If the oxygen level in the exhaust stabilizes, the catalytic converter efficiency will drop
considerably. In order for a catalyst to best clean up NOx the A/F ratio must be richer than
14.7:1 however, NOx emissions from the engine are highest when the engine is lean. For the
catalyst to best clean up CO & HC the A/F ratio must be lean, but CO and most HC is created
when the engine is rich! Three way catalysts (TWC s) overcome this problem by using cerium
for oxygen storage. This oxygen storage trick will greatly increase the efficiency of the TWC if
the oxygen level is cycling slightly rich and slightly lean.
The only way that proper exhaust oxygen level and oxygen cycling can be maintained is with O2
sensor feedback. This is called closed loop. After the computer has determined the proper fuel
metering (injector on time), the O2 sensor is used for feedback to fine tune fuel metering and to
make the oxygen content in the exhaust fluctuate slightly.