Vacuum leak on 2002 S60

A few weeks ago, I got an "emissions system service required" message
and warning light. Examining with an OBD-II reader, I can see a p1132
trouble code. This is an O2 sensor not switching (stuck rich). With
the
same OBDII reader, I can see that the O2 sensor output does switch
most
of the time, so I don't think the problem is with the O2 sensor.

What I think is happening is that there is a vacuum leak, that has
caused the ECU to en-richen the mixture, then when the boost pressure
rises, the leak causes air to leak out and the mixture to become too
rich, triggering the fault.

Does anyone know where these engines usually develop leaks on their
inlet side, or has anyone else had this fault code and found it to be
something else?

 

Normally a vacuum leak on a turbo car will make itself known with a
hissing noise under boost. Is the output from the O2 sensor switching
rapidly? Does this car have more than one O2 sensor? Many do.

 

I haven't heard any excessive hissing, but the engine is fairly well
sound proofed

Although I can see the sensor switching, the OBD logging system has an
update frequency of 1Hz, so I just see aliased data. The car does have
two sensors, but the second does not appear to be visible through the
OBD system. Do you think that the sensor might be getting "lazy" and
not switching at the correct rate?

 

EGR valve stuck open ?