[Sedoy]

Hi guys,
Just got 2015 X5 N55 with 51K miles and was about to start modding but keep getting annoying CEL - P0420 (Catalyst System Efficiency Below Threshold -Bank 1). Also I noticed slight decrease in fuel consumption about 2-3 MPG below EPA driving like a grandma on highway. No cold start or idling issues. Everything is 100% stock.

So I decided to replaced pre-cat 02 sensor as it was pretty cheap but stupid P0420 CEL is back (shows as pending with OBDII scanner). No other issues or CEL codes. Any ideas or suggestions?

[Tchao]

Cat efficiency is measured by the Post -Cat o2....and thats usually a narrow band sensor and cheap.

Pre-cat o2 is a wideband, used for engine fueling feedback, and sorta pricy.

[paimon.soror]

When you say 'just got' .. do you mean you recently purchased it from a dealer, or private sale?

If private sale, have you verified that the car wasn't modified beforehand and possibly had its catalyst removed?

[Sedoy]

So I replaced the wrong O2 sensor? I was under impression that bank-1 is pre-cat sensor per description for p0420 code.

Originally Posted by Tchao

Cat efficiency is measured by the Post -Cat o2....and thats usually a narrow band sensor and cheap.

Pre-cat o2 is a wideband, used for engine fueling feedback, and sorta pricy.

[Sedoy]

Yes just purchased from Manheim auto auction. Not sure who was the original owner but didn't see any evidence of modifications. The downpipe is stock, I know because I just spent freaking 3 hours under the car replacing the O2 sensor. Is there anyway to tell that stock dp cats were removed? If cats are removed shouldn't there be more CEL codes instead of just one?

Originally Posted by paimon.soror

When you say 'just got' .. do you mean you recently purchased it from a dealer, or private sale?

If private sale, have you verified that the car wasn't modified beforehand and possibly had its catalyst removed?

[paimon.soror]

Originally Posted by Sedoy

Yes just purchased from Manheim auto auction. Not sure who was the original owner but didn't see any evidence of modifications. The downpipe is stock, I know because I just spent freaking 3 hours under the car replacing the O2 sensor. Is there anyway to tell that stock dp cats were removed? If cats are removed shouldn't there be more CEL codes instead of just one?

Yea typically you would get a few but just making sure

Originally Posted by Sedoy

So I replaced the wrong O2 sensor? I was under impression that bank-1 is pre-cat sensor per description for p0420 code.

Ah yea, 0420 would be the downstream sensor (post cat) not the upstream.

[Tchao]

Originally Posted by Sedoy

So I replaced the wrong O2 sensor? I was under impression that bank-1 is pre-cat sensor per description for p0420 code.

Yes you need to replace the POST cat sensor.

[Sedoy]

Yeah, looks like I replaced the wrong sensor. Also while replacing the pre-cat sensor I noted that post-cat will be PATA to replace. Probably will end up removing downpipe for easier access.

[Tchao]

I mean....if you remove it.....might as well put in a cat-less pipe in there....

Just saying...

[jgoens]

Originally Posted by Sedoy

Yeah, looks like I replaced the wrong sensor. Also while replacing the pre-cat sensor I noted that post-cat will be PATA to replace. Probably will end up removing downpipe for easier access.

If your going to do that and have plans to add a catless, this would be the time to do it.

[FenixMike]

Did you first verify that A ) It was the sensor at fault, and not actually a catalytic converter failure that triggered the sensor, and also B ) that it was not something covered under the federal emissions warranty? The sensor may be fully operational and just doing its job, which is telling you the cat isn't operating within range... And if the cat is bad, it may be covered under the federal emissions 80K mile warranty.

[Sedoy]

Agree, already doing my research. Trying to find something which will pass state emissions test. Not many options available.

Originally Posted by jgoens

If your going to do that and have plans to add a catless, this would be the time to do it.

[Sedoy]

Thanks for the reminder, forgot about 80K warranty. Also I was under impression that bad cat will throw more codes. Anyway probably a good idea to pay for dealership diagnostics. I'm just not sure if replaced pre-cat O2 sensor which I just did will void my emission warranty.

Originally Posted by FenixMike

Did you first verify that A ) It was the sensor at fault, and not actually a catalytic converter failure that triggered the sensor, and also B ) that it was not something covered under the federal emissions warranty? The sensor may be fully operational and just doing its job, which is telling you the cat isn't operating within range... And if the cat is bad, it may be covered under the federal emissions 80K mile warranty.