Sydney Hoeltzli wrote in message IOW if a plane w/out mode C is at 9,500
will you get a TCAS in the flight levels?

Yes, you will get a warning. It happens quite often too. We had one today,
but already had a visual on the target. Another airliner apparently wasn't
so lucky. They received a warning and had to comply with the TCAS's commands
for a deviation (which can be quite upsetting to the pax and cabin
articles). Then the crew is required to inform ATC of the deviation. In this
instance, we listened on the frequency as the controller then confirmed that
the target's Mode C was inop and the target was never a real threat.

D.