If in doubt - do it. It's cheap, easy and great bang for the buck. I bought a 2001 3.0I running bad mpg (12ish) since the truck was an unkown I got a couple cheap Abbey brand online, $37 for 2.

Online there are many youtubes that all point you wrong. They all say get a O2 socket- its not needed. The one I got did not even fit in the space. Worthless.

I only did the 2 upstream here is the passenger side with new one on left obviously! lol
My guess is the old ones were original at 189K


driver side -



So here is the tip - both upstream O2's are accessible thru the wheel well. Online most guys say the passenger is accessible from the wheel well but then stand on their head for the drivers side -people are doing the most hairbrained antics, using mechanical arms and 3 foot screwdrivers to reach in from under the car. Don't. NEWS FLASH the driver side is even easier - ALSO thru the wheel well.
Just sit yourself a bucket beside the tire and pop them out. Mine turned like butter. I hit them a couple times with master blaster a few days before - but to me it felt like the expansion contraction in the exhaust
keeps these pretty loose. I used a 22mm box and a shorter 7/8 open end - next time I'd cut an old wrench in half bc shorter is better for this. 6" would be the thing.

Jack up the front you will need to get under there. But looking back I can't really say for what.

Just unplug the old, remove the 22mm nut, plug in the new - put a CCW pretwist in the line so that the pre-twist relaxes as it's tightened CW into socket.

Truck ran smoother after, my MPG went up to almost 15 but that was in the rain - I'm thinking it may be closer to 16 (I have low compression limiting efficiency). AND for the first time my CEL light went off which I think was the "401 exhaust gas insufficient recirculation flow" being effected - but thats just a guess. The CEL comes and goes now - but hell at 189K that's progress.

Anyway youtube alert! Take these things out from the side.

 

Never had any luck with O2 sockets either, except on rear O2s where there is usually more room, but I think I have only used it once in 10 years, lol, because just easier to use wrench

I cut the old O2's wires off and then use 22mm or 7/8" socket or wrench that works for that location

O2s last for 100k miles or 12 years which ever comes first, period
After that they are costing you money in wasted fuel, ain't much to start with but ain't 0 either
So changing them "on time" means they were FREE, actually better than free because no wasted money on fuel and no rich fouling of spark plug or clogged Cat converters

O2s use a chemical reaction to generate voltage
0.1volt is high oxygen, lean
0.9volt is low oxygen, rich
So under 1 volt

As they run low on the chemical voltage drops, so FALSE LEAN, and computer runs engine richer than it should
Lean or rich CODES are based solely on O2 sensor voltage, so computer has no way to tell if O2 voltage is false, there is no comparison "sensors"
This richer running causes carbon build up on spark plugs and hotter Cats or build up in the Cats

 

I thought to cut those wires it was tight from the side - I shoulda cut them from the top.

I guess I should go ahead and replace the rear now. I read that it's only a Cat monitor tho. Does the rear effect MPG as well?

 

Yes, rear O2s can effect MPG
Rear O2s should have a fairly stable voltage of 0.7 to 0.8 low oxygen
This is because the CATs burn up most of the oxygen when burning up pollutants

CATs use gasoline to get hot and stay hot, not much but thats what makes them hot/working
So every few seconds the computer sends a Rich fuel mix to the engine so extra unburned fuel makes it to the CATs keeping them hot
As O2 chemicals are used up voltage drops, so downstream O2 shows under 0.7v, so computer feeds more fuel to the CATs to make them hotter to burn up more oxygen, and MPG goes down a bit
But this will set a code sooner because it is an Emissions system, unlike the upstream O2s which are just operational

And changing the upstream O2s can then cause downstream O2 codes
If upstream O2 were too old so computer was running engine Richer than needed that makes the CATs run hotter so downstream O2s stay at the higher voltage
When upstream O2s are changed and engine has normal air/fuel mix again and CATS are now running at a "normal" temp, the older downstream O2s voltage may drop under 0.7, so computer needs to Richen the mix, which lowers MPG a bit, and would eventually set a downstream O2 code

 

Wow thanks for the complete picture. Darned interesting, I had no idea how a CAT works - its kinda ingenius!
I'm curious what changing the rear will do, been keeping pretty careful track of mpg- trying to restrain myself with the lost sheep
on the roads

 

Ok so now I went to replace my downstream and total disaster! used penetrating oil - tried 24 hrs later. Nope. used more oil tried next day- nope. Got under there got it plenty hot with a torch and went at again with a 22mm wrench - nope. got a chain n shackle on the round end of the wrench which stayed nice and snug on the flats - pulled steady and slow on the chain with a nice fairlead and felt it start to turn - but it was stripping.

any idea for next step on this ? I think that nut is brass which is why it stripped? its been on there 20 years, I can tell by looking. All I can thing of is channel locks or welding a cheater to it but I don't weld.

 

Independent Exhaust shop can get it out, and put in replacement, offer CASH payment, probably take them 15min, they do this daily and know ALL the tricks
Must be independent shop, chains just know how to change whole pre-made exhaust systems

 

Thanks you inspired me to give it another go. Got it. Sawzall cut the outer metal casing then worked the back off with channel locks, slipped a 22mm socket over the remaining flats with cheater bar on the back and it popped.

Put the the new one in and noticed an improvement. Will top up gas tank tomorrow and see how she does. I've got MPG up from 12.5 when I bought it to 15.3 last tank.

Could be like "oil change derangement syndrome" but she seems to run much smoother. I'll post here after I re-check MPG

 

Good work

It will take a tank or two of driving so get the computers Long Term Fuel Trims(LTFT) numbers back in line

LTFT allows computers to keep the engine running at its best as it ages, i.e. small air leaks, lower fuel pressure, lower compression, ect.................
LTFT is saved when key is turned off and then is loaded in to computer each time key is turned on again
So computer doesn't have to re-learn the whole system again
So it changes slowly as an average, if you monitor LTFT it looks like it changes fast but thats "real time" not the "saved" numbers
So with the newer O2s computer will change the LTFT to match correct feedback from all O2s, which means better running and better MPG

 

Thanks on that info. I noticed something seemed to smooth out after installing this sensor, started engine and let it idle and after about a min, it smoothed out pretty noticably. Might have been in my head - but drove awful well today seems smoother at idle. I've been keeping a MPG log at fill ups- went from 15.3 to 17.1 today - I stared at the numbers going "what the heck!" Is this new sensor backwards compatible?! I only drive 5 miles on it and I'm 17.1 already!! Then I realized I had been driving on the highway last week. ;- (