P0420 code and odd front O2 sensor readings

[Rogue38]

So, I have been getting the P0420 code for the last week or so on my 2007 Legacy Limited. Issues started with the P0171 code, for that I straightened the intake duct that was cocked and cleaned the MAF. That seemed to take care of that. Then the P0420 came up on the way home from work. Checked over the car with nothing obvious noted.

I cleared the code and decided to see if it returned, set up and my ran my Actron live drive on the way to work and for the next few days. After several days I took the scanner off the car.  Sure enough the code returned the following day. This was about 250-300 miles later, in the exact same spot on my commute. I viewed the freeze data and found the o211 mA at around 250.00. Once again, I cleared the code set up and my ran my Actron live drive on the way to work and for the next few days.

The o211 mA is jumping around 0 to 250.00, when at idle or below around 50mph. On the open interstate 60 to 75 mph the o211 mA is 240 to 258. When slowing from 70 the reading drops to 0 to 5, then starts jumping up and down again. Fuel Trims seem to be normal as does other indications. Once again the code returned about  two days and 170 miles later.

I’ve done a bunch of searching on this issue, but most talk about voltages, which on my car seem to be normal. I could not find much on what the milliamps are supposed to be, except one article that leads me to believe it should be in the 1-5 mA range.

I’m tempted to replace the front O2/AF sensor, but I am wondering if the high milliamps and jumping around is the sensor or something else. Car runs fine and 30 plus mpg.

Here is a link to the video of the live drive showing the sensor indications:

 https://youtu.be/rE_UYyz9Ezw

Any ideas?

Edited March 16, 2023 by Rogue38

[KZJonny]

Any idea how old the sensor is? Genuine Denso part or maybe not?

I did not dig as deep as you did, looking at trims and mA of the AF, but when I started getting P0420 codes I chucked in a new sensor, and have been all good since. I must have 6-7 years on my OBW, which may actually be an NTK part.

The GT got a Denso, and 2 years on that so far without issue. Not necassarily one to just start throwing parts at a problem, but if the sensor is sending bad readings and nothing else points at a problem, it's probably it. They're a wear item after all....

[silverton]

I would avoid NGK and NTK.  They make good spark plugs, but not much else. 60 day warranty on their parts tells me they dont stand behind them.  Denso O2's are 30 days, even worse!  Bosch and Delphi are 12 months.

[Rogue38]

22 hours ago, KZJonny said:

Any idea how old the sensor is? Genuine Denso part or maybe not?

 

I believe it is the original. Looks like a factory original. Popped the code again, coincidentally in the exact same spot on my commute. This time only one day later and about 70 miles.

I took pictures of the freeze data, but the O211 didn't show on it but these did:

ST FTRM1: 0.7

LT FTRM1: 4.6

ST FTRM3: -100.6

LT FTRM3: -100.6

I've never seen FTRM3 before

[KZJonny]

I’m not selling kool-aid about brands. Buy what you like and can afford! I think the NTK was the best I could afford at the time. I’ve had success with Denso in my Japanese cars over the years, but the Bosch sensor in my wideband has also been reliable. Ymmv.

Do you only get fuel trims for 1&3? Without knowing how your obd reader displays info, I have no insights, but a trim of -100% definitely seems to indicate something wonky… maybe someone who knows a little more about how to interpret that can help. I’ve become spoiled by having an Accessport now.

[Rogue38]

21 hours ago, KZJonny said:

Do you only get fuel trims for 1&3? Without knowing how your obd reader displays info, I have no insights, but a trim of -100% definitely seems to indicate something wonky… maybe someone who knows a little more about how to interpret that can help. I’ve become spoiled by having an Accessport now.

I can se Fuel trims 1, 12 and 3. Not sure the differences though. i only can see the FTRM3 with the Freeze Data when the code trips, I cannot see it in live drive like I can everything else. Watching the sensor readings everything looks fine while driving. The code has not tripped during the time I have had the scanner attached and driving.

Going to smoke check the car and see I can locate leaks before throwing parts at it.

[m sprank]

420 is catalyst over efficiency.  Have you checked your cat?  O2 reads off resistance.  Voltage and milliamps are the heater circuit.  When the heater fails, the O2 fails.  420 could be a bad sensor, or it could be what the code is designed to tell you, your cat is failing. 

[silverton]

There is a service bulletin (WVM-23R) for your MY, regarding catalyst efficiency.  It's an ECM update! Unfortunately if you're already producing a 420 code, you likely need to replace the catalyst along with the ECM update, if not already applied, to keep your new catalyst from failing.  I recently had a car that had a 420 code go away by replacing the spark plugs, BUT that car was also presenting fuel trims indicative of that, while your fuel trims seem fine.  That -100 is data that your meter can't interpret, the subaru diagnostic protocol is a modified basic protocol so it ends up with little errors like that on non-subaru scanners.  Torque would do the same thing with my ELM327.

Diag for P0420 is a bunch of wiring resistance/voltage checks, only part replacements mentioned are the catalyst itself, and the rear O2 sensor. alldata's identifix service show the coil pack and spark plugs as potential fixes as well, but I think if it were either of those we'd see that in your fuel trims.

Apparently #4 was too hard for someone to change, three of these have 60k on them.... The gap on the bad one was .077

[Rogue38]

Well it looks like the P0420 code is the least of the issues now. Decided to poke around the engine today and found this.

[Infosecdad]

Looks like a split boot from a CV axle flung a bunch of axle grease up there.

[Rogue38]

Unfortunately not.  CV Boot is completely dry and loss of a quart of oil over the last two tanks of fuel say it is engine oil.

[Infosecdad]

Where is that picture in the engine bay?

Weird that it's engine oil, don't usually see a splatter like that.

[Rogue38]

48 minutes ago, Infosecdad said:

Where is that picture in the engine bay?

Weird that it's engine oil, don't usually see a splatter like that.

The cv boot look fairly new. So some may be left over from a previous repair.  There is definitely fresh oil down there. I may clean the area up and watch it.

[KZJonny]

Bummer of a diagnosis dude. I am also in the 2.5i needs new head gaskets club. Looks like a simple enough job if you've got an engine crane tho...

Nice having the double whammy of both CV grease and dripping oil to make it easy to see the condition of the engine at the head gasket!

 

I've got a thread going on this elsewhere. I'll post up some pictures and my experience with the fix, if that would be in any way helpful to you.

 

Cheers!

[Rogue38]

It's going to the local shop Monday.  My family has used them for years and trusts them. I can do the job, but I down want to/have the time. Heading to Myrtle Beach in a few weeks and don't want to take my truck.