So, I was getting codes for the front sensor and figured it would be best to replace them in pairs. I got the upstream (ie. front) bosch sensors off of amazon and installed both earlier today. For what it's worth, these were advertised as "used-like new condition" and they arrived in original packaging, looked brand new to me, proper amounts of grease (dielectric?) were on the threading and they had little protective caps. They cost $36 a pop. I never would have gone this route except the ones I was going to order anyway shot up to $90 dollars each. I know what you're thinking, "Shoulda just ordered from AB/picked up new ones for full price, don't get parts on Amazon." Writing, this out now... I'm aware I made some mistakes there, so lets not get too stuck on that point unless you believe it to be a potential cause of the problem. For all we know, these used ones are functionally new, so I'd prefer to exhaust other possible problems before I buy a new set of sensors, only to have the same result.

After installing both sensors, I went to start the car. I hear the engine start cranking before cutting out followed by the horn howling, blinking security lights in the dash, hazards... the whole 9 yards.

Oddly, I had a shop do a transfer case swap for me years ago while I was working abroad (car left untouched for a few months). When I came to pick it up, the keyless unlock/lock function had stopped working but I didn't make a big fuss of it, and just went on with life manually. Well... After the failed start today, I was surprised to find that the keyless lock/unlock now works, but the car still won't start. Oh, Gods of Solihull: Why must thow taketh before thow giveth? Makes me suspect somethings up with the ECU

So my research points me to performing a reset on the ECU (tried that, albeit probably not correctly). I would greatly appreciate any help you wonderful people could offer.

 

Update: hooked it up to car battery starter.... and..well, it worked.

I know, feel free to call me names. At least I got my keyless entry back which feels like the pinnacle of luxury right now tbh.

 

It's anti-seize compound.

 

Makes sense. It's hard to get a proper tool on them when you need to remove them, so quite necessary. Managed it with channel locks but it was still tricky working around in there.

Now, my current battery is at least 5 years old and there is plenty of corrosion on both terminals. So a new battery is definitely in order. My guess is that with my limited travel these days, it's reasonable to think voltage was low.

The confusing thing is that I had driven the truck the day before all of this and it was reading around 13.7V on my monitor. I've had the battery die on me before and all that happens is it fails to crank. No anti-theft horn-blaring extravaganza. I wonder why the cars computer sent it into full on anti-theft mode after swapping the oxygen sensors....