Mystery downstream O2s on 2014 Dodge Caravan 3.6l

Hi all.

Working on 2014 Dodge Caravan 3.6l flex fuel.

I am trying to verify if anyone seen this and its normal and why?

Randomly one of the downstream o2s will go lean. It has done it while driving or parked and idling.

One smog tech explained it, it was due to different fuel/air mixture. But why don't the upstream o2 sensors show lean?? Another smog tech said it could be the PCM playing with the signals during testing. IF this is true, the PCM must be partially grounding pulling down the signal. The DVM on signal wire matches the scantool.

Trying to make sense of it. Perhaps its why O2 sensor monitor won't set.

I made a video about it.


Thank you in advance.

FWIW, I've seen several different known good vehicles with the same downstream sensor behavior. My own Scion xA (not an apples to apples comparison, I know) will do this repeatedly while driving.

When I asked other techs for their thoughts, the conclusion we came up with is that the catalytic converter is reaching its maximum oxygen storage capacity. The excess oxygen gets discharged into the exhaust stream and gets picked up by the downstream sensor.

Is that why the O2 monitor won't set? Why doesn't B1 do the same thing? I have no idea. :lol: It could also be that there's a small exhaust leak behind the upstream O2, and the PCM is on the borderline of setting a downstream fuel trim code.

FWIW, I've seen several different known good vehicles with the same downstream sensor behavior. My own Scion xA (not an apples to apples comparison, I know) will do this repeatedly while driving.

I appreciate you sharing this. Good to know there are other known good cases.

When I asked other techs for their thoughts, the conclusion we came up with is that the catalytic converter is reaching its maximum oxygen storage capacity. The excess oxygen gets discharged into the exhaust stream and gets picked up by the downstream sensor.

This sounds reasonable and logical. The cats can store so much.

Is that why the O2 monitor won't set? Why doesn't B1 do the same thing? I have no idea. :lol: It could also be that there's a small exhaust leak behind the upstream O2, and the PCM is on the borderline of setting a downstream fuel trim code.

Actually I have seen it do this on both banks. I've even seen both banks go low downstream at the same time. I had a shop smoke test the whole exhaust. I told them to look for a pin size hole around the cats, they didn't find any. Looked at the fuel trims, they are +-5%. Looked at Cams and they are within +-2 degrees of desired. I might be overlooking something.

Here are few more captures. Have you seen something like this?
Just got off the highway. Been sitting idling for about 5 mins. Then the B1S2 goes low. I waited about 1 minute and tapped the throttle gently (~1500rpms) and the banks flipped. Now B1S2 went high and B2S2 dropped low. Idled for few more minutes and we can see B2S2 gradually climbed high.



Here I was driving ~40mph, decelled to ~35mph, drove at that speed a little and gently came to a stop. We can see downstream o2s on both banks drop low with slight delay between the two.
Blue trace = Speed
Teal trace = B1S2
Purple trace = B2S2

I am adding Mode $06 data.

Does anyone know what part of Drive Cycle gets these Tests to run? I believe once these 6 run and pass, the O2 sensor monitor will go Ready.
TID:$81
- O2 Bank 2 Sensor 2 Slow Response Rich to Lean Result
Test incomplete or dependant test failed
--
TID:$82
- O2 Bank 2 Sensor 2 Signal Biased Rich Mass Flow Result
Test incomplete or dependant test failed

TID:$83
- O2 Bank 2 Sensor 2 Signal Biased Rich Voltage Result
Test incomplete or dependant test failed

MID:$02 TID:$07
Minimum sensor Voltage for test cycle(calculated) - O2 Bank 1 Sensor 2
Max: 0.568V Min: 0V
Test result value: 0.549V
PASS
----
MID:$02 TID:$08
Maximum sensor Voltage for test cycle(calculated) - O2 Bank 1 Sensor 2
Max: 65.535V Min: 0.725V
Test result value: 0.725V
PASS
----
MID:$02 TID:$81
- O2 Bank 1 Sensor 2 Slow Response Rich to Lean Result
Test incomplete or dependant test failed
--
MID:$02 TID:$82
- O2 Bank 1 Sensor 2 Signal Biased Rich Mass Flow Result
Test incomplete or dependant test failed
--
MID:$02 TID:$83
- O2 Bank 1 Sensor 2 Signal Biased Rich Voltage Result
Test incomplete or dependant test failed
--
MID:$05 TID:$91
- O2 Bank 2 Sensor 1
Max: 7.99% Min: 1.09%
Test result value: 5.06%
PASS
----
MID:$05 TID:$92
- O2 Bank 2 Sensor 1
Max: 1.99882 Min: 0.01501
Test result value: 0.26386
PASS
----
MID:$06 TID:$07
Minimum sensor Voltage for test cycle(calculated) - O2 Bank 2 Sensor 2
Max: 0.568V Min: 0V
Test result value: 0.549V
PASS
----
MID:$06 TID:$08
Maximum sensor Voltage for test cycle(calculated) - O2 Bank 2 Sensor 2
Max: 65.535V Min: 0.725V
Test result value: 0.725V
PASS
----
MID:$06 TID:$81
- O2 Bank 2 Sensor 2 Slow Response Rich to Lean Result
Test incomplete or dependant test failed
--
MID:$06 TID:$82
- O2 Bank 2 Sensor 2 Signal Biased Rich Mass Flow Result
Test incomplete or dependant test failed
--
MID:$06 TID:$83
- O2 Bank 2 Sensor 2 Signal Biased Rich Voltage Result
Test incomplete or dependant test failed
--