First Real Drive of the Year and A Few Questions

[AvusM5]

This past weekend, I took the M3 out for its first proper drive of the year. In the fellow company of my buddy's E39 M5, we did a ~300 mile loop from southern NH down to Newport, RI to specifically stop by the Audrain Auto Museum and check out the McLaren F1 and CLK-GTR that they had on exhibit. I had been fortunate enough to see both examples of the F1 and GTR running and up close at Amelia Island Concourse D'Elegance in 2021, but definitely wanted to see them again while they were local and a great excuse to take the Ms out for a day trip.

It was a great day and also felt amazing driving the M3 for an extended time after months of not getting to drive it (salt sucks). I had one close call where I didn't realize that I had failed to fully latch both sides on the hood after checking the oil level and one side popped up. It certainly would have been a very bad day if both sides had decided to release going triple digit speeds.

In classic 19 year old ///M car fashion, part way through the drive, the M3 threw a SES light. This was the first time I had seen the SES in over a year and a half and the fault codes point to the bank 1 post-cat O2 sensor. Here is where I am a little confused... The M3 has Fabspeed headers and is currently cat-less. The PO installed the headers under his ownership and says that the tune is factory. I've found it odd that the car has never thrown codes for catalyst inefficiency or other post cat O2 sensor related codes. The two codes being thrown are AfterCat 02 sensor aging, Cyl #1-3 and AfterCat 02 sensor voltage, Cyl #1-3 indicating that its likely just the post-cat bank 1 sensor and bank 2 is still fine.

My first question is, why would the ECU not throw cat related codes with stock software and are the omission of cats causing the post O2 sensors to fail more quickly? I'd like to get a proper tune with the Fabspeed headers but am finding that many local tuners in New England (such as BrenTuning) require cats. My first course of action is going to replace the failing sensor. Confirming that part #: 11781406621 is the correct sensor for bank 1?

Given that I'm still pretty new to the S54 world (long time E39 M5 owner), another question I have is if it is a common thing for the beauty cover securing nuts to loosen from vibrations of the engine or is it time for new motor mounts?

Thanks!

Luke

[DoubleSidedTape]

If you’re catless, you should be able to just delete the post-cat O2 sensors. I don’t know what if any emissions requirements you have in your state, but there is software you can use to code out the sensors yourself or you can work with one of the remote tuners to help you with that.

[repoman89]

If you have no cats you should delete all the post-cat O2 sensor related codes — they are meaningless when they aren’t monitoring what they’re designed to monitor. It’s easy to do with a hex editor (combined with mssflasher to automatically correct the checksum). Happy to help you with this in person or email or whatever. If the tune is stock then it is weird that it hasn’t thrown cat inefficiency codes. Not that important though either way, the post-cat emissions monitoring system doesn’t affect how the car runs and as I’m sure you’re aware state inspection is a free for all in MA for cars >15 years old.

That said I’d recommend putting in a decent set of cats in your section 1 — Doesn’t hurt power if you get quality ones and other drivers on the road will thank you!

[AvusM5]

Obioban I’m curious if you have any insight on why the DME is not throwing catalyst inefficiency codes and only one of the post-cat O2 sensors is now throwing a code?

[Obioban]

Obioban I’m curious if you have any insight on why the DME is not throwing catalyst inefficiency codes and only one of the post-cat O2 sensors is now throwing a code?

I would guess that either the PO codes them off, or you don’t have (long) enough drive cycles to trigger them yet.