Join Date: May 2006
Location: Gaithersburg, MD
Car Year: 2009
Car Model: Outback XT-B
The problem with having the sensor throw a code so that the service guy can "keep an eye on it" is, I dunno, annoying? Good, alert the service guy. But they can't turn the light off or change it's sensitivity. I dunno. There is probably some reason.
Here's the thing I've noticed about Subaru's. Unless something is catestrophically wrong, they aren't going to be horribly polluting cars. My 1994 Turbo Legacy passed the Maryland State Emissions test with no cat's what-so-ever. Totally catless, their sensors barely picked up trace amounts of anything remotely harmful even on a load-dyno, and that was with some 99,000 miles on it!
You will notice other symptoms of a bad catalyst far before the car begins emitting dangerous levels of anything. First and foremost, you'll probably notice a change in smell, followed closely by a change in fuel-economy. At that point, you may consider getting a new catalytic converter.
I too am an engineering student. I like to explore every possible option and do my research so I can make as few compromises as possible.
In all seriousness, the P0420 is only good for making your wallet lighter by either ~$100 for a new sensor or several times that for a new cat. I've never seen a P0420 actually point to a bad catalytic converter.
As far as how an O2 sensor works, I'm not entirely sure. There are a bunch of different types using 1, 3 or 4 wires. As far as I know, and I'm not really sure, it is essentially a miniature fuel-cell system. Exhaust gas passes through an element made of ceramic that contains platinum electrodes. From this sample is what powers the fuelcell (which is based off of Zirconium Dioxide). The voltage generated is, in fact, a comparison between outside air and exhaust gas. Low voltage means lean, high voltage means rich. BUT, here's the kicker. The ECU doesn't care what the rear O2 sensor sees or reads. All it does, is compare the numbers and their rate of change to the front O2 sensor. If the rear sensor's readings and output approach the front O2's readings and output, P0420 shows up thinking that there's not enough change from the catalytic converter.
So what might cause a failure? Contamination is a very big one. Race-gas kills O2 sensors because it coats them with lead. Same with oil residue and carbon deposits. They coat the sensor and cause problems.
But what about the front sensor? If it is coated and gross from buildups of carbon and oil, it would cause a delay in reading which could slow it's response time sufficiently enough to cause a P0420. That's not outside the realm of possibilities, is it? The ECU only monitors the rear O2 sensor for catalyst efficiency. It would never know that something was wrong with the front O2 sensor. Something that could be evidence to this would be poor gas mileage, espically in the city (as the ECU would respond too slowly to an overly rich condition). Since the ECU err's on the side of rich anyway, all the O2 sensor really does is let you know when it's WAY too rich (or if something is wrong, it'll say lean. If the O2 reads lean, it's probably too late anyway).
Here's what I'd do. Call around and find out if any shop has a Subaru Select Monitor. One of the older units with the cool graphical display. Plug it into the OBD2 port, and it will allow you to monitor O2 sensor output graphically. You should be able to note a significant lag in front O2 sensor performance, and that could help you narrow it down.
If that's too time consuming, or no one has one of the already rare Graphical Select-Monitor's, simply pull out your front O2 sensor and inspect it.
My Legacy only had 1 O2 sensor. Since I had no cat's, it was effectively getting abused as if it was the first Oxygen sensor. Like I said, when I pulled it out, it was REALLY gunky after 115000 miles. I'm sure there is some spray that is safe for Oxygen Sensors. I know there is some stuff called "Electrical connector cleaner" which is just a diluted spray of isopropyl alcohol (I think), so that couldn't harm it very badly. I don't know why some non-chlorinated brake-cleaner wouldn't work either. There might be some residue left over, but the O2 sensor heater should be sufficient to burn that off. I think the O2 sensor heater brings the sensor up to near 275-ish degrees celsius.
Just throwing some ideas out there. I'd strongly suggest pulling that front O2 sensor and simply looking it over. I could be wrong, but if I'm right it'll save you a lot of headache, time and money. If I'm wrong, at least you ruled out one possibility.
09 SWP Outback 2.5XT 5MT 263whp, 337 lb-ft