A few thoughts:

I agree with others that the O2 sensor is a primary suspect. Too bad the Check Engine light isn't coming on as it should (the FAQ may be wrong, but do make sure the bulb is not out). If it came on just after the engine warmed up then that would help point to the O2 sensor since the O2 sensor is ignored until the block temp sensor says the car is warmed up (exhaust gases are hot). I presume you've reset the code and watched for it to come back.

One thing I note is your use of an injector cleaner gas additive. If you did this before the 2-3-1 code appeared then I'd suspect a possible relationship. Both I and others have experienced heavy spark plug fouling immediately after cleaning (typically white ash). I always recommend checking and cleaning/replacing the plugs a few weeks (tank fulls) after using an injector cleaner. If plugs are fouled (for whatever reason) then the O2 sensor could be too. It might take a while for deposits to burn off the sensor so try for a highway tune up. The 16-valve B234F occasionally needs to clear its lungs with some serious highway running.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day