I finally had time to climb under the truck and get my eyes on these sensors. Holy shit guys, are you serious??? This is going to be a nightmare. Unless these little shits are somehow not stuck AT ALL, which seems very unlikely since they are probably original.
Good luck. I replaced what I think were the original O2 sensors on my new-to-me '97 K2500 plow truck. I had to remove one exhaust pipe to get the sensor out. All four of them needed to have the female threads reworked with the Lisle rethreading tool.
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Half the reason to remove O2 sensors on a semi-regular basis is that--completely aside from the fact that they get slow "lazy" as they age--they get seized in the female threads if left too long. Even if the sensor works perfectly--and it probably doesn't at that mileage--unscrewing the thing, replacing the anti-seize, and putting it back in prevents problems later.
When it's me, I tend to not leave them in more than 80K--100K miles, and once out, they're replaced with new.
I did spray them with some penetrating oil
ABSOLUTE WASTE OF TIME. The thing is sealed gas-tight using the same sort of gaskets that sealed older spark plugs--which could hold the enormous pressure of combustion. Penetrating oil cannot get into the threads to do any good.
Penetrating oil will make a lovely puddle on the ground where it drips off.
Keep in mind that when O2 sensors are not seized, an O2 sensor wrench "can" be very handy.
When they are seized, an O2 sensor wrench is as useless as penetrating oil, but even more dangerous. My limited experience with O2 sensors is that you're more likely to find seized ones than easily-removed ones; because nobody removes them on time--they're ignored until long past due. Therefore, O2 sensor wrenches are fabulous for INSTALLING sensors, and fairly worthless for taking them out. Add in the popularity of cheap-junk bottom-feeder tools from Communist China (TSOs--Tool-Shaped Objects, they're not really "tools".) and pulling O2 sensors can be an ordeal.
If you round-off the corners of the wrenching surface of the sensor because your O2 sensor wrench flexes at the split for the wire harness, you'll be in real trouble.
I bought a "special tool" for doing O2 sensors that don't want to unscrew. This is a deepwell, impact socket in 1/2 drive, and what makes it special is that most deepwell impact sockets aren't broached the full depth of the socket. This one is, so
the body of the O2 sensor fits inside once you clip the wire harness off. It's made by Wright, a family-owned tool company that makes quality product in the USA.
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I use an impact socket for strength and rigidity, not because I use an impact wrench on the thing. A long-handle ratchet works wonderfully.
Although...sometimes you get lucky. My most-recent O2 sensor debacle was on an '05 Ford Focus, owned by a friend-of-a-friend, semi-destitute. Everything I've touched on this car has been a disaster--rear brakes had failed wheel bearings in the drums, a tire had shifted belts, the car shook like it was having an orgasm at 60 mph. Doesn't idle right. I fix all that, take it for a test-drive, and the instrument cluster goes dead every time I get to 30 mph. Comes back to life at idle.
The alternator failed while I was working on the brakes, the instrument cluster would shut-down to protect itself because the alternator was throwing 17 volts when the engine revved-up. This alternator was two months old. The computer had a dozen "codes" and the O2 sensors are probably originals. I decide to replace the upsteam sensor "just because".
So I get under it, looking at the O2 sensor. I can just barely get a sensor wrench on it. And Wallah! It unscrews just like it's supposed to. It's a miracle! Praise God!
As I'm wiggling out from under the car, it occurs to me that I've just pulled the DOWNSTREAM sensor instead of the upstream like I'd intended. So back under I go, and reinstall the downstream sensor with fresh anti-seize.
The upstream sensor had to be accessed from the top side, with my Wright socket, and extensions, and a universal joint, and the longest 1/2" ratchet I own, with a cheater pipe. And harsh language. Lots and lots of harsh language.