This is a task that daunted me for years - yes years. I'm normally loathe to play around with expensive things that when broken leave me stranded - so I shied off until I couldn't wait any longer.

Disclaimer: Use proper safety measures when working on the truck. Support the vehicle properly, set the brake, block the wheels. Don't be an idiot, I've almost had my knee amputated by the rotor when the jack slipped -don't do the same!

At first glance the O2 sensor should be simple to replace - crawl under the truck, put a wrench on it and replace.... but it doesn't work that way. There are too many things in the way - the exhaust manifold, the catalytic converter, frame, transmission among other things. Can't get to it from the engine - unless your arms are 4 feet long and have an extra joint in them - you can't do it.

So - the solution is to go in through the wheel well. Unlike going in from under the truck, at first glance this is a tough one - but don't be fooled, it's simple. You just have to not be afraid to do a little body work.

First task is to remove the mud guard


It's held on with plastic rivets, just pop them off with a pry bar or similar tool. Don't get too attached to them - more than likely you'll have to replace them when it's over. Just make sure you support the wheel well and the guard so neither one tears. Those rivets hold it on tight!

At this stage a slight person could do it - but I'm not slight, so I next remove the bolt securing the brake line

Close up of the bolt


This lets you move it out of the way and keeps you from knicking it accidentially.

Now, because I'm not a slight man, I next remove the wheel well.
"WHAT??? You do WHAT???"
Trust me it isn't a tough job, it is in fact far simpler than you'd imagine - although Ford must have been out of their minds when they specified the screws holding it on - super oddball sizes and WAY too long. Take note: There is a vacuum resevoir attached to the wheel well liner - it doesn't have to come out - but you might do yourself a favor by unbolting it first. The mud flap needs to come off as well. Also, don't forget the plastic rivet holding the well on under the passenger door.


Ahhhh Nice and exposed - easy to get to, easy to unplug the wire, easy to (well not easy to get a wrench on, but doable). And because I still haven't seen it posted anywhere - the O2 sensor takes a 22 mm (7/8") wrench. You can use an O2 socket, and I recommend it, but I sure couldn't find one - and a decent box wrench costs 15 bucks max.

Getting the sucker off....
Well, it sucks, but it isn't on there super tight. Start the engine, let it run, let it cool down to a warm temp so you don't brand your arm unplugging the O2 sensor wire. Put your wrench/socket on the O2 sensor and give it a <whack> with a rubber or plastic mallet. Make sure you're going the right way. When it breaks free it will come out real smoothly and without too much trouble. I removed mine by hand.


So the tip is light grey - don't know what the means, doesn't matter since the sensor is bad anyway.

After its out follow the instructions than came with your new one, in fact read them before starting this entire thing! You did read the instructions first right????

 

Now's the time to replace the spark plugs and spark plug wires - with the well off they're right there!

 

I was going to post a thread on this very same thing for my 2000 3.0. I didn't have to move the brake line or remove the fender well. I did go to the hardware store and buy a MAPP gas/ oxygen torch set for $45. My son had a o2 socket that I could borrow. I removed the mud guard as shown, unplugged the o2 sensor, slipped my "blue wrench" in there and heated the pipe until it was red. When it was nice and toasty, I put the o2 socket on the sensor, then put a 1" combo wrench on top of it for the proper height and gave it a tug. It came out easily.
For the drivers side, I removed the shift cable bracket and moved it to the left out of the way(2 bolts for the automatic), unplugged the o2 sensor, heated the pipe with the torch, put a 7/8 combo on it and it easily turned out. Be very careful as the body panels meet in that small area and they very sharp.
For the 3rd o2 sensor, which is behind the cat, a little heat, a 7/8 combo and out it comes. The hardest part on that one was unplugging it! My truck has 170K on it so it was past due for all o2 sensors to be changed.