I was driving around the other night and suddenly my check engine light comes on. I pull over immediately and shut the car off. I do the flashing check engine light test and the code that comes out is 1221. I look it up and it is oxygen sensor. My car is a 1993 325is. This is not the first time it has randomly come on. The oxygen sensor was replaced with a new unit less than 5000 miles ago. The light will go off by itself after a day or so. What gives? :dunno

The ECU is getting a signal from the o2 sensor that is less than the ECU expects.

If the ECU signals to richen the mixture by increasing the phasing and it still gets a lean reading from the o2 sensor it gives that error signal.

It can be the sensor, or the heating circuits to the sensor, or something is preventing the mixture from going richer.

You can measure the output of the o2 sensor by using a VOM on the black lead from the o2 sensor.

.6 volts is dang rich .3 volts is dang lean.

http://www.kemparts.com/TechTalk/tt07.asp

Later,

My car experienced the same problem after changing the O2 sensor, it still flashed 1221. Does the check engine light go out if you rev the motor? For me the check engine light would go out if I reved the motor.

I my case I could hear a "sucking" sound under the hood. It turns out that I had a cracked intake gasket located between the intake manifold and the head. Maybe your car has the same problem.