The Curious Case of Clogged Cats For BMW N52 Engine Variants
I have seen a few examples of BMW vehicles with the N52 and N51 engine that consistently set the 2C31 Oxygen sensor before catalytic converter 1, trim control or P2097 Post Catalyst Fuel Trim System Too Rich (Bank 1) and/or 2C32 Oxygen sensor before catalytic converter 2, trim control or P2099 Post Catalyst Fuel Trim System Too Rich (Bank 2).
The usual scenario with these faults is that they have been coming and going for months or sometimes years as the car has floated between different shops getting smoke tested to death, adaptations cleared, plugs replaced, coils, guzzled various fuel injector cleaners, fuel pumps, mass airflow sensors fill the trunk, air filters, new valve cover, multiple sets of upstream and downstream O2 sensors and DME programming with no avail. Where to go when everything seems like it has been replace but nothing puts a dent in these codes?
It’s a good idea to look what your O2 sensors are telling you. Using a scan tool with the key on engine off, your O2 sensors upstream should read 2.04v and your downstream O2 sensors will read .42v. When you start the car and let the car warm up and idle down you should still be seeing about 2v on the upstream O2 sensors and the downstream O2 sensors will rise a little bit to about .6v-.7v with little to no fluctuation at idle. When suspecting a partially clogged catalytic converter you will usually see upstream O2 sensor numbers in the low 1.9v range sometimes even as low as 1.7v as the less air going by the O2 sensor the lower the voltage. On the downstream O2 sensor, check to see if the voltage is higher than .6v or .7v. This would indicate a "richer" mixture associated with a lack of airflow. Its a good idea to check your Multiplicative or long term fuel mixture readings here as well.
If you notice that your multiplicative values are trending negative, like -5% or -6% on either bank, it’s a good idea to pull the upstream O2 sensors and inspect the Catalytic converter element for partial or full clogging.
Most of the cars I have had experience with have had a partial clog with no mention of a lack of power or performance loss and some of the vehicle were in the 50k-100k mileage range with a pretty clean service history so be on the look out for clogged cats on the cars where you would least expect it.