileage, but change interval is a matter of opinion. And how much headache does a bad O2 sensor cause? It makes the car run non-ideally. That can be caught with a scan gauge or just by watching your mileage. I could bore you with how engineers determine ch ange intervals, but this post is already long enough.
I suppose perhaps if you'd known that my wiring actually was sound, and tha t I had ordered the right part, you wouldn't have called me a dumb-ass, but being that I never said I'd done any of those things, and you just assumed that I had, I guess your bias was really driving your view. People like yo u just look for people to call a dumb-ass, which is an unfortunate way to l ive.
So I'm frugal. I see no fault in that. I enjoy working on my cars. I don't like having others do for me what I can do myself, and I don't feel like fi nding an honest mechanic even though I acknowledge that there are several o f them out there.
So if you feel that it's always best to simply take your car to a mechanic, go for it. I've had experiences with good mechanics, and I've met mechanic s that I knew were trying to rip me off. After moving homes and away from m y known good mechanic to a house with a garage, I decided against gambling. So short of mandatory safety inspections, I haven't used a mechanic in at least nine years. That includes two clutches, a transmission swap, a head g asket, at least three subaru, two volvo, and one BMW timing belts with all associated seals, water pumps, a main seal, wheel bearings(take the hub car rier to the machine shop myself) CV joints, brake jobs, machining and insta lling an LSD on a car never offered with one, carb rebuilds, electronic ign ition retrofit in a distributor, multiple electrical issues much more compl icated than this, vacuum leak isolations, crash repair(not my fault), my ow n alignments, and I'm sure much more.
Despite all that work, this is the first time I've wasted significant money on a part I didn't need. No big deal. It barely borders on significance. A nd I still look at it as a win. IF you assume a 50-50 chance of finding an honest mechanic that would have looked past the code and tried fixing the e xhaust leak first by welding in a new flange or connector, then it's a 50% chance I could have been out of there for an hour or two of labor, and a 50 % chance I would have been at $300 for the retail price of the part, half h our or hour of labor, then still had the issue. Scenario 1 would be $80-$16 0, Scenario 2 would be $340-$380. So probable expense would be about $240. I got it done for $120 then tried sharing my experience with others that mi ght learn from it and save the $110. So I'm still $120 ahead.
So in conclusion, I still think that I do not need a mechanic, and that you need to act like an adult.
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