Hey guys just wondered if any of you all could give me some suggestions. Okay I just got a 95 explorer with 94k about 3 weeks ago. The CEL came on first during the 150 mile drive home from the previous owner, scanned the codes got EGR code, clogged/not switching, and 176 oxygen sensor bank 2 lean, then cleaned MAF, and put in new thermostat (it was stuck open, thought maybe that was why it was running lean). scanned codes got system pass. Went on 300 mile roadtrip yesterday, CEL came on, scanned codes, got (KOEO) 176 oxygen sensor, lean bank 2. Does this just mean I need a new oxygen sensor? Is their anything else that would cause this?, it only seem to trip the CEL on long extended highway runs. Any suggestions appreciated. Thanks

 

1) A 176 would be a continuous memory code, not a KOEO code. Two types of codes are output during the KOEO test: KOEO codes before the separator pulse and CM codes after the separator pulse. You're only reporting a CM 176, so I will assume that it gave you a pass (111) for the KOEO codes.

2) There are a lot of possible causes for a lean O2 sensor code: Bad sensor, bad connection between sensor and computer, exhaust leak upstream of sensor, incorrect fuel pressure, vacuum leak, and many others. A lot of people, including many shops, will "guess" and just replace an O2 sensor based on the code. Then, if the problem goes away, they fixed it. If the problem repeats, then they can go into the fuel control system in more detail.

3) I think the first thing I would do would be to perform the KOER test to see if you get a matching code from the KOER test. A lot of times it's easier to diagnose from the KOER test. Then check the wiring between the computer and the O2 sensor. Then check the fuel pressure. If that doesn't reveal any faults, I would then replace the O2 sensor and see if that fixes it. If it does, then great. If not, then, with a known good sensor, I could look into it in much more detail.

 

Ok sure - but stop and pull your plugs. What does they tell you opn the left side, bank 2? Are they white and truely lean or black and truely rich? knowing what is happening inside the engine is best determined by plug readings.

 

here we go again, replacing parts before we test them

Should the O2 sensor be replaced when the sensor light comes on in your car?
Probably not, but you should test it to make sure it is alive and
well. This assumes that the light you see is simply an emissions
service reminder light and not a failure light. A reminder light
is triggered by a mileage event (20-40,000 miles usually) or
something like 2000 key start cycles. EGR dash lights usually fall
into the reminder category.

 

Testing O2 sensors that are installed
The engine must first be fully warm. If you have a defective
thermostat, this test may not be possible due to a minimum
temperature required for closed loop operation. Attach the
positive lead of a high impedence DC voltmeter to the Oxygen
sensor output wire. This wire should remain attached to the
computer. You will have to back probe the connection or use
a jumper wire to get access. The negative lead should be
attached to a good clean ground on the engine block or
accessory bracket. Cheap voltmeters will not give accurate
results because they load down the circuit and absorb the
voltage that they are attempting to measure. A acceptable
value is 1,000,000 ohms/volt or more on the DC voltage.
Most (if not all) digital voltmeters meet this need. Few
(if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter
to look for 1 volt DC. Many late model cars use a heated
O2 sensor. These have either two or three wires instead of
one. Heated sensors will have 12 volts on one lead, ground
on the other, and the sensor signal on the third. If you have
two or three wires, use a 15 or higher volt scale on the meter
until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should
see a change in voltage on the meter in most late model cars. If
not, check your connections. Next, check your leads to make sure
you won't wrap up any wires in the belts, etc. then start the
engine. You should run the engine above 2000 rpm for two
minutes to warm the O2 sensor and try to get into closed loop.
Closed loop operation is indicated by the sensor showing several
cross counts per second. It may help to rev the engine between
idle and about 3000 rpm several times. The computer recognizes
the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts.
If you see less than 0.2 and more than 0.7 volts and the value
changes rapidly, you are through, your sensor is good. If not,
is it steady high (> 0.45) near 0.45 or steady low.

 

Hey aquanuat or anyone else who might know... I have code 41.

My O2 sensor reacts to changes in throttle but does not oscillate at all. This is the second new O2 sensor. I'm stumped. The voltages are correct from the EEC.

Any ideas?

 

mdtodd

code 41..No HO2S switch detected (right HO2S).

• Which o2 sensors did you change?
• did you bench test the sensors?,
• what yr is this related to?
• Have you checked for vacuum leaks, anywhere after the MAF?.

 

I'm working on a '90 Bronco 302

There is only one O2 sensor on this year (as far as I know)... it is located in the y-pipe which is just after the exhaust manifold.

I've tested every sensor except the TAB and TAD... all within spec

I've looked for leaks, tightened bolts at the throttle body and the manifolds as well as where the headers run into the exhaust Y pipe. Also pressure tested the vacuum reservoirs.

Listened for leaks with hose... not sure how else to test for leaks... suggestions?

The truck runs great just can't get the dang light to go away. It comes on after I've been driving for about 15 minutes.

According to the manuals (I have three) the O2 is within spec but they really don't say anything about the sensor switching back and forth which it does not do... the resistance is within spec at several temperatures and the sensor reads between .1v and .9v at different RPM.

I don't have a MAF I have a MAP... it's good.

Cleaned throttle body and Idle Air Bypass valve.

Changed fuel filter.

Tested fuel pressure at the injector rail... within spec with the KOEO, KOER

Changed plugs and wires.

Tested PCV... good

Four questions:

1. What is the best way to check for vacuum leaks on lines and engine?
2. Does the O2 sensor always oscillate, meaning is mine bad?
3. If it is bad... what could cause a new sensor to go bad... the EEC output to the sensor is 4v.
4. If the fuel rail has correct fuel pressure should I still check the pump?
Thanks for the help!

 

from earlier posting,

You are looking for voltage to go above and below 0.45 volts.
If you see less than 0.2 and more than 0.7 volts and the value
changes rapidly, you are through, your sensor is good. If not,
is it steady high (> 0.45) near 0.45 or steady low.

O2 sensors will go bad, if
you are running antifreeze through the exhaust (head or intake manifold leak),
burning leaded gas,
burning oil, or
running consistently rich,

did you check the colour of the plugs, that will tell you if you are out of range,
white ashy (lean),....
black (Rich)....
oil fouled ( self explan).....
light tan (where you want to be)

How does an O2 sensor work?
An Oxygen sensor is a chemical generator. It is constantly making
a comparison between the Oxygen inside the exhaust manifold and air
outside the engine. If this comparison shows little or no
Oxygen in the exhaust manifold, a voltage is generated. The
output of the sensor is usually between 0 and 1.1 volts. All
spark combustion engines need the proper air fuel ratio to
operate correctly. For gasoline this is 14.7 parts of air to one
part of fuel. When the engine has more fuel than needed, all
available Oxygen is consumed in the cylinder and gasses leaving
through the exhaust contain almost no Oxygen. This sends out a
voltage greater than 0.45 volts. If the engine is running lean,
all fuel is burned, and the extra Oxygen leaves the cylinder and
flows into the exhaust. In this case, the sensor voltage goes
lower than 0.45 volts. Usually the output range seen seen is
0.2 to 0.7 volts.

The sensor does not begin to generate it's full output until it
reaches about 600 degrees F. Prior to this time the sensor is
not conductive. It is as if the circuit between the sensor and
computer is not complete. The mid point is about 0.45 volts.
This is neither rich nor lean. A fully warm O2 sensor *will not
spend any time at 0.45 volts*. In many cars, the computer sends
out a bias voltage of 0.45 through the O2 sensor wire. If the
sensor is not warm, or if the circuit is not complete, the computer
picks up a steady 0.45 volts. Since the computer knows this is
an "illegal" value, it judges the sensor to not be ready. It
remains in open loop operation, and uses all sensors except the
O2 to determine fuel delivery. Any time an engine is operated
in open loop, it runs somewhat rich and makes more exhaust
emissions. This translates into lost power, poor fuel economy
and air pollution.

The O2 sensor is constantly in a state of transition between high
and low voltage. Manfucturers call this crossing of the 0.45
volt mark O2 cross counts. The higher the number of O2 cross
counts, the better the sensor and other parts of the computer
control system are working. It is important to remember that the
O2 sensor is comparing the amount of Oxygen inside and outside
the engine. If the outside of the sensor should become blocked,
or coated with oil, sound insulation, undercoating or antifreeze,
(among other things), this comparison is not possible.

 

Aquanaut20 maybe you can help me. I have a 01 sport trac and I am getting codes P0171 and P0174 in my ODB II reader. I was told to replace the O2 sensors (which I did) and I still getting the same codes. I was told to check for vaccum leaks, but I have not found one yet. What else should I check on.??

Thanks

 

P0171 and P0174 both mean "system too lean". Together they almost certainly mean a EITHER dirty MAF sensor or a vacuum leak in the intake (because it affects BOTH sides of the engine, the only way it can do that is of the fault is upstream of the engine itself).
It could be intake manifold gaskets or part of the many vacuum pipes around the intake rotted out or split.
I've explained here before how to do a vacuum leak check with carb or throttle body cleaner. If you do a search using my username you should find the method easily.
The instructions for a MAF sensor clean-up are there too someplace if you search, sorry I don't have references handy.
The GOOD news is that NONE of it is difficult, even a manifold stripdown, as long as you take your time and work methodically.

Oh, and don't take ANYBODY's word on replacing sensors just on a code reading unless they have tested the sensors on a bench, especially the guy who told you to replace both of those ones! I'd love to know how these guys go from their look-up chart which says "system too lean" to their diagnosis to you which was "buy 2 new sensors"? Anyway that's my personal rant about teaching staff to read codes but not teaching them to diagnose the fault!

Good luck, let us know how you get on.
MDTodd, this answers your question too. Your sensor is PROBABLY fine (only check it as a last resort after the vac leak checks) and if your fuel rail pressure is good, why do you need to check the pump? It HAS to be good, pressure-wise at least.

 

Thanks. The one who told me about the sensor was the same guys who sold me the ODB II reader (the guys at the auto part store).

 

I guessed.
The know how to sell.
They can sell sensors and scanners but they gave NO CLUE about diagnostics. It's just a damn shame that the store is too shallow to stand or fall by the quality of it's information and give you your money back on the sensors, especially if you search the forums here and you'll discover that, while new sensors may be needed every onece in a while, I'd guess 90% of the time the fault lies someplace else, so that's 9x more sensors they sell than they needed to. Not too shabby for a salesman on a commission.
Learn to diagnose your problems with the scan tool; that's where the real skill lies and some of the guys on this forum are awesome for that kind of info.
Anyway, you'll enjoy your troubleshooting and the snese of achievement in doing what is really a simple job that shops will charge a fortune for.
I'm glad i did mine last year, just last Monday a guy commented how nice it sounded while backing into a tight spot. THAT alone made it worth the effort for me!

 

Thanks all for the info!