This is not specifically MB related, its on a chevrolet impala 3.8 my uncle has, it triggered a P0420 code.

I tested the sensor via torque app and I see the rear sensor to be at about 0.15v constantly, if I rev the engine it does move, show a bit of small waves to 0.4 and then comes back down. This is quite wrong IMO. I tried to richen the engine by squirting some carb cleaner in it and the voltage rises for about 20 seconds then goes back down. The front one shows wave form but a bit odd - in short interval the waves are like bumps, long term shows consistent switching.

When I richen the engine the precat sensor spikes and sticks to rich condition. similarly when I lean it down (vacuum leak) it goes low.

Is this a cat failure or a sensor failure for the post cat sensor?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

You didn't mention what year, or how the car runs, mileage on car, mileage on the o2 sensors, actual voltage swing from sensor 1 when sensor 2 is very low, if there are any exhaust leaks before or just after the sensor, etc. It is impossible to even guess. Usually a P0420 is a bad converter but the readings you are giving are not the normal readings for a converter failure. What are you using to get these readings?

Paul

Edit: I see that torque app is an android app. I don't know anything about it but I guess it works. I have seen some of the cheap (less than $200-300) scan tools that weren't very good when working on problems like this. The one we were using before we retired was somewhere around $8000. (Snap-On)

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84 500 SEL (307,xxx miles)

It's probably just the O2 sensor unless the car has been showing signs of a melted cat (slow acceleration, glowing cat after a hard run, etc.)

Can you take the exhaust apart to look in at the catalytic media? If not I'd recommend a JY O2 sensor.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

The car is a 2002 impala 3.8. Its a V6 but the system is counted as one bank only, as in one pre cat o2 sensor, one cat and one post cat o2 sensor.

Im using the torque app on my android and connected it via a BAFX bluetooth OBD-2 scanner.

The voltage graph when seen at idle stays at lean position. about 0.15V, when tested in driving, it goes go to 0.45 - 0.50V in some conditions for a brief moment (hard run) and then comes down to 0.15V and mostly sticks there. The front sensor is switching between 0.2 to 0.8V constantly if I plot a graph for about a minute or two. I did try leaning and richening the engine and I can see the front one responding immediately. The rear one also responds by going to about 0.7V and staying there for a second or two.

The car runs freely, no strangling, no hiccup etc, the cat does not rattle and neither does it seem overheated.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Sounds like a sluggish o2. I read about the BAFX and it seams like a good inexpensive scan tool but with the type problem you have I would like to see the car hooked to a real scan tool that is set up for that car, not just generic OBDII. But with the information I have, I will guess it is the O2 sensor.

Paul

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84 500 SEL (307,xxx miles)

I got the car to a shop which has a nice snap on tool.

on the tool we see about 0 on the STFT and about -8.9 on the LTFT reading whilst at idle - meaning the ECU is seeing that the resultant exhaust is rich. The LTFT even whilst on driving was the same.

I later tweaked the andriod app and see the exact same readings.

The mechanic said that I should firstly clean out the MAF or replace it. then retest. If no dice then replace BOTH O2 sensors with genuine delphi items. GM ecus are very quick at adjusting the trim, this sort of LTFT can cause the CAT to overwork and maybe the cause of the P0420 code. Says that he has seen a lot of cars with this issue.

sounds like a plan does it?

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)