Anyone have a good knowledge of how the O2 sensors function on engines
with 4 sensors? The manual I have doesn't go into any detail. I'm seeing
error codes 171 & 172 which is sensor reporting lean condition on left and
right side. I would like to fix but not wanting to replace all 4 sensors
if possible (car is going to be sold in not to distant future).
96 CV 4.6L
My explanation here is admittedly simplistic, but, your
codes are related to signals from the O2 sensors at the
engine. The other two are pretty much used to measure the
performance and efficiency of the converters. With a lean
code on both banks, you have an overall lean condition which
would suggest a problem with either a vacuum leak or, a
fuel delivery problem. You need someone who has a scanner
that can provide realtime outputs to analyze the situation
causing the lean condition. With the same fault from two
sources, it is not likely that the sensors themselves are at
fault or erroneous.
The error codes are produced by the O2 sensors telling you
that something else in the system is causing a lean
condition. As Neil indicated, the MAF can and is known to
cause this on occasion and, can many times be corrected by
simply cleaning. There are instructions with pictures for
doing that on the net. If you choose to do this, you must
be extremely careful not to damage the small hair-sized
wires in it. Now, forget the O2 sensor and pay attention to
what they are telling you - they are only the messengers.
The mass airflow sensor measures intake air mass by a heated
wire which is monitored for amperage, as more air flows into
the engine, the heated wire cools off, the cooler wire
carries more amperage. If (when) dirt collects on the
heated wire, it becomes thermally insulated -by- the dirt
itself, this un-intended insulating effect manifests itself
as air entering the engine than what is actually reported to
the PCM by the mass airflow sensor.
The classic definition of lean (the codes you have stored in
memory) is too much air.
Additional logic; you have a lean condition effecting -both-
banks of the engine. What component can effect fuel mixture
to -both- banks of the engine. The Mass Airflow Sensor!
Why no MAF sensor code? Simple, the MAF hasn't failed
completely and yet it has no way to determine exactly how
much air -is- flowing past it. Coolant sensors act the same
way when they (or the circuit) slides out of calibration.
This is precisely why doing driveability repair work is
much, much more than reading a trouble code and shot-gunning
a part in the direction of the code! You -have- to know and
understand the whole system, what effects what and why.
Now, you can squander $400-$500 on nice shiney brand new O2
sensors, or you can go to Radio Shack and buy a $2 can of
tuner cleaner, give the MAF element a squirt, but if I've
seen this problem once, I've seen it a hundred times...
I have had problems WITH o2 sensors.... Cheapest test is to take o2
sensor out and torch it
take a blow torch and get the tip that goes in the exaust like pretty hot..
just not melting.. just before white hot.
Let it cool and try it. I got lucky you may too
Neil, thanks for the explanation this was what I was asking for. If you
read my other post (different subject line) you would see I am
looking/including things that are in common for both banks. This is the
first car I've worked on with a MAF sensor so lack of knowledge of this
component is present. Typ manuals don't go into very much detail.
On the other hand, I've read in several locations O2 sensors are good
for about 60k miles and this car has just turned 70k. If true they
could be the actual problem but before throwing $$$ at it they will be
tested for voltage output.
I don't swap parts until I stumble across the cause. As I said before
I'm just trying to gain some knowledge on this system.
As another poster, lugnut, suggested several times, checking vac and
fuel (common/shared components) was done before even posting the first time.
Again, thanks for the comments.
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