Oxygen sensor?

[brus brother]

My 05 auto outback has two precat air fuel sensors, two post cat oxygen sensors and one oxygen sensor on the rear cat. P0420 giving me a headache. No obvious exhaust leak. Got a cheap cat and will be installing tomorrow. Changed the post cat sensor at 105 k miles when code first appeared but that changed nothing. Now at 190 k miles fingers crossed. Could the rear cat and or sensor relate to the p0420?

[GeneralDisorder]

Pull up live data and watch the rear O2 readings. If they oscillate and aren't a steady 0.7v when idling at operating temp and your fuel trims are within normal parameters.... then you need cats. It's not necessary to guess. 

GD

Edited July 4, 2019 by GeneralDisorder

[brus brother]

Thanks GD.

I don't have live data ability.

Small correction in original post. the front a/f sensor was replaced at 105K miles. Cured nothing. Mis-diagnosis by wallet!

Today the passenger cat was replaced (Eastern Catalytic converter) and still throwing p0420. Their "direct fit" required additional gasket to fit!

I have a new o2 sensor for that cat which I will swap in as soon as the car cools down.

Subaru tech suggested that I might have to change out driver's side cat as well. Apparently that is sop in the shop as they want the $ and don't want the customer returning dissatisfied with same  CEL complaint. He also suggested that our winter gas here in CT that kills cats.

[GeneralDisorder]

A scanner that does basic live data is like $50 or less. I would humbly suggest that anyone attempting to appease the computer ought to highly consider asking it what it wants. 

Honestly you have no business working on that code without the proper tools. 

GD

[brus brother]

duly ashamed

[brus brother]

Car cooled down. placed new O2 sensor post cat on passenger side and.. p0420 came right back.

So to recap, new cat and O2 Denso downstream sensor.

Precat A/F sensor is 80K old. No obvious exhaust leaks other than the steam coming out of my ears.

I'd love to eliminate the cat reading from the ecu and would appreciate any off-line direction. Please PM me if you have know how for this 05 Outback AT 2.5 non turbo.

While our state of CT is hot on emissions, the wheels can be falling off your car and still pass inspection.

Edited July 4, 2019 by brus brother

[GeneralDisorder]

A live data scan tool is much cheaper than a Tactrix cable and the learning curve you will incur getting up and running to do a custom ECU reflash.

And in any case what if it the problem isn't the cat or the O2's/AFR's at all? What if your fuel trims are out of whack because of a MAF problem or intake leak? 

It's easier to just fix the problem. And for a 2005 that means knowing what inputs the ECU is recieving. 

I bet if you give the ECU a 0.7v signal back into the secondary O2 signal wires it would be happy. No guarantees but that's basically what it's looking for. 700 millivolts on the narrow band sensors. 

GD

Edited July 5, 2019 by GeneralDisorder

[brus brother]

12 hours ago, GeneralDisorder said:

I bet if you give the ECU a 0.7v signal back into the secondary O2 signal wires it would be happy. No guarantees but that's basically what it's looking for. 700 millivolts on the narrow band sensors.

Interesting suggestion. So If I were to cut the wire going back to the ecu and splice in a power source with the correct millivolts, it might do a trick? hmmm. splice in a  700 mv diode on the return line?

Car runs great and gas mileage hasn't changed any through the years.

Reason I dove in with both feet to replace the cat was that around 6 years ago, I did provide live data to CarDoc over on the outback site and he suggested bad cat. I will hunt around for the cable and software I used at the time and repeat.