About a month or so ago I got an engine warning light with code P0141
(2nd bank Oxygen sensor). I continued driving for another 10,000 Kms.
Did not notice anything i.e. no rough idle, no increase in engine
heat, oil consumption, and did not notice a reduction in fuel
efficiency. Dealer wants$300 to change it. I am not sure there's any
benefit to it especially now that I've passed the emissions test even
with this problem. Does anyone else have a similar problem/experience?
This is a 4 cylinder. This is in Ontario. I have expressed my concerns
to the mechanic before conducting the emissions test, but did it
anyways. I was expecting elevated pollution readings, but the test
came back with zeros in hydro carbons and carbon monoxide. I am
guessing that the computer was able to compensate for the lost sensor
- or the issue is a minor one and has negligible effect on the
emissions. I am not sure if the facility had the dyno test.
Maybe the wiring on the sensor broke. I am waiting for warmer days to
check it out. The vehicle has 400K, and I find it strange that a
sensor would malfunction before some other component that is under a
lot more stress or is more likely to break after this much mileage.
The heater has no role when an oxygen sensor is up to temp (about 750F).
In any case, the sensor /must/ be up to temp before any emissions test
is performed. One of the technician's jobs is to make sure of that, so
theoretically the O2 sensor heater should not even be a factor in a
No compensation needed.
The sensor itself is working perfectly. If it were not, you'd get a
different error code.
If your MIL was on when the car was brought in, the car would have
failed outright. I am guessing yours was off when it was inspected.
The Ontario DriveClean ASM sequence does not involve querying the OBD-II
computer for codes, so your stored code would not have been discovered.
Possible, but unlikely. It is, however, the first thing to be checked
before it's decided to replace the sensor. Corrosion on the connector
contacts is also a possibility.
Heater failure is /very/ common. The heater has a difficult job to do.
It is very likely your heater has failed and the sensor needs to be
replaced. It is not possible to replace the heater alone. I would
recommend an OEM sensor depsite the cost. Aftermarket will fail more
often, and more quickly than OEM.
The sensor itself is working fine. O2 sensor heaters ARE highly stressed
components and fail /very/ often.
You don't need to replace it now. If you like you can leave it forever,
so long as you know for sure the MIL is on with only P0141, and so long
as you can always pass smog. And so long as you don't mind the annoyance
of the MIL being on all the time...
The secondary O2 sensor can have a small effect on "fine tuning" fuel trim
within a very narrow range. That's intended to damp out any excessively
zealous tendency by the ECM to illuminate the MIL with a P0420.
As was said that is the secondary o2 sensor. It's only function is to
monitor the efficiency of the catalytic converter. It doesn't control
anything and is compared with the front sensor to make sure the cat is doing
it's job. It is interesting that he passed it because around here any
malfunction will fail the car. Anyway it is a minor fix. Just replace the
sensor. Check auto parts as the dealer will milk you about three times what
it should cost.
The ECM monitors the action of the secondary sensor. The ECM is /also/
allowed to "trim" the air/fuel ratio in order to /make/ the secondary
sensor flip-flop properly (or to NOT flip-flop, which is more accurate).
If the ECM is unable to force the secondary sensor into behaving properly
without exceeding the ECM's built-in limits, the ECM will set a P0420.
The reason for this "trim" function is to avoid excessive sensitivity to
The ECM cannot force the secondary O2 sensor to do anything. It's output is
only used to compare with the primary sensor to check CAT efficiency which
will generate the P0420 code. The primary O2 sensor is used to control fuel
Just one bit of evidence to support the contention that
secondary O2 sensors may indeed be used for fine tuning of
"Engines use either one or two oxygen sensors for fuel
control. Many later model 'V' engines use one sensor to
control each bank independently. This allows for correcting
any bank-to-bank air/fuel deviations or imbalances.
Additional oxygen sensors can be located downstream of the
catalyst. These downstream sensors are used for catalyst
system on-board diagnostics and may often be used for
'fine-tuning' fuel trim."
More dialogue and documentation on this from Stephen H and
others in 2005: http://amermlrs.com/cars-367.html
Well the wiring checked out, so I got a new o2 sensor. I tried to
unscrew the old one. This was wishful thinking. Even though I had a
long 22 wrench this sucker is holding on to the CC with more than
1000lb pressure. I am planning to use a torch on the outside part of
the o2 sensor thread. The theory is that the cat converter thread will
expand, and then with some hammering of the sensor to loosen its grip,
the wrench will be adequate to unscrew it. Has anyone done something
like this with success?
On 3/17/08 11:51 AM, in article
I'm curious -- how were you able to apply 1000 lb pressure?
I've never needed anything more than my trusty 4 ft piece of gas pipe on the
end of the breaker bar to remove these things (also works on Honda crank
I used a pulley that I use to pull a downed Moose when I go hunting.
It has a scale on it. I hooked the end of the wrench, wrapped the rope
around the fixed pipe in the garage and turned on the motor that wraps
the rope. I stopped when the motor registered about 125lb. It is
eactly 1/8 of the pressure at the hook. The wrench is about a foot
(that's the radius R). Torque = R x F.
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