Downstream O2 sensor diagnostic

[wsteele]

I thought it makes sense to post another thread as this topic isn't related to the thread P0442 MIL conversation.

In doing a live data run looking at my upstream O2 sensor readings for the P0442 thread, I noticed when my engine was warming up, the Bank 2 Sensor 2 reading was stuck down low like 165 mV, while B1S2 was sitting up around 750 mV. After the engine (and I assume the exhaust system as well) had warmed up fully, the B2S2 came back up to like 646 mV.

Neither was hopping around following the S1's, so I assume it likely isn't a cat issue and maybe one of the downstream O2 sensors is having trouble with its heater circuit?

Right now, I am thinking of swapping the two downstream sensors and seeing if the voltages moves with the sensor. Any input from anyone who actually know this stuff would be greatly welcome.

 

[Tonyrodz]

I thought it makes sense to post another thread as this topic isn't related to the thread P0442 MIL conversation.

In doing a live data run looking at my upstream O2 sensor readings for the P0442 thread, I noticed when my engine was warming up, the Bank 2 Sensor 2 reading was stuck down low like 165 mV, while B1S2 was sitting up around 750 mV. After the engine (and I assume the exhaust system as well) had warmed up fully, the B2S2 came back up to like 646 mV.

Neither was hopping around following the S1's, so I assume it likely isn't a cat issue and maybe one of the downstream O2 sensors is having trouble with its heater circuit?

Right now, I am thinking of swapping the two downstream sensors and seeing if the voltages moves with the sensor. Any input from anyone who actually know this stuff would be greatly welcome.

View attachment 275402

Might just be a bad or lazy O2 sensor. I just went through the same ordeal. Swapped it out and everything is good now.

 

[wsteele]

Might just be a bad or lazy O2 sensor. I just went through the same ordeal. Swapped it out and everything is good now.

These all four are new (like a few weeks) OE. I guess I will swap them side to side and see if it moves. I wish I knew how the heat affected the S2 readings, might just be a weak heater on one. Maybe I will wait til its cold again, start it and watch the heater voltages, before sliding under there and getting all dirty again swapping them. The good news is the downstream are a lot easier to get to the connectors.

 

[Tonyrodz]

These all four are new (like a few weeks) OE. I guess I will swap them side to side and see if it moves. I wish I knew how the heat affected the S2 readings, might just be a weak heater on one. Maybe I will wait til its cold again, start it and watch the heater voltages, before sliding under there and getting all dirty again swapping them. The good news is the downstream are a lot easier to get to the connectors.

You might've just gotten a bad O2 out of the box. What brand are they? I used Denso.

 

[wsteele]

You might've just gotten a bad O2 out of the box. What brand are they? I used Denso.

It is Delco OE, but not sure that means anything as it pertains to DOA. It does seem to respond and reports a similar voltage to the other side once things warmed up.

The picture above was after the engine coolant temps had gotten to normal operating temps. During warm up, it was reporting a solid 160's mV.

 

[FrankU]

I had the secondary O2 sensors shut off on mine. Simple.

 

[Fless]

The swap would tell you. If you do that, measure the resistance of both sensors' heater circuits. I'd venture a guess that the heater in the slow one isn't working properly, or maybe not at all.

 

[wsteele]

I had the secondary O2 sensors shut off on mine. Simple.

Unfortunately, we have emissions checks here and I think anything that isn't Jake they bust you.

 

[wsteele]

The swap would tell you. If you do that, measure the resistance of both sensors' heater circuits. I'd venture a guess that the heater in the slow one isn't working properly, or maybe not at all.

Good idea on checking the resistance of the heater circuits on each. I can do that without even swapping them.

 

[Tonyrodz]

I had the secondary O2 sensors shut off on mine. Simple.

Same here.

Unfortunately, we have emissions checks here and I think anything that isn't Jake they bust you.

Same in NJ, but they're still tuned out.

 

[Fless]

You can also load test the heater circuits with the right test bulb(s). Eric O at South Main Auto YouTube shows that in some of his videos. I need to make up a couple of test jigs like he has.

 

[wsteele]

Same here.

Same in NJ, but they're still tuned out.

Even with the downstream O2's tuned out, your Cat monitor passes?

I wonder how that works?

 

[Tonyrodz]

Even with the downstream O2's tuned out, your Cat monitor passes?

I wonder how that works?

Well.....it's tuned out. I haven't actually been to an inspection. All they do is plug into your obd and do a cat visual. No cats and a cat--you pass.

 

[wsteele]

Well.....it's tuned out. I haven't actually been to an inspection. All they do is plug into your obd and do a cat visual. No cats and a cat--you pass.

Here in ID, they usually only do the plug in ODB. I have had them walk around with the mirror checking to actually see the cats in place, but not every time, seems random.

The reason I asked about the Cat monitor is that is one that has to show “passed”. The only monitor we can have that is not completed and still pass the test is evap.

I don’t know squat about EFI tuning, I didn’t realize they could tune out (or at least make happy), the Catalyst monitor without the downstream O2 sensor there.

 

[kbuskill]

Here in ID, they usually only do the plug in ODB. I have had them walk around with the mirror checking to actually see the cats in place, but not every time, seems random.

The reason I asked about the Cat monitor is that is one that has to show “passed”. The only monitor we can have that is not completed and still pass the test is evap.

I don’t know squat about EFI tuning, I didn’t realize they could tune out (or at least make happy), the Catalyst monitor without the downstream O2 sensor there.

I just remembered seeing a section in the Tech 2 when I was playing with it yesterday for the O2 sensors heater circuits.

I didn't go into that menu, as I had no need to, but I think it was in special functions so it is probably some kind of test.

You may want to check it out. You might not even have to slide under the truck if you cam test them from the driver's seat.

 

[Fless]

I posted this in a different thread but it might belong better here.

This article might shed some light on what to expect.

https://www.aa1car.com/library/o2sensor.htm

What I've been reading is that, generally, the mv reading should move around between 100 mv (lean) and 900 mv (rich), but the desired value is around 450 mv. Obviously the sensors are always changing, which is the point of their function.

If you're seeing consistent values lower than 100 mv, I would think that the sensor might be somewhat out of range (weak?) or there might be a problem on that bank. It might be worthwhile to swap the two that are in their respective positions to see if the pattern follows.

 

[wsteele]

I posted this in a different thread but it might belong better here.

This article might shed some light on what to expect.

https://www.aa1car.com/library/o2sensor.htm

What I've been reading is that, generally, the mv reading should move around between 100 mv (lean) and 900 mv (rich), but the desired value is around 450 mv. Obviously the sensors are always changing, which is the point of their function.

If you're seeing consistent values lower than 100 mv, I would think that the sensor might be somewhat out of range (weak?) or there might be a problem on that bank. It might be worthwhile to swap the two that are in their respective positions to see if the pattern follows.

Thanks for the article, pretty thorough explanation. I didn’t know about the breathing through the wire ports. I wanted to do some reroute of the wiring after I finished replacing the O2 sensors recently, mainly because I didn’t like how hard it was to get to a couple of the connectors. I think while I am there, I will make sure each set of wires coming out of a sensor, go downward to assure water doesn’t run down into the port.

 

[wsteele]

You can also load test the heater circuits with the right test bulb(s). Eric O at South Main Auto YouTube shows that in some of his videos. I need to make up a couple of test jigs like he has.

I found an article that called out a #7740 Incandescent bulb as the right wattage (22W) to simulate the heater circuit. If I can't diagnose the heater circuits with the Tech 2, I will slide under and check the resistance on the sensor heater lines (my old sensors show 11.7 ohms). If that all checks out in the green, then I will go and find the bulb and socket and make up a test fixture for the heater circuits. I hate the idea of having to find a fault in the O2 heater wiring...

 

[wsteele]

As @kbuskill suggested, I spent some time with the Tech 2 and the O2 sensors this morning and the heat circuits seem to be working as designed, all four heater circuits reported a similar amperage draw.

My conclusion is that as @Tonyrodz suggested at first, one sensor must just be a little slow. After getting up to full temp and running the engine above idle in cycles, both number 2 sensors are reporting very similar voltages and track throttle applications.

I may swap in one of the old downstream sensors into Bank 2 position and see if it responds a little more quickly during the warmup cycle, although it appears the new unit works fine after the engine gets warm and is in driving mode. This last bit will be just for my own edification.

 

[swathdiver]

Here in ID...

Idaho? Whereabouts? We just drove our new Sierra back from there.