Diagnosing P0420 CELs

[Ric]

I'm occasionally getting P0420 codes thrown in my manual 3.0RB.
It only happens when on cruise control doing about 80 on a freeway.
All my reading points to an O2 sensor, but I'm not sure if it's one or both of the front sensors, are maybe the rear.
Subaru suggest it's a failed cat, which they are happy to replace for only $5,000

I have a Bluetooth OBD2 interface, and Torque-free installed on my Android phone.
I think by logging the O2 sensors, I should be able to establish if they are the problem.
Only a few of the O2 sensors listed by Torque actually seem to give a reading.
Can anyone advise precisely which PIDs I should be watching, and what for?

[Kekotic]

I'm occasionally getting P0420 codes thrown in my manual 3.0RB.
It only happens when on cruise control doing about 80 on a freeway.
All my reading points to an O2 sensor, but I'm not sure if it's one or both of the front sensors, are maybe the rear.
Subaru suggest it's a failed cat, which they are happy to replace for only $5,000

I have a Bluetooth OBD2 interface, and Torque-free installed on my Android phone.
I think by logging the O2 sensors, I should be able to establish if they are the problem.
Only a few of the O2 sensors listed by Torque actually seem to give a reading.
Can anyone advise precisely which PIDs I should be watching, and what for?

Ric FYI Matt disables this code on his tunes because it is basically for emissions on the rear cat and doesn't do anything.

viewtopic.php?f=16&t=21348&p=304333&hilit=P0420#p304333

$5000 lol

[jakey]

Ric FYI Matt disables this code on his tunes because it is basically for emissions on the rear cat and doesn't do anything.

[Kekotic]

Ric FYI Matt disables this code on his tunes because it is basically for emissions on the rear cat and doesn't do anything.

S Hat Icture F Arth?

[KiDo_Tuning]

It is the rear O2 sensor detecting higher than normal Exhaust Gas Temps based on closed loop feedback from the Heater circuit. ECU assumes that your cat has failed and thus throws code. It is easily turned off, even if you don't want a tune or have an existing tune

[jakey]

Ric FYI Matt disables this code on his tunes because it is basically for emissions on the rear cat and doesn't do anything.

S Hat Icture F Arth?

Es. Ad arth.

[Kekotic]

Es. Ad arth.

Kay Hanks Akey.

Ow Ack N Opic!

[Ric]

It is the rear O2 sensor detecting higher than normal Exhaust Gas Temps based on closed loop feedback from the Heater circuit. ECU assumes that your cat has failed and thus throws code. It is easily turned off, even if you don't want a tune or have an existing tune

Thanks Matt.
I may well get the code disabled soon, but I would like to identify the root cause first.
Do you think I can establish that by logging all the sensors? I can only find two that appear to be active in Torque.
Alternatively, do you think one of your tunes would clear it up anyway, even without disabling the code?

[KiDo_Tuning]

It is the rear O2 sensor detecting higher than normal Exhaust Gas Temps based on closed loop feedback from the Heater circuit. ECU assumes that your cat has failed and thus throws code. It is easily turned off, even if you don't want a tune or have an existing tune

Thanks Matt.
I may well get the code disabled soon, but I would like to identify the root cause first.
Do you think I can establish that by logging all the sensors? I can only find two that appear to be active in Torque.
Alternatively, do you think one of your tunes would clear it up anyway, even without disabling the code?

Tune won't fix it unless I turn off the rear O2 sensors influence on the tune which is essentially the bandaid fix to turning off the CEL anyway

[Ric]

Thanks.
So is it in fact the Cat slowing dropping off in efficiency, or a sensor dying that causes these errors to start appearing?
From my research, it's triggered by a lack of difference between the front and rear readings (which a bad cat would cause), but can also be triggered by a slow responding front sensor.
My car has done 188,000 km, with a stock tune the whole time.
(Although to be pedantic, the engine was replaced at about 28,000k, so it has only done 160,000)
I usually run Shell-95, with an occasional tank of Optimax, or the BP/Caltex equivalent

[KiDo_Tuning]

Thanks.
So is it in fact the Cat slowing dropping off in efficiency, or a sensor dying that causes these errors to start appearing?
From my research, it's triggered by a lack of difference between the front and rear readings (which a bad cat would cause), but can also be triggered by a slow responding front sensor.
My car has done 188,000 km, with a stock tune the whole time.
(Although to be pedantic, the engine was replaced at about 28,000k, so it has only done 160,000)
I usually run Shell-95, with an occasional tank of Optimax, or the BP/Caltex equivalent

I have turned the codes off for dealers with secondhand vehicles who have replaced sensors and cats to get them off their used car lot... nothing worse than a car that throws a CEL during a test drive lol

[<GB>]

I disabled mine as it used to come on lots it's a pain on a long drive if it comes on and then can't use cruise

[Robbks]

mine never came on with any consistency.
sometimes it would show up after a flogging, other times was just after a long flat cruise
it's gone now

[seadevil]

If the code is very easily disabled can someone please post the process?

[HardwareBoB]

Read tune from ECU
Go into romraider and tick the code as excluded (or whatever the term is)
save the image
reflash.

.. someone else or I can post a more detailed version, but as long as you've played with romraider and can flash your car it's pretty straightforward