Hey everyone,

I got my truck when it had about ~50k miles on it. I currently have about 129k now. I can't for the life of me recall if the o2 sensors have ever been changed. I know its as easy as a turn of a wrench and connecting two plugs together.

I have read that mpg will suffer among other things prior to the o2 sensors failing and throwing a code up. Would it be in my best interest to go to Ford and get 4 sensors, and replace them just for "preventative maintenance"?

 

Quick answer.

No, it would not be in your best interest.

 

I am going to respectfully disagree. O2s can wear out and switch slower without causing a CEL. At almost 130k, it's time to replace them if you are experiencing low MPG or other driveability problems. The good news it you really only need to replace the front ones. The rears only monitor cat efficiency and the fronts are the ones that monitor the exhaust gas and report back to adjust pcm values like fuel trims and etc. If you decide to replace them stick with motorcraft, Tasca or rockauto are good sources for these parts.

 

Replace all 4 for the best mileage. The rears do more than monitor. Careful with the rears tho, if you take out the bung threads, that's a much larger job for repair.

 

REAR FUEL TRIM

The Post-cat O2S has been used since 1988 to control fuel trim (to
some extent)

Used to fine-tune the A/F ratio to maximize catalyst efficiency

Will also compensate for a degraded Catalyst

Every manufacture today uses the rear O2S to trim fuel today


The post cat O2S is a fuel control input. Toyota and Saab have used the
post-cat O2 to control the fuel trim since 1988; Before OBD-II. This sensor
is used to fine-tune the air-fuel ratio to maximize catalyst efficiency. It
can also adjust the air fuel ratio to compensate for a degraded Catalyst. If
you were to measure the actual amount of time the post-cat O2 is used during
the running of the catalyst monitor (maybe once per trip) and compare that
to the amount of time the post-cat O2 is used fine tune the air-fuel ratio
(almost always in closed loop), you will see that this sensors major
function is fuel control. Every vehicle manufacture today uses the rear O2S
for fuel correction; even if it is undocumented.


According to these manufactures, the post-cat oxygen sensor has minimal
control over fuel trim under normal operating conditions. GM material states
the rear O2S has +/-1.0% authority over fuel control. The Ford OBD-II
manual leads us to believe this sensor is of little consequences in regard to
drivability as it only has +/-0.5 fuel control authority. Toyota allows for
a whopping 2.0% +/- correction. However these claims are under normal
driving circumstances. What happens if there is a problem? We have
documented up to 30% correction in fuel trim from the rear O2S in abnormal
conditions.


http://www.autodiagnosticsandpublish...or-testing.htm

 

Bung threads?

Also when looking on the Motorcraft website there are 3 different o2 sensors for my 01 screw w/ the 5.4l

Oxygen Sensor
5.4L; With "Green" electrical connector

Oxygen Sensor
5.4L; With "Gray" electrical connector

or

Oxygen Sensor
5.4L; 393mm long

anyone know or should I just go to Ford and find out?

 

Threaded bung:




Unthreaded bung:

http://2.bp.blogspot.com/-FpnHAZtMNf...topper-001.jpg

 

Gotcha, so basically don't break the weld the bung is in or I'm screwed.

 

Holey bung holes batman... I had to adjust my screen. That damn thing took some real estate.

 

Sorry - fixed it (just a link now)...


MGD

 

Thanks for post #5 as well.

 

Fine. Spend $240-$300 for four sensors. Since you don't know how much improvement might be possible, let's assume you gain a 5% improvement in fuel economy. For a vehicle currently getting 15 MPG and gas at $4 per gallon, calculate the number of miles to reach the breakeven point. Repeat for several other values of improvement.

I can find absolutely nothing in Ford's literature that would support the claim of downstream sensors affecting fuel trims except during diagnostic routines. I'm skeptical that such an effect exists for the vehicle in question. See the 2001 Ford OBDII manual at http://www.motorcraftservice.com/vdi...f/obdsm107.pdf

 

^^

A couple things. First that's a 2001 publication/revision. So applicable to the OP's truck, certainly.

Looks like this is now in play, though for newer MY vehicles ...

In the 2010 revision, page 54:

"FAOSC (Rear Fuel Trim) Monitor
As the front UEGO sensor ages and gets exposed to contaminants, it can develop a rich or lean bias in its transfer
function. The rear bias control (also called FAOSC – Fore/Aft Oxygen Sensor Control) system is designed to
compensate for any of these bias shifts (offsets) using the downstream HO2S sensor. The "FAOS" monitor looks
for any bias shifts at the stoichiometric point of the front UEGO sensor lambda curve. If the UEGO has developed a
bias beyond the point for which it can be compensated for, lean (P2096, P2098) or rich (P2097, P2099) fault codes
will be set. "

Second: http://www.modularfords.com/f149/aft...2/#post1199372

https://www.f150online.com/forums/2076304-post18.html (where primary infers the presence of a 'secondary' function - confirmed with a phone call)
BTW - the rest of that thread also has some good info regarding O2's in general ...

I've heard the same thing from other tuners as well.

Odd find: From Page 34 of the 2004 MY manual:

Some 2004 Partial Zero Emission Vehicles (PZEV Focus) will utilize three sets of HO2S sensors. The front
sensors (HO2S11/HO2S21) are the primary fuel control sensors. The next sensors downstream in the exhaust are
utilized to monitor the light-off catalyst (HO2S12/HO2S22). The last sensors downstream in the exhaust
(HO2S13/Ho2S23) are utilized for very long term fuel trim in order to optimize catalyst efficiency (Fore Aft Oxygen
Sensor Control). Ford's first PZEV vehicle uses a 4-cylinder engine so only the Bank 1 DTCs are utilized.

Wow - 6 sensors,

 

$195 out the door for all four.

 

i have a 2002 f150 with a 5.4. i have changed all 4 o2 sensors, but i still get s.e.s light to come on every so offten. i have had it scanned and every time its the same. bank one sensor two heated circut. i have replaced this one twice and still the same. all 4 are ford parts. any thoughts. thnx..