There's been a few cases (e.g. this thread) where a bad rear O2 sensor seemed to have been a the root of misfires in some 2005+ H4s. I don't recall any with the earlier H6. Nevertheless, I believe the rear O2 does have a role in fuel management (it's not only to monitor cat converter function) and a malfunctioning sensor could upset the fueling and cause misfiring.

Incidentally, do you have a way to monitor the activity of the sensors? (In the linked thread, the rear O2 sensor voltage was bad, but wasn't recognized immediately -- a learning experience.) Given the work you do on the cars, a program such as Romraider (RomRaider - Open Source ECU Tools | RomRaider / RomRaider) might be well worth having. It's free, and if you have laptop (Windows 7 or later) all it needs is an inexpensive Vag-Com KKL 401.9 OBD-usb adapter cable readily available on Amazon or eBay. It can display and record (permanent log file) all the same parameters that the Subaru dealer's Subaru Select Monitor covers.

 

There's been a few cases (e.g. this thread) where a bad rear O2 sensor seemed to have been a the root of misfires in some 2005+ H4s. I don't recall any with the earlier H6. Nevertheless, I believe the rear O2 does have a role in fuel management (it's not only to monitor cat converter function) and a malfunctioning sensor could upset the fueling and cause misfiring.

Incidentally, do you have a way to monitor the activity of the sensors? (In the linked thread, the rear O2 sensor voltage was bad, but wasn't recognized immediately -- a learning experience.) Given the work you do on the cars, a program such as Romraider (RomRaider - Open Source ECU Tools | RomRaider / RomRaider) might be well worth having. It's free, and if you have laptop (Windows 7 or later) all it needs is an inexpensive Vag-Com KKL 401.9 OBD-usb adapter cable readily available on Amazon or eBay. It can display and record (permanent log file) all the same parameters that the Subaru dealer's Subaru Select Monitor covers.

I’m game. If it’s only a couple bucks, I’m definitely game. I don’t have a laptop yet though. It’s on my to do list. Can it work with Apple?

 

I won't say it's not possible, but it really shouldn't. And if you're not getting other symptoms/codes for the rear sensor, it's probably a waste of time/money.

 

@plain OM I returned my blue driver. I’m going to try romraider and once I’m ready I’ll probably summon your help with it.

 

Sure.

Read the instructions on the RR site about downloading and setting up the linked ECU and Logger definition files. These are necessary for it's operation.

There's others here who are also familiar with Romraider (RR); I recall that @traildogck has a thread here somewhere on setting it up. And there's a RR Forum with discussions related to getting it going.

(Incidentally, when RR is booting up, it might request downloading a "cars" definition file; this is optional -- not needed for logging operation -- but if it is downloaded and installed, it avoids RR triggering that pop-up each time it loads.)

As noted earlier, RR has access to the same data stream as the dealer's Subaru Select Monitor and that stream has the most comprehensive coverage of engine and AT transmission parameters; more than the OBD data stream that most phone/tablet apps use. Nevertheless, apps such as Blue Driver and Torque Pro can still be useful, as they can be set up easily on a phone or tablet like a gps nav app, and can be used to display engine operation continuously, e.g., an enhanced instrument panel.

 

Back to the misfire, is there an exhaust leak? Do you have a thread with troubleshooting you've done so far?

 

 

@eagleeye @plain OM I'm going to the garage later today so I'll see what data I can pull with the scanner and see about uploading it to the computer. gotta change oil today and do a couple other things, might switch out a strut. may as well get stuff done while the weather is nice and I'm not infected with COVID19

 

So I hooked up to the O3 L.L. Bean today, but when I got home realized I might of accidentally erased the data I saved. Here is a general look at what the scanner says is a lot of things wrong. I think my next step is getting some starting fluid and see if I have any leaks. The voltage readings on the O2 sensors looked the same as the readings I took off the O4 Bean. Oh heck, I think it's a bad O2 sensor at this point anyway, maybe two I dunno. Maybe I can buy one with my stimulus check.
@plain OM @eagleeye

I'll try again another time to grab data after I get a little tutorial from Autel on how to get it uploaded to my computer. Anything specifically you guys want to see, provided the scanner can show it to me?

 

do you have used matching sensors from your previous cars ?

check rock auto and see if you can nab a denso / toyota label one out of a toyota camry with the o2 sensor on top front of the engine. (vs. crawling underneath in the muck,.)

and are the pick and pulls / autoparts stores open after tomorrow ? (....pick / pull being a nice place to excerise at 6 feet apart from the world at large)

 

do you have used matching sensors from your previous cars ?

check rock auto and see if you can nab a denso / toyota label one out of a toyota camry with the o2 sensor on top front of the engine. (vs. crawling underneath in the muck,.)

and are the pick and pulls / autoparts stores open after tomorrow ? (....pick / pull being a nice place to excerise at 6 feet apart from the world at large)

Great minds think alike, I thought about that before too, taking one from a Toyota. Toyota Camry's are always in the yard, but the H6 OBW's are only seen once in a great while. The two O2 sensors I currently have upfront on the 03 Bean now came out of the exact same car from out of the junk yard, which was an 03 Bean. They look like OEM. The downstream one is original I think, but I can't remember for sure, the car has sat for so long. I think I might of switched out the downstream one with the one I had on hand from the 01 VDC a long time ago. I don't think they are compatible between the 01VDC and 03 L.L. Bean. I truly wish that was my problem. It's hard to remember what you did when you have two L.L. Beans, and you've worked on the exhaust on both of them over the last 6 years.

 

To the OP... I skimmed through the posts here... OBD2 only uses the downstream oxygen sensor for monitoring catalyst efficiency. The O2 sensor itself cannot affect how the engine runs. However, the catalyst can. If you have a failing catalyst due to blockage, which would lead to increased backpressure, then the backpressure alone can cause a misfire, as exhaust gasses may not be able to escape fast enough inside the combustion chamber.

If you would like some piece of mind, then just unplug the rear O2 sensor from its connector and see what happens. Your misfire will remain the same. If it doesn't.... well, I'd be very surprised. I've worked on all makes and models and never seen or heard of this nor is it theoretically possible based on how OBD2 works. If we were talking about a 2016 or something then maybe I would yield quicker with some of the more modern ECMs and their advanced features, but there's just no way on your vehicle. BTW I read the post that someone else posted... It's very difficult to me to believe that someone fixed that issue by replacing a downstream sensor. I don't believe it. Plus, that was on an entirely different engine and ECU so just take that into account.


Here's an article worth reading.

WHAT THE HOME MECHANIC NEEDS TO KNOW ABOUT O2 SENSORS

 

I skimmed through the posts here... OBD2 only uses the downstream oxygen sensor for monitoring catalyst efficiency. . . I've worked on all makes and models and never seen or heard of this nor is it theoretically possible based on how OBD2 works.

Not what Subaru says about the rear O2 sensor.

From the 2003 SOA FSM publication G2300BE, (2018), H6 Engine, Diagnostics, Subaru Select Monitor section, Read Current Data for Engine (NORMAL MODE) chart [highlight added]:

"Normal Mode" is the Subaru Select Monitor scanner data stream. The same reference to the rear sensor is in the accompanying chart of parameters available using OBD II scanners:

From SOA Subaru Technician Reference Booklet, Basic Emissions and Fuel Systems, Module 405, page 38:
"The Zirconia Post Catalytic converter (downstream) Oxygen sensor is used for minor fuel trim modification. If there is a problem with the Post Catalyst sensor there could be an emissions or driveability fault. The sensor influence is difficult to identify however a variation from the known good reading could help identify an emissions or driveability air/ fuel fault. The Post Catalyst sensor is primarily considered a catalyst verifying input however in the history of Subaru fuel control there was an indication called “Phos” which identified an engine management role by the Post Catalytic converter Oxygen sensor."

And on page 47:

The linked "What the Home Mechanic . . ." article is correct as far as it goes, but that doesn't necessarily mean that the rear O2 sensor can't be used for more than just monitoring the converter’s operating efficiency, as Subaru is clearly indicating.

 

Well I'll be.... today I learned something! This is something I expected of post 2012 vehicles, not an early 2000s.

Also... this seems like an awesome piece of documentation you have! Would you have the ability to share? H6 and H4?

 

H6 are not H4,

and 2003-2004 US spec h4 had EGR,...so a little smarter than 2000-2002.