I got an 850 off Craigslist for $600. So far I put new rotors on all four wheels, and new pads on the front.

Also fixed:
airbox thermostat
new air filter
headlight switch

Still needs fixing:
e-brake shoes (I have the parts, just need to slap 'em in)
tailgate struts ($60 each! That can wait a bit)
Odomoeter (got the gears on order. I'll replace all the lamps while I have it out)

I am specifically asking for help form the experts here on these issues:

Tailgate latch:
- The metal housing is cracked and the latch won't engage. There are NONE available at any junkyards near me. What do I do? Braze the broken one back together and hope I don't melt the plastic parts? Where can I get one of these?

Steering u-joints:
- The classic 850 steering binding issue. I am 90% sure it's the lower joint, the one in the engine compartment. My local volvo dude said 'dismantle the steering column, blast it with WD-40 and spin it using a drill for an hour'. Is there any other way?

Using the blinky-lights ODB-I code, the only error I get are 521 and 212.

521 is 'Front O2 Sensor Heating Faulty'

212 is 'Front O2 sensor missing or faulty.

Now on this 850, there seem to be two exhaust sensors, one before and one after the cat converter.

Do I have it right that both of these codes are about the same sensor, the one upstream of the cat converter?

Upon close inspection I can see a hairline crack around the base of the sensor, so I can understand if it's not working right. BUT, there's also an exhaust leak after the cat converter, where that joint is in the exhaust system.

Would I get these two codes just from the leaking exhaust? Once I wiggled the connectors and the sensor and the codes cleared up, but they came back...