00 E46 330i O2 sensor heater puzzle

[whiskychaser]

Emissions warning light comes on occasionally and the fault is pretty clear:

DME: (191) BF insufficient heat output, oxygen sensor after cat., bank 2. Sensor heater faulty(P0161)

One after cat sensor has a resistance of 2.7 ohms and the other is 2.9 ohms. OK, this is not conclusive. But DIS live data for all four heaters says they are all 99.3%. If that figure isn't efficiency, what is it? Advice appreciated

[MIKYZZ4]

Right, the 02 sensors are reading fine, the heater circuit is not.

[whiskychaser]

Right, the 02 sensors are reading fine, the heater circuit is not.

Agreed. I propose to swap the leads and fully expect the heater fault to 'move' to bank 1.

But I'm curious why all four heaters read 99.3% if one is faulty

[bmwdirtracer]

The O2 sensors are 17 years old, including their little heating coils. All of this lives in ridiculously hot temperatures. I'd say it's time for new sensors.

But in answer to your question, I'd guess that the 99% which DIS is reading refers to the control circuits for the heaters being at "full on". This would be the likely state when the car is idling. If you rev the engine, I'd expect the primary sensor's percentages to drop, as the exhaust provides more heating.

[whiskychaser]

I already have a new sensor but access is very limited without a lift. I didn't want to risk rounding off the old one either. So I asked an exhaust fitters and a BMW Indy. They both took one look and promptly turned the job down. TBF, the nuts and studs on the cat/exhaust flanges do look in poor condition. I don't think they fancied the complications of removing the cats if it came to it.

The engine was hot but it was at idle. So that makes perfect sense. Will see what difference revving it makes.

Many thanks for the responses

[whiskychaser]

Update:
Exhaust specialist confirmed the flats on the sensor had rusted off. They said they could remove it if the manifolds were off the car - they don't do mechanical work as such. They would even re-tap threads if they stripped. They removed the seized and rusty studs and put some new bolts in to make life easier for me to do it.

I didn't have time today so I put on the new valve cover gasket I had instead. Fired it up and it ran like a pig. Half a dozen fault codes but the O2 sensor heater wasn't among them. Strange. (I didn't really say that. Imagine some expletives).
I ran the diagnostic again and got only two codes - bank 2 lean and bank 2 after cat O2 heater. That's more like what I have come to expect. By now, it is idling very nicely and it occurs to me that maybe the DME is learning. So I run the diagnostic another 3 times. Each time I get NO FAULTS.

Will run it for a few days and see what happens...

[thejlevie]

You don't say how many miles are on the car, but be aware that the pre-cat sensors are a scheduled maintenance item with a life of 100k. The post-cat sensors are in a more benign environment and rarely fail. In this case I'd check to see if power is present at the post-cat heater and go from there.