[DreamCar5683]

I've had my 2006 MO for 13 years and over 240k miles. I've done all my own service including a replacement engine 100k miles (6 years) ago. I also have a YouTube channel partially dedicated to the projects on this car. It currently has 283k on the odometer with the original CVT. I recently changed all the catalytic converters and Air/fuel and O2 sensors to eliminate a P2A00 code. When I changed the engine before I also had this code and a new set of converters and sensors seemed to solve that issue.

Here's where I need some information: After changing the cats and sensors this time, and the intermediate resonator/muffler, I have persistent P0420 and P0430 codes telling me the catalyst is inefficient on both banks. I smoke tested the exhaust and found only very small pinholes where the new intermediate muffler connects to the rear pipe before the muffler. I found no exhaust leaks anywhere else. I've also driven it and cleared the codes several times in hopes that the ECM needed to adjust to the new sensors. I've also recalibrated the throttle body, accelerator pedal position and Air Fuel ratio calculations according to the FSM. Again, in hopes the ECM needed to be "taught" there are new sensors in place.

I am wondering if I bought defective cats or sensors. I bought generic cats but OEM equivalent (Bosch and NTK) sensors. I'm trying to narrow it down better with someone else's experience. I don't really WANT to swap new cats in again because it's not a small project but if that's the best choice I will do that. I'd like to get at least another 20k miles out of this car.

Other suspicions: Although I don't see any smoke escaping during smoke test I do know that the bank 2 Cat - Manifold joint doesn't have a gasket. It never did and the new cat converter kit did not come with one. I see now that some kits include a gasket there. Also, although I see no smoke from the bank 1 cat -Manifold joint I'm now questioning myself if the little metal donut gasket is jammed improperly and allowing some leakage.

The codes appear sooner if I'm driving highway/city combined at lower speeds (about 50 miles distance or less). It takes many more miles for the codes to appear if I'm driving straight highway speeds (160 miles). Not sure if that's relevant or not.

I have a good scan tool but am not skilled at interpreting what I'm looking at. One of these attached photos shows a portion of time when the sensor readings seem to bottom out. I've also attached the freeze frame data from the most recent code setting.

I'm hoping for some insight/suggestions on how to proceed with the data I have. Of course I could always just drive around with a check engine light on but that infuriates me given the experience and time I have invested in this car. I'd much rather fix the issue.

Any help anyone can offer would be greatly appreciated.

 

[I need coffee]

You're probably a better diagnostician than I am so not sure how much I can help you, but those codes are typically thrown when the ECM sees that the downstream O2 sensors are switching high/low more than expected. The last pic you posted shows a bunch of waveform traces--is that from live data or are those graphs pulled from the O2 sensor monitors (...I'm not familiar with your scan tool)? I ask because the waveforms look really noisy for a downstream O2 sensor. I would expect to see relatively smooth traces during normal driving except under acceleration or deceleration conditions where the voltage would rise or drop, respectively. See for example a snapshot of my downstream sensors just cruising down the road (...taken via Torque Pro--not quite as capable as your Snap-On scan tool ).

Other suspicions: Although I don't see any smoke escaping during smoke test I do know that the bank 2 Cat - Manifold joint doesn't have a gasket. It never did and the new cat converter kit did not come with one. I see now that some kits include a gasket there. Also, although I see no smoke from the bank 1 cat -Manifold joint I'm now questioning myself if the little metal donut gasket is jammed improperly and allowing some leakage.

How high was the pressure set for your smoke machine? If it was at an EVAP setting (e.g. less than 1 psi) than that may not be enough to simulate exhaust backpressure. Regardless, I personally don't like leaving gaskets missing, but this still doesn't explain why codes are thrown for both banks.

 

[DreamCar5683]

So, I think our scopes may be set at different settings. If mine is set for milliseconds, maybe we’re seeing fluctuations you can’t see on your graph. It kinda looks like your screen is measuring in seconds or every couple seconds. That’s probably something I should narrow down: what scale I should be using to view this sensor data.

I thought the same about the smoke test. It’s an inexpensive version made from an ammo can. Got it on EBay. It uses a propane regulator for pressure which is supposed to be 2psi. Maybe another smoke test midway up the system. My first test was injecting at the tail pipe with one outlet plugged shut.

I’m also researching online about interpreting this data. Thanks for your help. Fingers crossed.

 

[I need coffee]

The width of the screen probably equates to about 8-10 seconds. I have other "real" scan tools and that's pretty typical when just viewing live data unless the settings are modified. You definitely need to find out what the x and y settings are to make sense of those graphs.

Anyway, one thing you could do to test the O2 sensors, which you'll see quite a bit on YouTube is a propane enrichment test. Pull a hose from the intake manifold and connect a small propane cylinder and graph the O2 sensors and short-term fuel trims (STFT) for each bank. Open up the valve to dump large quantities of propane into the intake forcing a rich A/F mixture and you should see the downstream sensor voltages go high near 1 V and STFT go negative as the ECM trys to correct the rich condition. This confirms both that the O2 sensors signals are behaving as expected and the ECM is receiving those signals and responding appropriately. Then, immediately disconnect the propane cylinder leaving the house disconnected, which would induce an acute lean condition (vacuum leak + ECM already cutting fuel from rich condition). You should see the O2 sensor voltages drop like a rock to zero and the ECM respond by increasing STFT high positive to counter the lean condition. Again, this would confirm the signal reaction from the sensors and the ECM response. You can even try jiggling the harness connectors if you can get to them while monitoring the voltages to rule out a loose pin/connection. If all this checks out then your O2 sensors are probably normal. Even without propane, you can get an idea if your O2 sensors are reacting as they should by just doing a couple of wide-open throttle (WOT) tests while driving. O2 voltages should go high when your foot is to the floor and when you release the gas pedal they should drop to zero under decel.

If you're confident that your O2 sensors are good (...with brand new Bosch/NTK sensors I suspect they probably are), then you can use O2 sensor live data to confirm the condition of the catalytic converter as well. After driving the car for 20 minutes or so, park it and just let the engine idle with no loads. Keeping in mind that O2 sensors only read oxygen in the exhaust stream and the job of a catalytic converter is to remove oxygen from the exhaust stream, the graph of the downstream sensors should read above 0.5 volts (higher the better) and the trace should be relatively flat or with a smooth upward trend. Unfortunately, I can't find an old graph for my 2nd gen, but it should look something close to the middle of the graph I posted earlier. But, I did find an old graph from another car that compares an older-style narrow band upstream O2 sensor and downstream sensor. The waveforms of upstream narrow band sensors look completely different from the A/F sensors in our Muranos and show peaks and valleys at idle based on the constantly-changing oxygen levels in the exhaust stream caused by the ECM switching rich/lean to achieve a stoichiometric A/F ratio, but this actually makes it a lot easier to see if a catalytic converter is working. You can see that with a healthy cat (and O2 sensors), the hilly-looking up/down waveform of the upstream sensor (red) is basically transformed to a flat line (green) trending rich because the cat has removed most of the oxygen from the exhaust stream. A bad cat would result in the traces looking nearly identical. A failing cat would show a downstream trace that has fluctuations similar to the upstream trace although not quite as dramatic, but still noisy-looking. Again, this graph is not from a Murano, but the point is to show that a healthy catalytic converter in any car should result in a downstream O2 sensor trace with similar characteristics with engine warmed up at idle and no loads (i.e. relatively flat and trending rich).

Here is another graph from my 2nd gen Murano that compares the bank 1 A/F and O2 sensors during a WOT test. Because A/F sensors have a different stoichiometric voltage point and behave the opposite of O2 sensors (i.e. go high during lean condition and low during rich condition) it makes it more difficult to compare them, but still possible at the extremes. I've annotated the graph to show where WOT begins/ends and another section that's very important in determining whether or not the cat is working efficiently. Section A in the graph is WOT. You can see the A/F sensor voltage drops during WOT and immediately after my foot comes off the accelerator pedal (2nd yellow line) the voltage climbs quickly to a very lean voltage--this is because under decel, the ECM shuts off the fuel injectors so the exhaust stream is basically atmospheric air with an abundance of oxygen. The importance of Section B is this--you'll notice that the downstream O2 sensor voltage doesn't react as quickly as the A/F sensor and doesn't actually go lean until noticeably later. This is because a good catalytic converter has the capacity to store oxygen so it is absorbing a large amount of the oxygen in the exhaust stream immediately after the vehicle is in decel until it becomes saturated at which point the O2 sensors will then show a steep drop in voltage. I've marked the estimated saturation point in the graph as well so hopefully you can see what I'm talking about. If the downstream O2 sensor showed a steep drop in voltage at the same time the A/F sensor showed a steep increase then that would suggest that the catalytic converter has little/no oxygen storage capacity and it needs to be replaced.

BTW, for your reference your 1st gen Murano's stoichiometric voltage for the A/F sensor is 1.5 volts while 2nd gen is 2.25 volts. I don't know why Nissan changed it, but if you do research on A/F sensors it's more or less arbitrary anyway since the ECM is not actually getting that voltage signal from the sensor.

Here's an example of a trace with both downstream sensors of my Murano under WOT conditions. Again, at WOT end (foot off pedal and beginning of decel), there is a noticeable lag for when the O2 sensor voltages drop lean.

The point of this post is to give you some examples of known-good graphs for the upstream and downstream sensors and what those graphs should look like with a good catalytic converter. Hopefully, you'll find them somewhat useful. Definitely keep us updated on what you find.

BTW, I meant to ask in my previous post, but what brand of catalytic converters did you buy? Is there a link to look at the fine details (if available)? I've read that manufacturers of cheap generic catalytic converters often skimp on the rare metals in the reactive substrate and this can result in people getting cat efficiency DTCs because the converters are not working as efficiently as the ECM would expect (...at least as expected based on an OEM converter).

 

[DreamCar5683]

The width of the screen probably equates to about 8-10 seconds. I have other "real" scan tools and that's pretty typical when just viewing live data unless the settings are modified. You definitely need to find out what the x and y settings are to make sense of those graphs.

Anyway, one thing you could do to test the O2 sensors, which you'll see quite a bit on YouTube is a propane enrichment test. Pull a hose from the intake manifold and connect a small propane cylinder and graph the O2 sensors and short-term fuel trims (STFT) for each bank. Open up the valve to dump large quantities of propane into the intake forcing a rich A/F mixture and you should see the downstream sensor voltages go high near 1 V and STFT go negative as the ECM trys to correct the rich condition. This confirms both that the O2 sensors signals are behaving as expected and the ECM is receiving those signals and responding appropriately. Then, immediately disconnect the propane cylinder leaving the house disconnected, which would induce an acute lean condition (vacuum leak + ECM already cutting fuel from rich condition). You should see the O2 sensor voltages drop like a rock to zero and the ECM respond by increasing STFT high positive to counter the lean condition. Again, this would confirm the signal reaction from the sensors and the ECM response. You can even try jiggling the harness connectors if you can get to them while monitoring the voltages to rule out a loose pin/connection. If all this checks out then your O2 sensors are probably normal. Even without propane, you can get an idea if your O2 sensors are reacting as they should by just doing a couple of wide-open throttle (WOT) tests while driving. O2 voltages should go high when your foot is to the floor and when you release the gas pedal they should drop to zero under decel.

If you're confident that your O2 sensors are good (...with brand new Bosch/NTK sensors I suspect they probably are), then you can use O2 sensor live data to confirm the condition of the catalytic converter as well. After driving the car for 20 minutes or so, park it and just let the engine idle with no loads. Keeping in mind that O2 sensors only read oxygen in the exhaust stream and the job of a catalytic converter is to remove oxygen from the exhaust stream, the graph of the downstream sensors should read above 0.5 volts (higher the better) and the trace should be relatively flat or with a smooth upward trend. Unfortunately, I can't find an old graph for my 2nd gen, but it should look something close to the middle of the graph I posted earlier. But, I did find an old graph from another car that compares an older-style narrow band upstream O2 sensor and downstream sensor. The waveforms of upstream narrow band sensors look completely different from the A/F sensors in our Muranos and show peaks and valleys at idle based on the constantly-changing oxygen levels in the exhaust stream caused by the ECM switching rich/lean to achieve a stoichiometric A/F ratio, but this actually makes it a lot easier to see if a catalytic converter is working. You can see that with a healthy cat (and O2 sensors), the hilly-looking up/down waveform of the upstream sensor (red) is basically transformed to a flat line (green) trending rich because the cat has removed most of the oxygen from the exhaust stream. A bad cat would result in the traces looking nearly identical. A failing cat would show a downstream trace that has fluctuations similar to the upstream trace although not quite as dramatic, but still noisy-looking. Again, this graph is not from a Murano, but the point is to show that a healthy catalytic converter in any car should result in a downstream O2 sensor trace with similar characteristics with engine warmed up at idle and no loads (i.e. relatively flat and trending rich).

View attachment 56317

Here is another graph from my 2nd gen Murano that compares the bank 1 A/F and O2 sensors during a WOT test. Because A/F sensors have a different stoichiometric voltage point and behave the opposite of O2 sensors (i.e. go high during lean condition and low during rich condition) it makes it more difficult to compare them, but still possible at the extremes. I've annotated the graph to show where WOT begins/ends and another section that's very important in determining whether or not the cat is working efficiently. Section A in the graph is WOT. You can see the A/F sensor voltage drops during WOT and immediately after my foot comes off the accelerator pedal (2nd yellow line) the voltage climbs quickly to a very lean voltage--this is because under decel, the ECM shuts off the fuel injectors so the exhaust stream is basically atmospheric air with an abundance of oxygen. The importance of Section B is this--you'll notice that the downstream O2 sensor voltage doesn't react as quickly as the A/F sensor and doesn't actually go lean until noticeably later. This is because a good catalytic converter has the capacity to store oxygen so it is absorbing a large amount of the oxygen in the exhaust stream immediately after the vehicle is in decel until it becomes saturated at which point the O2 sensors will then show a steep drop in voltage. I've marked the estimated saturation point in the graph as well so hopefully you can see what I'm talking about. If the downstream O2 sensor showed a steep drop in voltage at the same time the A/F sensor showed a steep increase then that would suggest that the catalytic converter has little/no oxygen storage capacity and it needs to be replaced.

BTW, for your reference your 1st gen Murano's stoichiometric voltage for the A/F sensor is 1.5 volts while 2nd gen is 2.25 volts. I don't know why Nissan changed it, but if you do research on A/F sensors it's more or less arbitrary anyway since the ECM is not actually getting that voltage signal from the sensor.

View attachment 56318

Here's an example of a trace with both downstream sensors of my Murano under WOT conditions. Again, at WOT end (foot off pedal and beginning of decel), there is a noticeable lag for when the O2 sensor voltages drop lean.

View attachment 56316

The point of this post is to give you some examples of known-good graphs for the upstream and downstream sensors and what those graphs should look like with a good catalytic converter. Hopefully, you'll find them somewhat useful. Definitely keep us updated on what you find.

BTW, I meant to ask in my previous post, but what brand of catalytic converters did you buy? Is there a link to look at the fine details (if available)? I've read that manufacturers of cheap generic catalytic converters often skimp on the rare metals in the reactive substrate and this can result in people getting cat efficiency DTCs because the converters are not working as efficiently as the ECM would expect (...at least as expected based on an OEM converter).

Thank you. This is all helpful. It gives me some ways to move forward with diag.

The converters I bought are from EBay…they’re likely filled with Chinesium instead of enough precious metals. The last set I also bought from EBay with no issues for 100k miles, but that’s the gamble sometimes on EBay right? Haha.

 

[chidog]

Things like cats, I don't think I would do mail order for them, except maybe Rock auto. Get locally then there is a place to return them to if problems come up.

 

[PaulDay]

Things like cats, I don't think I would do mail order for them, except maybe Rock auto. Get locally then there is a place to return them to if problems come up.

You can find one of the three Nissan approved manufacturer's cats used in production on EBay, usually about 1/2 of the dealer cost. The knockoffs usually have fitment issues, along with going cheap on the inner core coatings causing them to fail sooner than later during emissions testing.

At one time I had a list of the three Nissan OEM cat manufacturers' but can't find it at this moment. Sorry.

My understanding is that all Nissan OEM cats for NA are manufactured to CA emissions standards. Stick to the CA approved setups and it's more than likely that it was manufactured by one of the OEM manufacturers.

Have a good day.

 

[DreamCar5683]

Things like cats, I don't think I would do mail order for them, except maybe Rock auto. Get locally then there is a place to return them to if problems come up.

You're right. Technically best way to get problems solved. In my case I'm trying to push through one more NYS inspection and get maybe another 20k miles on the car. So I'm not willing to spend what it takes to buy these locally.

 

[DreamCar5683]

The upstream sensors are pretty steady around 1.5 V at idle.

The downstream ones are switching a lot. The blips In the upstream sensor data is me increasing RPMs at idle.

So, are the downstream sensors trying to compensate for converters not scrubbing well enough? Is that what I’m seeing?

 

[I need coffee]

The downstream ones are switching a lot.

That's definitely way too much switching. The downstream graph should be relatively smooth at idle and greater than 450 mV.

The blips In the upstream sensor data is me increasing RPMs at idle.

It appears to me that during your throttling the upstream and downstream sensors look like mirror images of each other with no differentiation. The rise in the A/F sensor voltages immediately coincide with a drop in the downstream O2 sensor voltages. Assuming the downstream sensors are good, there's something wrong with the converters as they don't seem to be removing much oxygen from the exhaust stream.

So, are the downstream sensors trying to compensate for converters not scrubbing well enough? Is that what I’m seeing?

The only thing an O2 sensor can do is sense the level of oxygen in the exhaust stream. The ECM takes that information and responds accordingly. With the engine just idling the voltages are switching lean way too much. This shouldn't happen with good catalytic converters because any oxygen in the exhaust stream would be used to catalyze emissions byproducts or would be stored in the converter substrate for that purpose.

One thing I forgot to mention is there should be a live data PID for the bank 1 and bank 2 converter temperatures. I would be curious to see what those are when the engine is at operating temperature.