2C31 Code

[isarkisov]

I have 2009 BMW X5 with N52 engine and 151k miles. Check engine light is on and getting '2C31 Lambda probe in front of catalyst, trim control' error. Is that pre-cat O2 sensor on Bank 1?

[andrewwynn]

https://bmwfault.codes/

Searching though it shows bank 2 eventually.

Searching though some threads with the same code at least one case said the fault never cleared until the rear O₂ also changed.

[ard]

If those are original O2s on a 2009, Id replace all of them....

[isarkisov]

I am planning to replace this vehicle with V8 as I need something more powerful to trail my boat, so replacing all 4 O2 sensors won't be economical. This vehicle is probably worth $3k tops, and I won't be able to sell it with check engine light on as it won't pass smog inspection.

I really want to figure out what's wrong and hopefully fix it. My other scanner showed 'P2096 Post Catalyst Fuel Trim System Too Lean Bank 1'. I also got these voltage values from O2 sensors:

Before Catalyst Bank 1 - 1.76V
Before Catalyst Bank 2 - 1.99V
Behind Catalyst Bank 1 - 0.61V
Behind Catalyst Bank 2 - 0.72V

Based on this article upstream O2 sensors should be at 2V, so it looks like voltage for upstream Bank 1 is too low which, according to this article, is indicative of clogged catalytic converter.

What do you all think?

[ard]

Is that article based on reading values from 15 year old O2 sensors?

[andrewwynn]

You can confirm worn cat by charting a graph with the same bank pre and post. The pre should flip high/low/high/low many times a minute. The post cat should work towards a valve and hover near a constant value. If it tracks lockstep with precat the cat is usually shot but If it's just a little too active you can add a defouler to get a couple more years out of a tired cat.

https://a.co/d/09Qk6poG

An example. A totally worn out cat it won't help but a weak cat that still will moderate nox etc just not as well as new it will squeeze some life out of the cat.

Follow your conscience; the car will grate a little more pollution but from what I've seen not a lot because when the cat totally fails the defouler trick will not help at all.

[ard]

Originally Posted by andrewwynn

You can confirm worn cat by charting a graph with the same bank pre and post. The pre should flip high/low/high/low many times a minute. The post cat should work towards a valve and hover near a constant value. If it tracks lockstep with precat the cat is usually shot but If it's just a little too active you can add a defouler to get a couple more years out of a tired cat....snip


.

IMO, when you have worn out sensors, trying to do 'rifle shot' diagnoses seems .... foolish. Maybe he gets lucky, but most of the published work on these oddball diagnoses and fixes tend to be done AFTER someone has installed proper O2s. At the least the pre-cat sensors. Massive $150 investment, I know....



JMHO

[andrewwynn]

Upstream sensor is before the cat will have no connection to monitoring wear of the cat. I don't remember my n55 pre cat sensors read but you can find a YouTube video where somebody shows a graph of pre/post sensors for your exact engine. Look for different chassis if can't find in X5.

I've had very little luck with non oem O₂ sensors (rounds to zero), but I've installed $20 Amazon sensors that worked for months.

If further diagonals leads to pre cat are shot get the highest rated knock off with a 12mo warranty if you sell the car jus tell the buyer the O₂ were replaced and have a year warranty you'll give him the receipt for exchange.