I have heard that sometimes the Front O2 Sensors can get "lazy" and that can affect gas mileage, etc. Is that also true of the Rear O2 Sensors - can they also affect gas mileage?

I have about 52,000 miles on mine.

 

Rear's are there to monitor the CAT's and have no input that would affect mileage that I am aware of.

 

Yes the rears only report the condition of the cats
However--they also report the "heat of the cats as well---When they overheat-- the ECM dumps extra raw fuel into the intake to cool them down---And YES it can cost you fuel mileage !!! Most tuner simply disable or turn OFF the cat overtemp system (COT)
A lazy Front 02 of course may cost you some fuel mileage as your fuel trims won't be able to correct the fuel mixture acurately or fast enough--You can check for a lazy 02 with any basic scanner that shows "real time data" The front 02 voltages should be oscilating very rapidly between .100- .950 millivolts

 

Unless your front sensors were ever contaminated by unapproved combustion cleaners, they should be good for 100k miles or more.

 

Other than the length of the leads and the connector style, what exactly is the difference between the front and rear sensors?
Fronts are AFS75 and the rears are AFS139. I couldn't find any definitive technical data on either.

Ron

 

Nothing else.

 

I was under the impression, right or wrong, that the fronts and rears also have different heaters (rear are heated more) and the rears also switch at a different rate than the fronts.

 

That is why I asked. I have "heard" the same, but w/o getting at the actual technical data on the two, all it is is heresay.

Ron

 

I don't see how the switching rates of the sensors, from front to rear, can be any different. All O2 sensors are designed to only generate a millivolt output, that is proportional to the amount of oxygen in the exhaust stream.

Any perceived switching rate differences, from front to rear, are likely due to the sampling rates of the ECM.

The fronts are likely to be sampled at a faster rate, by the ECM, because they are responsible for the actual control of the air/fuel mixture. The rears only measure the catalytic converter efficiency.

 

100% correct!!! The PCM samples the output of the O2 sensor. It adjusts the duty cycle of the injectors to ADD or REMOVE fuel to maintain a 14.7/1 AFR. It’s the PCM changing the output of the injectors that causes the oscillation of the O2 sensor output.

Something that I didn’t know until recently is the PCM supplies the signal wire with 450 mv. The oxygen content or lack of oxygen content of the exhaust will either cause the output to increase or decrease. That’s why a damaged or worn out O2 sensor shows a constant 450 mv output.


I have an ACTRON scanner that reads the O2 sensor data. When you stomp on the accelerator the O2 sensor output will peg high (enriched mixture and when you decelerate it pegs low (lean) It can stay high or low for quite a while until your no longer in a Power Enrichment or Decel condition.

Both of those conditions DO NOT rely on O2 sensor data as the PCM switched to a PE or Decel "TABLE" to control fuel.

From all the info that I have seen, the major difference between the Front and REAR O2 sensors is the rear sensors have a more robust heater element to keep it operating at the correct temp as it is far removed from the heat of the combustion chamber.

Kick me if I stated anything stupid!

BC

 

I certainly don't think you stated anything stupid, Bill, but O2 sensors certainly can become 'lazy.' If the mixture swtiches from lean to rich, and the O2 content in the exhaust changes, BUT the O2 sensor doesn't REPORT that change for a certain period of time....you get the P1133 and P1153 codes, insufficient switching O2 bank 1 sensor 1, and bank 2 sensor 1 (I just went thru this).

 

CORRECT. Thats why I stated :

" That’s why a damaged or worn out O2 sensor shows a constant 450 mv output. "



Heres the HO2S schematic and you can see the HIGH and LOW reff from the PCM:

 

Well, I have both available. I might power them on a bench supply and measure the heater current. With a constant voltage, if the rear draws more current, then it will get hotter.

 

EXCELLENT!! More substantiated data!

 

I will report back tomorrow.

Ron

 

Measuring the heater current on both will solve the remaining mystery.

Good idea Ron!

 

Drumroll please...........Cymbol crash!

Ok, so I measured my original rear sensors and one front sensor. I seem to have mispalced the other.

I set the supply at 14v and measured the current draw.
The rears draw 900mA (0.9 Amps)after fully warmed up.
The front draws 950mA (0.95 amps) after fully warmed up.
Essentially, no difference between them.

Of course the current is higher when cold as the heater resistance goes up with temp.

I also measured a brand new front sensor that I will use in my build.
Very interesting. It measures 600ma (0.6 Amps) fully warmed up. It also gets hotter.

Also, note the difference in construction from old to new. The new one just might fis better in tight areas like with headers. also note the change in the tip.
My originals are pretty much plugged up!



Hope this helps clear up some mystery.

Ron

 

Hi Ron!

Where have you bought the new front sensor?

//C5Swede

 

the new one looks like an NGK brand. I tried running those and had to remove them and go back to my originals. Maybe I got some bad ones but I couldn't get them to work right and I think it was from not getting hot enough with headers.

BTW, with software you can change the switch points of the O2's to make it run richer or leaner. I had to adjust my switch points because of the ethanol in the fuel I have here.

 

GM part number 19178930.
AC Delco part number AFS75
Right out of the box.
There are no identifying marks on the sensor itself.

So this is what you get from GM now.

Have a new one in the box for my 06 Cobalt SS.
Made by NTK, has the gM part number on it.

Clearly from my measurements, the new part is less wattage. I don't know about the actual temp. That deals with the element, can't measure it from the outside.

Ron