This is a well maintained 2007 Subaru with 140k km on the 3.0 litre six cylinder EZ30R engine. She's running a little scratchy most noticeable when cruising although no codes or anything.

I decided to take a look at the airflow situation and noticed bank two seems to be running richer with less responsive data for the pre-cat wideband AFR and post-cat oxygen sensor.

Couple of screenshots attached we can see AFR on bank two is less responsive and also the O2 sensor reaction on bank 2 can be a second or two delayed compared to the bank 1 signal.

I understand the two banks won't be perfectly in sync but more than a second difference seems long with EFI. Could be a normal situation too I guess. Perhaps this is an airflow problem internal to the engine.

Some tests I've done;

  • Switched sensors between banks, no change
  • Smoke tested exhaust and intake, no leaks
  • Replaced exhaust with a junkyard system as mine was rusty, no change
  • The removed exhaust didn't have signs of oil or or coolant cleaning
  • Checked fuel trims, all 3% or lower

The graphs below show data sampled at 20 rows per second;

Screenshot One Screenshot Two

Appreciate your help. I'll post responses and comments, hate to see a question die here.

EDIT 2019/06/16 Removed previous propane graph to save space. Adding results of my next propane test from @Zaid's help. The propane bottle may have been running a little low but the results tie in with @Zaid's image regarding a bad cat. Also added a chart showing AF & O2 sensors whilst driving steady and gentle on the highway, bank #1 O2 sensor is all over the place!

Propane test, second attempt Highway cruising

EDIT 2019/06/20 Now logging STFT to find bank 1 is cycling additional STFT at cruise, would this mean an air leak someplace ?

Bank One Bank Two

  • Great detail in the question! When you replaced the exhaust did it including replacing the catalytic converters?
    – Zaid
    Jun 11, 2019 at 9:55
  • Thanks @Zaid, I replaced the exhaust from the manifolds to the middle of the car, this included both left & right catalytic converters and the Y piece that joins the banks together under the middle of the car. I was surprised to see the same symptoms although it's likley the replacement parts were from a high mileage car.
    – ajayel
    Jun 12, 2019 at 3:06
  • 1
    I would use the propane test to confirm if the Bank 1 cat is bad.
    – Zaid
    Jun 12, 2019 at 7:43
  • OK I read the propane test Q&A it looks very interesting and will try it over the weekend
    – ajayel
    Jun 13, 2019 at 8:04
  • 1
    I think the chart you removed is clearer than the current one in showing the erratic behaviour of the Bank 1 cat. It does look like the Bank 1 cat is on its way out. It looks fine for the most part at idle, but even at part loads it doesn't seem to be able to function well.
    – Zaid
    Jun 16, 2019 at 20:42

1 Answer 1


It looks like there is an issue with Bank 1, not Bank 2.

Given the stellar job you've done with ruling out false readings due to sensors or leaks, it looks like the catalytic converter on Bank 1 is not doing a good job scrubbing out the oxygen, which is why the Bank 1 post-cat O2 is cycling between rich and lean.

Lambda probe test

You can see more information in my answer to this Q&A:

How can I tell if I have bad cats?

  • Would this situation affect short term fuel trim ? I found bank 1 is adding STFT cyclically but bank 2 is not
    – ajayel
    Jun 19, 2019 at 20:37
  • 1
    Yes, it can. See this post for more information.
    – Zaid
    Jun 19, 2019 at 21:02
  • TIme to follow up, found replacement cats at the wreckers the bank 1 rear o2 is reading way more stable now thankyou @Zaid
    – ajayel
    Jul 15, 2020 at 4:02

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