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.