2000 Outback Wagon w/P0420 code, already replaced both 02's

[DEVO]

2000 Outback Wagon w/P0420 code (105K miles). I did a search (yeah, lot's of hits), but my question is..I already replaced both the front and rear O2 sensors (as well as the knock sensor) before the light came on, when the light came on a few months ago for the O2 sensor .

 

Have had the CEL on for a week or so now. For both O2's I used Bosch units from Autozone. I am hoping it is just the PCV valve and not the cat. Could the front Bosch O2 sensor be the cause? I have read you should only use front O2's directly from Subaru (although I have heard they are made by Bosch?). Thinking about just replacing the pcv valve to see if that does it.

 

The actual code is P0420: Catalyst efficiency below threshold - bank 1

 

Does "bank 1" refer to the front O2 sensor or front cat?

Edited July 7, 2010 by DEVO

[GeneralDisorder]

The code refers to the difference in reading between the front and the rear O2 sensors. It's looking for a specific percentage effeciency from the cat.

 

Since you replaced both sensors, it is likely that you either have an exhaust leak that is reducing the temperature (and therefore the effeciency) inside the cat, or the cat is actually bad.

 

Aftermarket cat's don't work out for a lot of people - they either aren't effecient enough to keep the code away or they don't fit well enough to seal and stay hot. Subaru cats are horrifically expensive but generally solve the problem.

 

You can install a spark-plug non-fouler between the rear O2 sensor and the exhaust bung. This will cause the rear sensor to read lean and eliminate the code. The non-foulers are about $4 to $8 at any auto parts store.

 

This problem has been discussed at length on this forum - do some searches and you'll find whole discussions about it's repair and how morally bankrupt a bunch of us are since we opt for the $4 fix and get on with our lives.

 

And there is only a single bank on Subaru's in general with respect to O2 sensors. "Bank 1/Bank 2" refers to larger engines that have (typically) a 4 sensor system where there are two front's, two rear's, and two cats. Then you will have a right/left bank associated with either side of a V6 or V8 engine. On a Subaru you will only ever see "Bank 1" related codes.

 

Oh - and for sensor brands - I use NTK, which is NGK spark plug's sensor division. I have never had a problem with any of NGK's products and I use their plugs/sensors exclusively for all my Subaru work. I also have one of their wide-band tuning sensors and it's a great product:

 

http://www.ngksparkplugs.com/products/oxygen_sensors/index.asp

 

GD

Edited July 7, 2010 by GeneralDisorder

[idosubaru]

if you don't have any exhaust leaks then you need the non fouler fix or just buy a ready made part off ebay for a few dollars.

 

bank 1 is just the front like he said. bank 2 is used on the H6 engines, not the H4's, as they have two front O2 sensors.

[porcupine73]

Sometimes if you have any little pinhole exhaust leaks it can cause issues too. Just curious, on that Bosch sensor you bought, did the connector match up with the vehicle, or did you have to splice the wires to the original connector? The '00obw has an interesting connector, with like 6 wires on a 7 pin plug iirc.

[J A Blazer]

I would also suggest checking your knock sensor. I know that you replaced it, but on my 2000 4cyl OBW, when the 420 code comes on it is accompanied by a knock sensor code at the same time. I will replace the knock sensor and hope that I don't see the 420 anytime soon.

[Steves72]

I would also suggest checking your knock sensor. I know that you replaced it, but on my 2000 4cyl OBW, when the 420 code comes on it is accompanied by a knock sensor code at the same time. I will replace the knock sensor and hope that I don't see the 420 anytime soon.

 

That would be a complete waste of money unless he too has a 0325 code in the computer.

 

I would suggest trying the PCV valve and examining the exhaust has been previously suggested but be prepared for that new cat. And, do not go with the a cheap unit. When it comes to the cat you really do get what you pay for.

 

Steve

[wirelessenabled]

I have had an on and off P0420 on my '00 OBW for about 36 months.

When it started I replaced the plugs, coil and air filter and front O2 sensor,and cleaned the idle air controller. After that it would come on a few times a week and then go off. Two weeks ago I replaced the plugs again, after only 30K miles on them, and the check engine light has been off since, over 1500 miles.

 

Maybe I had a bad/defective plug? My mileage also increased about 3 MPG to right at 27 MPG.

 

Who woulda thought?