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EQ ratio is a command NOT a measurment
...."You ALWAYs want to measure lambda pre-CAT.
The sensors don't really measure AFR, but equivalency ratio
(which is inverted to give lambda). Basically O2 is added or removed to the test chamber until stoich is reached. Since CAT is supposed to be removing hydrocarbons, the post cat sensors are there to see rather or not there is a proper shift from the real (pre-cat) equivelency ratio measurements.
When you get a 'cat efficiency' DTC it generally means that one of the sensors has gone bad, or the shift between pre and post CAT is below expected.
"....
The sensors don't really measure AFR, but equivalency ratio
the above statement is not really true
AFR sensors do measure AFR and
EQ ratio is a command NOT a measurement
in the real world
for actual cars driven on the road -
the ECM does not add or remove 02
the ECM changes the amount of fuel from a base value depending on inputs from various sensors
the amount of change is represented by fuel trim values
short term trim and long term trim on most cars
not all cars
when you get a cat efficiency DTC it almost always means bad cat
because
the cat test WILL NEVER RUN if there is a problem with the 02 or AFR sensors
the 02 or AFR sensor testing must be complete
and it must pass before cat testing will be enabled and
if a 02 or AFR sensor should set a DTC , cat testing will be disabled until the DTC has been cleared
and
then
the 02 or AFR sensor testing will have to run to completion and pass in order to enable cat testing
which would be the reason we save the last run mode 6 test for the cat
prior to clearing DTCs , so we have an idea if the cat will fail when testing is re enabled
if you have a cat efficiency DTC and no other DTC and
all mode 6 and mode 5 tests (if present) have passed
except the cat tests
the cat is bad -
a 2000 jeep has 02 sensors , plain jane bang bang sensors
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