Unless you're literally dumping coolant into the intake, i can't imagine you could get enough through the engine to significantly lower the O2 content of the exhaust stream. The usual causes of coolant through the engine, say a leaking head gasket, generally just adds a small amount with each cycle of the cylinder and that gets mixed with the output from ...
Easy to find out: unplug the sensor (electric supply) and see what the scantool tells. If it tells B1S2 you got your answer.
Is your sensor warm enough?
Does it react (correlate) to changes in throttle position
especially cut off?
does your throttle(flap) work correctly? (by cable or wire?) clean if neccessary