Repeatedly killing PRE-CAT O2 sensors — 274A / 274B every 1–2k miles. HELP

[supergoji]

TL;DR

Every 1,000–2,000 miles my front (pre-cat) O2 sensors throw 274A / 274B (both banks). Minor side issue: slight hesitation at low RPM under high load. Trying to determine if this is tune/calibration behavior, wiring/harness, sensor environment, or worst-case DME.


*edit***-replaced 02's 3 starts ago over 8 miles and codes are back
Car Info

2008 E92 M3
44k miles
DCT
Car is very well maintained

Mod List

1. Dinan Intake
   
2. Dinan ITBs
   
3. Schrick 284 cams
   
4. Port & polished heads
   
5. ARH long tube headers + midpipe (catless)
   
6. M Performance exhaust
   
7. RK tune on 93 (no burbles / no crackles)
   
8. Cold start removed

Symptoms / Pattern

• Repeats consistently: 274A/274B every 1–2k miles
  
• Happens on both banks
  
• Pre-cat O2 sensors replaced multiple times
  
• Minor drivability issue: slight hesitation low RPM / high load

Codes Seen

274A / 274B which translates to P2626

Fault text Oxygen sensor pump current calibration fault before cat. converter, bank 1/2
Fault description O2 Sensor Pumping Current Trim Circuit/Open (Bank 1/2 Sensor 1/2)

Condition for fault identification Test condition: Oxygen sensor is available, oxygen sensor voltage > 4951 mV (K_LAVK_OBD_SA_DIAG_OL)

Terminal condition: Engine running - overrun condition Voltage condition: none Condition for fault memory entry after a defined SA phase the lag period K_LAVK_OBD_SA_T (2.5 sec) must be completed

Action in service 1. Check plug and wiring harness on pre-cat oxygen sensor LSVK B1 (especially trim circuit wire IA), 2. If OK, replace oxygen sensor, replace DME as indicated Fault effect and breakdown warning MIL lamp on after engine runs 2x /Small influence on drivability

Why I don’t think it’s just “bad sensors”

1. Sensors have been replaced multiple times
   
2. Condition has been present since initial exhaust + tune changes
   
3. Both banks faulting together makes isolated sensor failure less likely

Working Theory (FCP Euro convo + my own thinking + talks with other owners who have the same mods minus cams/headwork with zero issues)

This seems more consistent with an operating-parameter / calibration mismatch than confirmed component failure.

Key observations:

Cold-start removed → changes startup exhaust temp behavior

Catless → changes exhaust oxygen stability vs factory assumptions

BMW guidance suggests these can occur when the DME sees sensor behavior as implausible, not necessarily electrically “dead”

It’s possible the DME is detecting warm-up behavior outside expected thresholds during startup and setting 274A/274B even if sensors are functioning

Other notes:

I've spoken with at least 3 other owners who have ARH long tubes with no issues. one well known person had the same setup on 2 cars.

2 well respected people in the BMW circle have mentioned pre-cat O2 angle / moisture potential due to long-tube placement. I agree it can be a thing, but I don’t think it’s my primary cause (I’d expect other symptoms/codes).

I’ve read long-tube collectors can take longer to reach operating temp than OEM, delaying closed loop / readiness.

Tools / Data

I have a TOPDON Phoenix Plus 2

Plan is to capture:

Freeze frame for 274A/274B

Live data: O2 readiness, warm-up behavior, open loop → closed loop (if available)

Next Steps I’m Taking

Continuity test both pre-cat O2 harnesses (haven’t done this yet)

I purchased a re-tune from RK and sent him the above info

Hoping this ends up being tune/warm-up/heater logic vs harness or DME

Questions for the community

Anyone else get 274A/274B repeatedly on catless long-tube S65?

If yes — what fixed it? (tune / harness / DME / sensor placement / exhaust leak?)

Any recommended ISTA/INPA parameters to confirm warm-up plausibility vs heater circuit fault?

If it was wiring: where did the harness fail? (connector pins, heat damage, shared power/ground, etc.)

If anyone wants, I can post screenshots of freeze frame + live data once it faults again.


yea yea, another 02 sensor post.

[spammysammich]

Are you absolutely sure there are no exhaust leaks at the header/xpipe junction? Leaks will absolutely kill the front O2 sensors eventually.

[omgzirra_exe]

check for leaks, check to make sure your SAP valves are open and closing, or delete it all together, the SAP being stuck closed can cause the o2s to die also.

I've also been told some BOSCH O2 Sensors are "questionable" and replacing with BMW branded one has helped.

[supergoji]

Originally Posted by spammysammich

Are you absolutely sure there are no exhaust leaks at the header/xpipe junction? Leaks will absolutely kill the front O2 sensors eventually.

No leaks. I could imagine there would be other codes. But this is for the Heater circuit



I will check the SAP system. The Bosch sensors im buying are from fcp euro.

[GORDON.M3]

My suspicion is the tune.

[DrFerry]

Originally Posted by supergoji

Mod List

1. Dinan Intake
   
2. Dinan ITBs
   
3. Schrick 284 cams
   
4. Port & polished heads
   
5. ARH long tube headers + midpipe (catless)
   
6. M Performance exhaust
   
7. RK tune on 93 (no burbles / no crackles)
   
8. Cold start removed

The exhaust header gaskets need to be replaced on both banks or loose header bolts. Just my two cents. Trying to be helpful.

[supergoji]

Originally Posted by GORDON.M3

My suspicion is the tune.

ok, got it, why do you think it's the tune?, I also need to grab a Mr 12 volt android / phone charger from you.

Originally Posted by DrFerry

The exhaust header gaskets need to be replaced on both banks or loose header bolts. Just my two cents. Trying to be helpful.

This would fall under exhaust leak, but the thing is the rearward header exhaust donut gasket is aft of the pre cat, as in behind so i dont think that would be the issue.

a lose header bolt could absolutely play with the A/F ratio however.


Just got another tip from Partee racing that it's probably DME or rear voltage distributor , could happen when it blows it's internal links.
thing is if it's DME related..I'd imagine it should pop other codes sooner no?

[DrFerry]

Originally Posted by supergoji

This would fall under exhaust leak, but the thing is the rearward header exhaust donut gasket is aft of the pre cat, as in behind so i dont think that would be the issue. A lose header bolt could absolutely play with the A/F ratio however. Just got another tip from Partee racing that it's probably DME or rear voltage distributor, could happen when it blows its internal links. Thing is if it's DME related..I'd imagine it should pop other codes sooner no?

.

[supergoji]

understood. Thank you for the visual. i'll smoke test exhaust and see if leaks.

[omgzirra_exe]

Originally Posted by supergoji

understood. Thank you for the visual. i'll smoke test exhaust and see if leaks.

In the tech world it’s always DNS for even the most smallest of issues, but here it’s almost always the donut gaskets. Most people’s problems get solved the moment they replace the OEM donut gaskets. But if that doesn't solve it, then it's can usually be traced back to bad O2 Sensors/ SAP errors / DME errors

Also, the clamp eventually stretches over time and stops giving the header-to-X-pipe a proper seal, so I’d recommend replacing that too.

[BPMSport]

Flash back to stock and see if issue persists.

We’ve seen this before and flashed our software, problem gone.

Not to say this is definitely the situation in your case but it’s a very logical first step. If you get any other additional errors (other than cat efficiency), then you have your answer.

[GORDON.M3]

SYT_Shadow thoughts on OP's problem?

[supergoji]

Originally Posted by BPMSport

Flash back to stock and see if issue persists.

We’ve seen this before and flashed our software, problem gone.

Not to say this is definitely the situation in your case but it’s a very logical first step. If you get any other additional errors (other than cat efficiency), then you have your answer.

I've got dinan ITB's, cams, headwork and long tubes. you're the tuner here, will flashing back to stock even safe?

In your eyes, what specifically do you suspect is the issue in the tune?

[Redd]

Originally Posted by supergoji

Symptoms / Pattern

• Repeats consistently: 274A/274B every 1–2k miles
  
• Happens on both banks
  
• Pre-cat O2 sensors replaced multiple times
  
• Minor drivability issue: slight hesitation low RPM / high load

How long have you had the tune? If you just installed it then I would go back to stock and see. If it's been there for a while then it's not the tune. Tunes don't just suddenly go bad.

Have you done any intake mods/changes recently? My guess is that you have an intake vacuum leak. I had the exact same thing happen to me before. Hesitation/stumble specifically at low rpm high load. DME showed misfire codes on all 8 cylinders. I smoked/water tested the plenum 3x, each time there were no leaks. Replaced O2 sensors 2-3x (thank you FCP!) cos the misfiring would kill them.

It took me months to finally find the issue. I had a small vacuum leak at the IAT sensor bung (pic below) on my intake plenum. It was small enough that it would only appear at high engine vacuum conditions (low rpm, high load). At WOT it drove fine.

How I finally localized the issue was I removed the plenum entirely and drove the car. This forces the car to run in alpha-n mode - no sensors, just throttle-fuel maps. Lo and behold, no more hesitation/stumbling! So try this first. If your problem goes away then you know it's something in the intake system causing your issues.
.

[supergoji]

Tune has been on the car for years, only changes have been axle back exhausts.

I was recommended to a gentleman who can help. Will troubleshoot tune and report findings.

The only time i got misfires were just once randomly high load, low gear and then the emissions light came on for that code.

The smoke machine arrives in a few days so I'll see if there's any leaks.
That's good insight ( what codes were you getting?)

[DrFerry]

Originally Posted by Redd

How long have you had the tune? If you just installed it then I would go back to stock and see. If it's been there for a while then it's not the tune. Tunes don't just suddenly go bad.

Have you done any intake mods/changes recently? My guess is that you have an intake vacuum leak. I had the exact same thing happen to me before. Hesitation/stumble specifically at low rpm high load. DME showed misfire codes on all 8 cylinders. I smoked/water tested the plenum 3x, each time there were no leaks. Replaced O2 sensors 2-3x (thank you FCP!) cos the misfiring would kill them.

It took me months to finally find the issue. I had a small vacuum leak at the IAT sensor bung (pic below) on my intake plenum. It was small enough that it would only appear at high engine vacuum conditions (low rpm, high load). At WOT it drove fine.

How I finally localized the issue was I removed the plenum entirely and drove the car. This forces the car to run in alpha-n mode - no sensors, just throttle-fuel maps. Lo and behold, no more hesitation/stumbling! So try this first. If your problem goes away then you know it's something in the intake system causing your issues.
.

Super good info ^. Thanks for posting Redd

[SYT_Shadow]

Originally Posted by GORDON.M3

SYT_Shadow thoughts on OP's problem?

Flashing to the stock tune. Or switching tuners.

[supergoji]

Originally Posted by SYT_Shadow

Flashing to the stock tune. Or switching tuners.

currently in the process of reading the tune file currently.

I'll even post it up for people to read.

Identifying ECU...

VIN: WBSWD93568PY41097

ECU: MSS60

Hardware: 7848125

Firmware: 05690TØZ240E

Unlock Detected: True

Status: Normal operation

Identifying ECU...

VIN: WBSWD93568PY41097

ECU: MSS60

Hardware: 7848125

Firmware: 0569QTOZ240E

Unlock Detected: True

Status: Normal operation

Preparing ECU. This may take several moments...

Reading Tune...

Read blocked by ECU. Contact your tuner or perform a clean full flash.



I used the Mss6x flasher to try to read the ecu, wouldn't connect with the flasher or cable.

Used rk tunes and it connected but it's locked.. ��

[SYT_Shadow]

Since the mss6x came out, most tuners protected their files

[OmairK]

That ARH System is a forward X-Pipe configuration from appearance. I trust the experts above more than my keyboard warrior knowledge but I don't think the car will run right with the all of the modifications and a stock tune (to potentially remove variable) so it may be a moot point to revert the car back to a stock tune. I also have heard reliability issues with the non-OE Bosch Sensors from another thread. I would do as suggested above if all else fails and change tunes.

[Redd]

Oh I didn't catch that. Forward x-pipe systems are known to cause hesitation/stumbling with some tunes. There's quite a few threads on it and one of the reasons I've never experimented with a forward x-pipe. You may require a forward x tune.

[spammysammich]

Can confirm the BW forward X caused driveability issues on my car without a tune. PO was going nuts trying to troubleshoot it.