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Old 07-17-2015
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O2 sensors..

Hey guys -
Been gone from the forums for a looong while. (Truck's now up to 125k, so I've NOT stopped driving..!)

I figure I gotta replace my O2 sensors with the CEL light, but wanted to get a second opinion - because I'm often wrong (according to my wife.)

Getting a P0171 error. Engine's running perfectly, though MPGs have dropped.

So, two questions -
-- With 125k miles, should I replace them all? Or just the one that's throwing the error?
-- How many are IN the friggin' truck? Doing a google search tells me between 3 and 4.

Thanks! :D
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Old 07-17-2015
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1995 and up 4cylinder Rangers will have two O2 sensors.
Sensor #1 is the one closest to the exhaust manifold, it is also called the "Upstream sensor"

Just FYI, on a V6 or V8 there are two sensor 1's, Bank 1, sensor 1 is on the passenger side, Bank 2, sensor 1 is on the drivers side

Sensor #2 is located behind the Cat Converter, it is there to test if the Cat converter so working, cleaning up the exhaust.

A V6 or V8 engine would have three O2 sensors, two #1's and one #2(full dual exhaust with 2 cats would have four O2 sensors)
4 Cyl would only have two O2 sensors

P0171 is usually not the code for a bad O2 sensor, here is how it works:

The MAF sensor is used to get the amount of air coming into the engine, lets say there is 14 ounces of air, everything is by weight not volume.
Computer gets this data and calculates the correct amount of fuel it should add, air:fuel ratio for gasoline is 14:1.
So with 14 ounces of air the computer calculates it will need to add 1 ounce of fuel, and does so, this is called 0 Fuel Trim.

O2 sensor sees the Oxygen in the exhaust, not fuel, high oxygen means Lean and low oxygen means Rich.
Computer watches Upstream O2 sensor(sensor #1), if it shows too much oxygen(Lean) computer add a little more fuel, this is +1 Trim, if still Lean then +2 and then +3, ect....
If O2 shows too little oxygen computer reduces fuel, this is called -1 Trim, then -2, -3, ect....
Engine in good tune will cycle between -5 to +5 trim while driving, +3 to +7 at idle, idle is richer on purpose.

So 0 trim is set by MAF and computer calculation based on MAF data
+ and - Trim is set by O2 sensor

When Fuel Trim gets to about +20 computer will set code P0171(Lean) and turn on the CEL to let the driver know that something is wrong, engine will still be running 14:1 air:fuel ratio, so it should be running fine, the code and the CEL are a "heads up, something isn't right".

Could be the O2, but could be the MAF as well, could be a vacuum leak so engine is getting more air than reported by the MAF because air is bypassing MAF.

Could even be low fuel pressure, computer can't monitor fuel pressure, so expect 40psi at the injectors, it calculates the amount of fuel based on that pressure and how long it opens each injector, an injector with only 20psi wouldn't let as much fuel in as an injector with 40psi.
All computer knows is the it thinks it is adding more fuel than it needs to based on 14:1 calculation.

First thing to do is free or close to it, Clean the MAF sensor.

After engine is warmed up unplug the IAC Valve's wires, engine idle should drop down to 500rpms or engine may even stall, either is fine it means there is no vacuum leak.
If RPMs stay up at 800 or higher you have a vacuum leak.


At 120k miles Upstream O2 could be changed just on spec, they say 100k-150k is life expectancy, but they can work fine to 300+k.
Downstream O2's tend to last the life of the vehicle, unless there was an engine problem, it was burning oil or running very rich for awhile.
The Downstream O2 "sees" much cleaner exhaust being after the Cat.

The lower MPGs could point to Upstream O2 sensor, O2 sensors monitor oxygen by the millisecond so switch numbers very fast, computer monitors the numbers of course, but also the speed that they change, when O2 sensor's starts to fail it will slow down, and there is another code number for that.

Last edited by RonD; 07-17-2015 at 11:24 PM.
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Old 07-18-2015
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Originally Posted by RonD View Post
1995 and up 4cylinder Rangers will have two O2 sensors.
Sensor #1 is the one closest to the exhaust manifold, it is also called the "Upstream sensor"

...snip...
Thanks for this. I remember reading some of this in other threads that came up while searching.

So despite having 125k miles on both O2 sensors, it's likely still the MAF (ore leak, etc)? Interesting!

I was stuck running a train all day, so I wasn't able to do anything today. Tomorrow, however, I'm off, and will - per your recommendations - clean the MAF and then run the IAC test. (I do NOT think it's a leak, personally, since I have had experience with leaks on another vehicle, and I could head a difference in the engine noise and feel.. in this Ranger's case, the engine is running perfectly normal..) But it certainly can't hurt running the test!

Fuel filter was changed about 40k miles ago - it's getting near time for a new one, as well. But I also don't believe this is the cause, as if it were, my MPG wouldn't have dropped either.

Curious, is there an ODB code/PID that displays the 14:1 trim steps? (I'd be curios to watch it change..)

Thanks!
-Mike
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Old 07-18-2015
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Well, thanks again. That was a quick fix..!

I bow to your knowledge - I was wrong, it was an air leak -

I must have popped the crank-case air feed (just above the oil fill, don't know the proper term?) that goes back to the intake pipe off slightly when I changed my oil the last time. DESPITE having looked in the engine more than once, I missed the darker oil staining around the pipe's snap-mount. When I went to remove the air filter cover, I heard a wheezing (as the intake pipe was squeezed and bent, air was wheezing out of the loose connection).

I went ahead and cleaned my MAF anyways, and snapped this pipe back on completely.

Reset CEL errors, and will drive it the next few days to/from work, and follow up here if I need to re-visit the issue!

So, my two main questions still stand -
After 124k miles, should O2 sensors be changed? I have one, can easily get a second.
Is there an OBD2 pid for the trim level? :D

--Mike
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Old 07-18-2015
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Good eye

It won't hurt to change upstream O2 sensor, it is at the upper limit for recommended change time, it will improve MPG assuming old one was biased towards Lean, which is what they do as the chemicals inside get used up.
Downstream should be fine, but really up to you.

Yes, look for STFT(short term fuel trim) Bank 1, sensor 1
With engine at idle you should see very fast numbers changing from +3 to +7
While driving it should go into - trim and back to +, also shifting numbers very fast.
But running in the + all the time isn't bad, as long as it stays under +15
This is while cruising, when accelerating or decelerating trims jump all over the place.

LTFT(long term fuel trim) won't change much, and downstream O2 is part of this one, small part, LTFT tells the computer how to run the engine when it is cold and the O2 sensors can't work.
O2 sensor needs to be above 650degF to work, so it takes a few minutes before computer can get feedback on Lean/Rich, so LTFT is used to bias the fuel calculation for condition of the engine.
Older engine may have small vacuum leaks or lower fuel pressure, nothing bad just older used parts, so LTFT might be +5, computer runs cold engine rich anyway but adds +5 to be sure.

What do the "+" and "-" numbers really mean?
They represent the time an injector is open, this is called the Pulse Width, - means shorter Pulse Width, + is longer Pulse Width

There may also be an O2 sensor voltage readout, .1 to .9volts
.1v is lean
.9v is rich

.4v to .6v is where it should "hover", switching very fast


Just an FYI:
One of the benefits of fuel injection is that the fuel can be shut off when coasting.
So never shift into neutral when going down hill, you did this in a carb engine because the high vacuum when using the engine as a brake caused more fuel to be sucked in.
With fuel injection the computer will shut off the injectors when your foot is off the gas pedal and RPMs are above 1,400, so as you go down a hill with foot off the gas and high rpms you are using 0 gas.
If you shift into neutral then computer turns on injectors to idle level.
It ain't much but...........it ain't nothing, lol.

Last edited by RonD; 07-19-2015 at 12:37 AM.
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Old 07-19-2015
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Quote:
Originally Posted by RonD View Post
Good eye

It won't hurt to change upstream O2 sensor, it is at the upper limit for recommended change time, it will improve MPG assuming old one was biased towards Lean, which is what they do as the chemicals inside get used up.
Downstream should be fine, but really up to you.
Very cool, thanks. I'll heck the readings today as I go out for a short drive (relatively speaking), and watch the fuel trim and O2 voltage. I may change it out today, I may wait, I'll see.

Quote:
Originally Posted by RonD View Post
Just an FYI:
One of the benefits of fuel injection is that the fuel can be shut off when coasting.
So never shift into neutral when going down hill, you did this in a carb engine because the high vacuum when using the engine as a brake caused more fuel to be sucked in.
With fuel injection the computer will shut off the injectors when your foot is off the gas pedal and RPMs are above 1,400, so as you go down a hill with foot off the gas and high rpms you are using 0 gas.
If you shift into neutral then computer turns on injectors to idle level.
It ain't much but...........it ain't nothing, lol.
I wasn't aware the Ranger did this! I have a '92 Mini Cooper, one of the first models to have a small little ECU controlling a single fuel injector spraying fuel into the intake manifold. One of the 'features' this little ECU had was turning off gas completely when the throttle was closed (via a switch on the pedal!) and engine RPM was above 1500. The pedal switch caused huge issues if, say, it was flaky, and you're cruisin around and the ECU loses the 'input' from the switch - thinking you let off on the pedal. You can imagine what happens. :D
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Old 07-19-2015
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Most fuel injection, if not all, have that.
Most also have "Clear Flooded Engine Mode" as well, which can cause a no start.

TPS(throttle position sensor) is used now instead a switch on the gas pedal.
Computer sends TPS 5 volts with key on, TPS is a variable resistor like a light dimmer or volume control.
TPS sends computer under 1volt(.6-.9) when throttle is closed, and above 4.5volts when throttle is open all the way, and, of course, 1v to 4.5v for other throttle positions.
If computer sees under 1v from TPS and RPMs are above 1,400 then injectors are shut off.

If Computer gets above 4.5v from TPS at 0 RPMs then it enters "Clear Flooded Engine Mode", it leaves spark on but shuts fuel injectors off.
It assumes you have pushed gas pedal all the way to the floor and are holding it there before cranking flooded engine to clear it.
As soon as you let up the gas pedal injectors will restart, try it some time.

If TPS should fail or 5volt and return wire should short together, injectors are shut off, causing a no start.

Most people are unaware of this and it is in almost all fuel injection computers, when buying a used car, and you are starting it for the first time, press gas pedal to the floor and hold it down, casually, lol.
Crank it a few times with a puzzled look on your face at the no start, hmmmmm
Then let it start.
Knowledge gives you an edge in life, knowing something the other guy doesn't is often worth something.
And I am sure the seller is probably not telling you everything, not dishonest(we hope), just economical in their description of the vehicles past.

Last edited by RonD; 07-19-2015 at 11:12 AM.
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Old 07-19-2015
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Well, I'm bummed.

Surprisngly, that did not fix it! (Or, my light cleaning of the MAF sensor did it in..)

The two readings mentioned above were mostly in spec mentioned above -
STFT jumped all around. When I STARTED my trip, it would sometimes get as high as 17, but this only happened once in a while. The loweest it ever went was -11.

Do not know O2 voltage readings on the first part of the journey.

Half-way, the CEL light came back on - same error.

I added the O2 voltage sensor, and headed back home a different route.

On the way back, STFT readings never went above +11, and never went below -11. It mostly stayed within -3 and +7 during normal driving. (Both highway and back country roads.)

The O2 sensor never went outside .1 an .8v. This reading was not jittery or random, but every 1-2 seconds it would drop to a low value (.1 - .4) and then raise up to .6 - .8v, before dropping back down. This was repeated the entire return of my trip.

When I got home, no CEL, BUT I pulled codes, and I have a P0171 pending again.
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Old 07-19-2015
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After I wrote it I saw 2010 engine and thought you may have wide band O2 sensors, they have voltage range of 1.5v-3.0v

Looks like you do have narrow band, which is fine.

+20 is just a number I used, the computer is programmed to react to a specific fuel trim number and the time the computer has to maintain that number to keep O2 voltage near .45v, so on your year computer could react to anything above +15.

Computer is always changing Pulse width of injectors even when you are cruising along on flat ground.
-3 to +7 is normal, 10 point spread is expected, i.e. -5 to +5 would be new car, hopefully, lol.
So yours is fine.

The drop and rise of O2 is odd but 2010 computer may do that or if it was still the old O2 sensor new one may change that.
Staying between .3 to .6 would be expected, computer is always changing fuel mix so fluctuations are not that unusual, but the drop is.
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Old 07-20-2015
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Originally Posted by RonD View Post
The drop and rise of O2 is odd but 2010 computer may do that or if it was still the old O2 sensor new one may change that.
Yeah, I forgot to mention, I have NOT replaced the O2 yet, so this is the old sensor. This week I'm stuck in the office all day, but this coming weekend I'll slip in the new sensor.

Thanks again!
-Mike
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Old 07-23-2015
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Well, color me confused.

By this morning, I expected the CEL light to illuminate from this weekend's driving. So when I got to the train station this morning, I plugged in my reader.

No more pending code. All tests report as passed.

I hate it when I can't pinpoint the exact fix, or when I fix something and the result isn't clear. So, no idea why after re-connecting the air hose and cleaning the MAF sensor it threw the error again, but.. all better.

I'll know on gas mileage this weekend, as well.

Thanks guys. I love this stuff. :D
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Old 07-23-2015
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Thanks for the update.
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