[Brian86]

'06 N52 with gadzooks miles.

It urks me that the cat conversion below threshold codes always come as a pair. Not just me, but pretty much every post I've read about it.

Was looking at live data on my Foxwell today, and there is a significant difference between the response in bank 1 and bank 2.

Both upstreams show the same ripple. Bank 2 downstream is nice and flat, but bank 1 downstream varies from a small ripple that roughly matches it's up, to a rather large ripple about 3x the magnitude.

I can watch that data and understand that the bank 1 cat's are probably shot, but it really annoys me that BMW has the car throw codes for both banks simultaneously every time.


Had a catastrophic failure of the oil separator about a year ago and I thought I had lucked out and the cat's were mostly spared. The cats could have been fouled by the oil, or it could just have worn out.

I have my fingers crossed that a plug anti-fouler spacer will tidy up the bank 1 downstream signal enough to keep it smog legal.

[Brian86]

I did a little more staring at my live data from the O2 sensors.

Bank one is really erratic. Really all over the place. I'm starting to wonder if the sensor is bad instead of the cat. One moment it will read steady, then it'll be oscillating with the rhythm of the upstream sensor, then it'll just wander...

I'd expect a cat, performing a passive chemical reaction would do whatever it is going to do in a stable and consistent manor.


Over the years I've pretty much never found a downstream sensor that actually failed, so I'm skeptical, but hopeful.

Think I'll try swapping the left and right harness couplers to see if the problem chases from one side to the other.

[Brian86]

So, continuing this monologue:

BMW in all their wisdom, decided to use different couplers for the front/rear banks on the downstream O2 sensors. I just reversed the sensors in the exhaust pipes.

Results are a little inconclusive, but I'm leaning toward a bum O2 sensor on the bank 1 harness. Much of the anomalous behavior stuck with bank one, suggesting sensor. However the slightly larger ripple moved to bank 2.

Car doesn't need smogged again until next summer so I'm going to sleep on it for now.

[Brian86]

Bringing this monolog to a close in case anybody searches...

I popped two new Denso downstream sensors on and after 15 or 20 odd drive cycles the computer is still happy with the emissions performance.

[ome91]

Very interesting to hear these, I just changed out for a 330 Intake Manifold in my 328i and tuned it to the free Bimmerlabs tune, not a couple days later I got these cat codes. I'm thinking it could be a oil leak somewhere or a crack but I figured it would give me lean/rich specific codes.

Hope its not my cats but I'll do some more diagnostics tomorrow. What specific sensor data did you read for the cats/O2 sensors, might do some diagnostics tomorrow with my Carly adapter.

[Brian86]

I was using a foxwell/schwaben handheld to look at O2 sensor voltages in real time.

I was clued into a bum O2 sensor because the trace on the bank 1 harness would look normal, then start wandering and oscillating and periodically dropping to zero volts before snapping back to a steady 0.6 or 0.7v to match the other bank and it's accompanying upstream.

Best to look at O2 sensor traces while driving under steady speed and light throttle. It gives the engine time to find closed loop and 'settle' into a continuous running condition. All the traces should be relatively steady and smooth. If the engine is changing throttle, or RPMs the O2 sensors traces will be all over the place.

Ideally you'd see a very small oscillation on the upstream sensors, and less oscillation, or a completely smooth line on the downstream sensors.