CEL code P0420

[ryanw]

98' Subaru Outback with 197,000 on it. Bought it 6 months ago and immediately had the head gaskets go out (after driving/owning it for 4 hours). Did the 2.5/2.2 swap and she has ran fine except only gets 22-23 mpg... seems the same city or highway.

 

A strange side note is, the 3 times I filled up at half a tank, I was getting 25-26 mpg, which is what I would expect... do you think this is just a coincidence or does this signal a problem? I guess I should start filling up at half tank every time for awhile to see how it goes.

 

Back to original problem, P0420... from what I've read, replace the front O2 sensor first, which I did and the CEL went away for about two weeks and appears to have improved my gas milage by about 1 mpg as I get 23-24 now which I have not gotten in a long time.

 

But CEL P0420 came back on... from what I've read, its the Cat (are there 2 cats and O2 sensors on this car?). Should I replace the cat or should I take it in to have someone further diagnose the problem to ensure it is the Cat causing the problem?

Edited September 30, 2008 by ryanw

[ryanw]

Okay, I RTFM, and there are two O2 sensors and I replaced the rear one, which is the wrong one right? Normally the problem is caused by the front one (they are just like a foot or two apart)? However I also read on here that the front one HAS to be a Subaru O2 sensor, is that correct? If so, then I guess its good luck because I used an aftermarket for the rear O2 sensor, and it seemed to help gas mileage a bit, so I'm not worried about it.

[Jerry DeMoss]

Yes you are correct that you are better off getting a new O2 for the front one from subaru. I think that the rear that you replaced with a different one should be fine.

[idosubaru]

might want to check the O2 sensor plugs (sensor and engine wiring harness side), make sure none of the wires/plugs were damaged during the engine swap. i've seen pins pull out before. also make sure the plugs are fully seated.

 

if you end up getting a new cat, your old one is worth $120 at a scrap yard. new converters can be had for cheap (aftermarket).

[ryanw]

I have replaced the plugs/wires (which really needed it), fuel filter, PCV valve, and the rear O2 sensor (mistakenly thinking it was the only one). However, when I pulled that rear sensor, it was covered white and the slits were somewhat clogged, so I really thought that would fix it because (based on my non-expert opinion) it looked pretty bad. So if the front looks anything like the rear, I think that is probably the problem (again based on my non-expert opinion). Can you tell anything from looking at the O2 sensor, like since the rear was covered in white powder, does that mean anything or is that normal for 197,000 miles? Or could that just have been from the previous 2.5L engine going through several sets of head gaskets and having coolant leak into the engine?

 

On a side note, can someone tell me which front O2 sensor I need? I looked it up and subaruparts.com and came up with 22690AA290 for 96-99 ej22 and ej25s. However, when I called the local dealer and told them I had a 95 EJ22 engine with a 98 ECM, they said I would need 22641AA00A. Anyone know which O2 sensor I need to replace the front with?

[johnceggleston]

Anyone know which O2 sensor I need to replace the front with?

 

 

www.subarupartsforyou.com

 

engine parts, sensor o2 = 22690AA290

[ryanw]

www.subarupartsforyou.com

 

engine parts, sensor o2 = 22690AA290

 

Thanks, I came up with that part number as well, but when I called the dealer and the guy told me it was the wrong one I got worried.

 

I just want to be sure I have this right... if I have a 95 EJ22 engine in a 98 outback I use part 22690AA290 for the front O2 sensor right?

[johnceggleston]

Thanks, I came up with that part number as well, but when I called the dealer and the guy told me it was the wrong one I got worried.

 

I just want to be sure I have this right... if I have a 95 EJ22 engine in a 98 outback I use part 22690AA290 for the front O2 sensor right?

 

95 used a different o2 sensor i think, 2 of them , front and rear were the same.

 

you need the o2 sensor that matches your ECU, so unless you changed the computer as well as the engine, you need the one for the 97-98.

 

when buying parts, never mention the 95 2.2L engine, unless you are buying engine parts. you'll just confuse the parts guy.

Edited October 1, 2008 by johnceggleston

[ryanw]

95 used a different o2 sensor i think, 2 of them , front and rear were the same.

 

you need the o2 sensor that matches your ECU, so unless you changed the computer as well as the engine, you need the one for the 97-98.

 

when buying parts, never mention the 95 2.2L engine, unless you are buying engine parts. you'll just confuse the parts guy.

 

I did not change the ECU. Lesson learned Thanks John!

[ryanw]

So I replaced the front O2 sensor with one from the dealer and cleared the code. So far it has not come back, but time will tell I guess. If the code does come back, the next step in this process is to replace the Cat right?

[ryanw]

Thanks nipper, but I did that on accident first :-) so the rear one has been replaced with generic and front one replaced with OEM. It's my wife's car and she says its driving much better since I replaced the front one, so I hope that was the problem... but I was just wondering if CEL p0420 did come back, if the Cat is the next step since both O2 sensors have now been replaced.

[ryanw]

I replaced the PCV shortly after I installed the 2.2L. However, I did use a generic on that. Should I go buy one from Subaru?

[Bmm001]

I second Nipper on the PCV Valve. Even the Subaru ones are cheap, and take about 2 minutes tops to swap in. I also agree that there are plenty of other things to look at before a new converter if the code won't go away. Find a mechanic who will diagnose the problem and not just pull and read the code and tell you the converter is bad.

 

Example:

My girlfriend's '02 Corolla has been plagued with PO420 since she's owned it. She's had 3 different catalytic converters that she probably didn't need in the 3 years she's owned the car. Then she found an honest mechanic who actually hooked up a scan tool and let it plot the O2 sensor readings rather than just pulling the code.

 

Next, he took into consideration the fact that her car burns about a quart of oil every 700 miles, and started pulling the spark plugs found them soaked with oil. He determined that leaking valve guide seals were allowing oil to be sucked into the combustion chamber and blown out through the exhaust. The oil being burnt was skewing the readings and triggering the code repeatedly. Nothing wrong with the converter, nothing probably ever was.

 

Brian