[mwebb]

the system will go to open loop
when faults are set for Mr 02 sensor

open loop will not ever result in lean condition
================
you may try adding resistance to the 02 sensor ground enough to bring the ground up 100 to 200mv which
MIGHT have the effect of having the ECM compensate for the
"rich bias condition" demonstrated by the 02 sensor

the 02 sensor "average" when cycling as designed will be 450mv
drive the average high and .....
as long as the 02 sensor is still cycling , you MIGHT be able to trick the ECM
to lean bias the system .
but
in theory
it will not work , the system will stay at stoich until the ECM becomes angry with the 02 sensor . when it sets a fault and reverts to
open loop

to see if it will work ...
use your scope to monitor injector pulse width average as you tamper with
02 sensor signal ground - to see if IPW average decreases as you bring the ground voltage higher .

in reality
the AFR sensor will not do what you seek in your system
================================

EQ ratio is not a measurement of an actual condition
it is a
command

EQ ratio will be equal to and opposite of CURRENT lambda
when / if the system is attempting to maintain
stoich
14.64 to 1
at the time

EQ ratio is not usually a valid / active PID on systems with bang bang 02 sensors
such as your system has ?