As SB states , the system will correct for Reg pressure , but if the reg diaphragm is ruptured and allowing fuel to pass to the intake, this richness can be large enough to over-ride the systems corrective limits..[ adapt values ]
... on the same note, a vac leak can do the same thing in the other [ lean] direction...the leak, of course , having to be large enough to be non-corrected by the ecu limits..[ again,adapt values]
An easy test for o2 sensor reaction on these EFI systems
is to monitor an isolated o2 signal [ meaning unplugging the feed/sig line to the ECU] and watch the sig while disconnecting
and plugging the vac line to the Fuel Regulator..this causes an instant rich condition and should show up on your 02 signal as high voltage.
This verifies o2 sensors ability to recognize rich mixture condition..
For lean recognition test, simply creating a vac leak by removing a vac hose and watch for low voltage reaction...here one can simply put your thumb on/off the vac hose and watch the reading
a little experince and you can tell a decent sensor , although a scope is killer for absolute diagnosis..