1996 S-10 2.2,I recently had a 141 code O2 sensor at the rear of the
catalytic and after replacing the sensor, I notice the scan tool was
then showing close loop, but moments later " 5 mins" it would go to
open....... monotering the LT trim was at 114 and the ST Trim it was
128.... the O2 sensor at the rear of cat was in the .833 mv all of the
time. Should the voltage toggle for the sensor ? keeping it in close
loop ? The code has not returned since replacement of sensor, engine
temp is at 207 F .. seems to operate fine.
When cold the engine runs in closed loop using preset defaults for fuel
trim. As the engine warms up it goes into open loop where the front O2
sensor and other conditions control the fuel. The O2 sensor after the
converter is used to determine the efficiency of the catalytic converter and
does not switch much. If your OBDII scanner has the ability graph the two O2
sensors while driving and you will see the relationship. If the aft O2
sensor starts switching as much as the front one you will get a cat
Update for more info, 130,000 miles but in superb condition, start up
is good "open loop", 3 to 5 mins later, light comes on and PID shows
"close loop"........then in about 5 mins time, returns to open on PID
and light on the snap-on scanner.
engine temp 207............ monitoring the proccess from cold to hot
front O2 around 500 mv when hot, switches more when cold
rear O2 steady at .837 when hot............ starts cold with lower mv
like 190, 288
LT @ 114
ST @ 128
at 50 mph injector pw 3.9
idle @ 1.9 all inj. equal on all the injectors
no DTC's since the O2 has been replaced.
if the Cat wasn't operating properly, could that cause the rear O2 not
to switch, and being in the open loop, is that why the LT Trim is at
But here is what is confusing to me, when the engine is cold "open
loop" I see no PID for the LT & ST trim, which is okay. After it goes
to close loop, I can then see the PID for the LT & ST trim...... but
when it returns to open loop the PID for the LT & ST continue there
display. So it's obvious that it is still in close loop if the LT & ST
are still displaying, no ?
No, not a post cat O2 sensor.
A wash coat of cerium is added to OBD2 catalytic convertors, the
cerium stores oxygen, a high voltage reading from an O2 sensor
equals low oxygen content in the exhaust, the steady high voltage
reading is telling you that the catalytic convertor is still
capable of storing oxygen. periodically, you may see the O2
voltage go low, this is the cat burping off excess oxygen,
Rear O2 has no closed loop authority.
The P0141 was for a heater circuit failure.
Good odds that the rear O2 sensor internal heater burned open and
replacement fixed the problem.
Thanks to the tips everyone posted, the FIX was the front O2 sensor,
when it went back to open loop, I noticed the mv in the front O2 was
fixed at .535mv and did not switch. I purchased a new one, installed
it and now the close loop is steady without dropping out.
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