Downstream HO2S Sensors

for the brainless *twit* known as Bob, or bob, who questioned what the
2nd O2 sensor in each bank was for, as I answered, it is to monitor
catalytic converter efficiency.

bob questioned that, and said that was true for cars "10 years ago."

bob the nitwit, never came back with an answer regarding what they are
really for.

I was in fact correct, and here is the proof:

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D1.) Catalyst Efficiency Monitor - Information
The Federal Test Procedure Catalyst Monitor monitors for deterioration
in the catalyst system and illuminates the MIL when tailpipe emissions
exceed the appropriate HC emission thresholds. The Catalyst Monitor is
enabled after the upstream and downstream HO2S sensors have been
tested and verified to be functional. This monitor relies on the front
and rear heated oxygen sensors (HO2S) to infer catalyst efficiency
based upon oxygen storage capacity. Under normal closed loop fuel
conditions, high efficiency catalysts have oxygen storage which makes
the switching frequency of the rear HO2S quite slow compared with the
frequency of the front HO2S. As catalyst efficiency deteriorates, its
ability to store oxygen declines, and the rear HO2S begins to switch
more rapidly, approaching the frequency of the front sensor. In
general, as catalyst efficiency decreases, the switch ratio increases
from a switch ratio of 0 for a low mileage catalyst to a switch ratio
of 0.8 or 0.9 for a low efficiency catalyst.

Some vehicles will monitor substantially less than the entire catalyst
volume in order to meet the stringent catalyst monitoring malfunction
thresholds. In many cases, only the front, light-off catalyst is
monitored.

Front and rear HO2S switches are counted under specified closed loop
fuel conditions. After the required number of front switches are
obtained, a rear-to-front HO2S switch ratio is calculated. The switch
ratio is compared against a threshold value. If the switch ratio is
greater than the calibrated maximum limit, the catalyst has failed.
The test entry conditions for the Catalyst Efficiency Monitor are as
follows: ECT or CHT (warmed engine), IAT (not at extreme ambient
temperatures), MAF (greater than minimum engine load), VSS (within
vehicle speed window) and TP (at part throttle) are required.
Because an exponentially weighted moving average is used for
malfunction determination, up to six OBD II drive cycles may be
required to illuminate the MIL.
NOTE: THE CATALYST MONITOR ON SOME EARLY OBD II VEHICLES (SOME
1994-1996 VEHICLES) WAS REFERRED TO AS THE "STEADY-STATE CATALYST
MONITOR" AS OPPOSED TO THE "FTP CATALYST MONITOR" (DESCRIBED ABOVE)
THAT IS MOST COMMON FOR VEHICLES BUILT AFTER 1996.

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so there ya go *bob.* shove it up your exhaust pipe and put a cork on
it.

Lg