Converter efficiency, and one O2 sensor...

Mazda RX8, here goes:

CEL code is "P0420".  Catalyzer efficiency.

OEM cat was gutted when I bought the car.  No CEL thrown, so I didnt know.  I've replaced it with an off-brand unit just to get me through inspections.  The morning of my most recent inspection, this CEL popped up. 

I stuck my dongle in the slot (snicker) and thought I'd monitor the situation on the "Torque" app.  The goal was to watch the leading and trailing (pre vs post cat) O2 sensors and compare the values.  If they matched,I'd but a new catalyzer.

Problem is, when analysis was attempted only the "bank 1, #2" sensor gave any values...

Definitely not what I was expecting.  BUT, having never done this before, I wasn't sure if this was normal.

Here are the screen shots:

In the photos, I've selected every O2 sensor option available for display.  Only "Bank 1, #2" gives a reading.  I'm interpreting this as "post cat" (single exhaust).

I'm penniless and overdue for an inspection, so I'd like to get this right the first time (and for as little money as possible).

Questions are:

Am I seeing a failure of the pre-cat sensor, (Normally I'd expect a code for such things, right?) or is this normal operation for this here make/model when read by this dongle/app?

If failed, would a pre-cat sensor cause this CEL? (Read, should I replace the pre-cat sensor as a fix for this issue?)

If failed, why would my car be in closed loop?  Is it reading it's tailpipe sniff (snicker) off the aft sensor?

Of note, I think I can hear the slightest of exhaust leaks coming from the header to cat gasket.  I have a spare and plan to replace.  Please advise if that's a contributor to said sensor reading (I do believe it may be a factor in the CEL, but not 100%).

Cheers!

I don't know much about the RX8 ECU specifically, so this is general advice:

1) A bad sensor seems like a likely cause, but there are other ones.  Sensors are not too expensive, but if you're strapped for cash then it's worth doing some additional diagnosis first.  I would start by carefully inspecting the wiring to see if there's anything that's rubbed through or been attacked by rodents (or hacked up by previous owners).  If the connectors are the same on both sensors then I would also experiment with swapping them around and see if the failure follows the sensor (suggesting a bad sensor) or the position (suggesting either a problem in the ECU circuitry or perhaps an incompatibility between the OBD2 implementations in the code reader and the ECU)

2) The catalyst efficiency is monitored by comparing the values of the pre- and post- cat sensors as the computer changes the fuel mixture, so yes, a failed sensor will mess with this monitoring.  That said, I'm surprised there isn't a code for the sensor itself if this is the case.

3) I think it's unlikely that the ECU is running the engine off the post-cat sensor.  Usually this is used only to monitor cat performance.

4) Theoretically an exhaust leak before the O2 sensor can cause oxygen to be drawn into the exhaust and make the O2 sensor read incorrectly (or, well, it's correctly reporting the presence of oxygen, it's just no longer correlated to the combustion process).  I don't think this would make it report "no data" though.  Physically swapping the two sensors would help with diagnosing this too.

Really odd that you are not getting a first sensor failure. It should not be able to run a cat efficiency test with that signal. I, too, would suggest looking into why that is a dead signal before throwing stuff at it.

Its quite possible that its running closed loop off the rear sensor- those are in a very safe spot for most O2 damage. So they make good back ups. Just slow to respond with a catalyst taking out all of the O2 in the exhaust. 
 

look for the sensor to be disconnected. One plugged in or with a dead heater would show something. 
 

Also, while you should deal with leaks, the sensor is reading rich, so it's not really affecting the signal. 

That's a scantool error.  Even if a sensor is unplugged, you should get a reading, erroneous may it be.

Are you sure you are reading the right PIDs?

I would try a different scan tool that has the correct definitions loaded and go from there.  If you have no exhaust leaks and your rear O2 is switching properly, you likely have a dead catalyst.  

Is there an air fuel (AF) ratio sensor?  That could be B1S1.

Also, a P0420 is pretty much a junk cat.  If it were missing a signal from one of the sensors, it would have codes for bad sensors. 

If the rear sensor is cycling rich/ lean at any speed at all, the cat is junk.

Thanks guys.  I got the cars swapped and the Mazda is in the driveway and ready to be worked on.

I can try swap-tronics on the sensors (and I'll inspect the connections, etc while in there).  I can also try a different scantool (I got local help with a proper laptop for such things).  I'm getting the feeling that Streetwise is right, though.

The exhaust leak is pretty minor, but I'm going to order a new gasket/hardware kit to correct it.  I'm looking at cheap options on e-bay just to stuff a mid-pipe under it for the purposes of inspections (I just missed an OEM unit from Italy for $350).  If this one is dead, it might be my fault as I let the car sit for a couple long periods and flooded it when I re-started it (I did that twice).  Both times resulted in the typical rotary smoke screen.

More to follow.

Well, I happen to have an rx8 cat in my storage.  It is from 2004 and oem.  I have no idea how to tell what sort of condition it's in.  

I can try to get a shipping quote if it's something that might help you!

wvumtnbkr said:

Well, I happen to have an rx8 cat in my storage.  It is from 2004 and oem.  I have no idea how to tell what sort of condition it's in.  

I can try to get a shipping quote if it's something that might help you!

ah, heck to the yizzah!  Are you serious???  That would be AWESOME!

I really wasn't looking forward to buying another e-bay unit

Streetwiseguy said:

If the rear sensor is cycling rich/ lean at any speed at all, the cat is junk.

It was explained to me that the rear sensor cycles, just not as quickly.  The ratio between the front and rear sensor switching is what's referenced for cat efficiency.

Streetwiseguy said:

If the rear sensor is cycling rich/ lean at any speed at all, the cat is junk.

Except for those calibrators that did that intentionally to better optimize the emissions. I made it switch pretty regularly to make sure the post sensor catalysts are working. 

In reply to alfadriver :

That's why it's glorious to be able to HAVE that information in the service info.  Because everyone does things different.

I don't think anyone actually knows what Toyota does 

In reply to Pete. (l33t FS) :

It's been years, but the Toyotas I've dealt with used a wideband "air-fuel ratio" sensor upstream of the cat and a narrowband "oxygen" sensor downstream. The downstream unit could afford to be simpler because it was more or less reporting a binary value.

Easy to tell the difference, because the part number for the former started with 89467, and the latter with 89465.

DarkMonohue said:

It's been years, but the Toyotas I've dealt with used a wideband "air-fuel ratio" sensor upstream of the cat and a narrowband "oxygen" sensor downstream. The downstream unit could afford to be simpler because it was more or less reporting a binary value.

FWIW, my understanding is that even a wideband is still an oxygen sensor, just a more precise one.  This matters when you get misfires -- since it didn't burn in the cylinder there is still oxygen around and the sensor will report lean even if there was plenty of fuel.

In reply to codrus (Forum Supporter) :

In the real world, yes, you're right. In the Toyota parts catalog, they use those terms to differentiate between wideband and narrowband. I was trying to indicate that with the quotation marks, but it didn't really come across.

In reply to DarkMonohue :

'Wideband" is strictly an aftermarket term like "OBD-II port".  In service information they are called air/fuel ratio sensors.

Oddly, I can't even remember what they are called in the underhood emissions sticker.  I generally only even look at that anymore to see if an engine is SFI or DFI.

Lambda sensors work by having exhaust gases on one side of a special cell (Nernst cell) and outside air on the other.  If there is more oxygen on the outside than the inside, this generates voltage - it's an oxygen sensor.  It's called a lambda sensor because it has a switch point where the air/fuel ratio of the engine is at 1.0 lambda, or stoich.

Wideband oxygen sensors are the same thing, but they can pump current into or out of the Nernst cell to alter the switch point.  This is why AFR sensor readings are in microamps, not volts, and they can be a negative number.  And why you need an expensive controller for them to work - that microamps is not coming from the sensor, that's the current needed to send TO the sensor to make the Nernst cell read .5v.

O2 is also pumped in for a wb sensor, which is needed for a correct measurement. And, technically, it's not specifically looking for oxygen, it's looking at the relative ratio of oxidants (O2&NOx) and reductants (HC,CO,H2) in the gas. Or more correctly, the reaction that makes voltage is the reductants to the oxygen inside the sensor- you get a voltage when it's rich, not lean. Also, most of the time, there isn't any O2 for the downstream sensor. It gets combusted in the first brick with the excess HC and CO.  One thing to add- the rear sensor is normally dealing with very little CO,H2,HC,and NOX- as it's normally in the under 100ppm for those when they are in the 2000-10000ppm range upstream of the first catalyst- that brick does ~95% of the work. 
 

WB sensors have been around for a long time- but in production, somewhere near 2000 someone did them, and by 2010, pretty much everyone used them for the first sensor in the stream. 
 

And I would not really compare switch rates, as one of the systems we used in 2008 switched the target a/f based on the rear sensor- so if all things were perfect, the sensors would be in pretty good sync. How fast the rear would change would tell you the health of the catalyst. 
 

People were experimenting with using a UEGO for the second sensor when I retired. I really never saw the benefit. 

Well, the laptop was a bust...

Probably something I'm doing, but this is a Delphi interface unit that I've seen do some pretty trick stuff in the past.  Unfortunately it wouldn't give me any real-time info from the sensors with the engine running and would instead freeze every time I tried to connect to that part of the program...

Ok, up next:

1) physical inspection of connections and components

2) swap-tronics to see if reading follows (it wont.  My faith in that laptop is strong enough to believe that it would have detected that fault)

3) replace cat.

I'm convinced #3 is the fix, but it's not payday yet so I have time to fiddle.  (Also:  WVUMTNBKR, I sent ya a PM.  It may have gone to spam)

Goot times

I got your email and responded.

I just want to reiterate that this is a stock 20 year old cat.  It might not be any better than what you have.  

At the very least, you might be able to scrap it and get some money for it when it gets to you.

Pics and dimensions incoming tonight or tomorrow.

In reply to wvumtnbkr :

I'll post some pics so the forum can participate...

dimensional pics:

It's ALIVE!!!

Ok.  I got readings from the front O2 sensor on my second go with the laptop.  A side-by-side comparison of the front v rear sensor wasn't possible on this platform, but it's enough for me to feel confident in a cat replacement.

Graph sequence is: cold idle, 3 blips of the throttle (well, four in the first one.  I was learning on the fly), back to idle, next sensor reading, repeat:

those were all the sensor values I could find (minus commanded cat temp, which was 640-something degrees).  All went positive with a throttle blip except for the post cat sensor, that went negative.

When you put the replacement in, get the data while driving. Tapping the throttle isn't as good as a steady 45mph for that test. The monitor mostly runs in a quasi steady state range. 

In reply to alfadriver :

Copy and will do. 

I still have the laptop so I may try to snag a graph of "before" replacement this weekend.  It's going to require a co-pilot though, so one of the Hungarlings is going to have to buckle in and play camera man