1994 APV 3.1: Recommended Coolant & O2 Sensor Missing?

[jaysonsperling]

Vehicle: 1994 Lumina APV, 3.1, 102k miles (see end of post for work done since under my wing)

Greetings, all!

Two issues I'm working with on this van:
1. (Coolant) A few weeks back, the drain plug decided to die and pop out (it snapped apparently), draining all of my coolant as I was parking. Fixed the plug (put a new one in) and refilled with Dexcool 50/50 pre-mixed. Recently, I put in another type of coolant, and now I really want to flush and refill it. I try to stay away from mixing when I can. What's the recommended/best coolant I could put into this van given the milage? Changed the oil last night, and I did not see evidence of coolant in the oil, so I'm praying I'm safe from having blown the head gasket or such. But knowing my luck...

2. (O2 Sensor) Per the Haynes manual that I bought when I got this van, it says I should have 2 O2 sensors: 1 on the exhaust manifold (changed that one), and one downstream from the cat. I can't find this one. Physically ran my hand over the entire length of pipe/muffler after the cat, and not a single hole or mount for a sensor. Thinking this is an aftermarket exhaust pipe. Apparently has passed California Smog before, but not this time (high HC during 25MPH test this past week). Any ideas on this one? Or should I pull the exhaust pipe, and cut and weld a bolt/hole into place to put the second O2 sensor (that opens up a whole 'nother can of worms...)?

Any information would be greatly appreciated, and I've already learned a great deal of information from these forums. You guys rock!

Work Done:
-Yesterday
Changed engine oil and filter (added some Lucas Oil Stabalizer)
Changed the one O2 sensor I could find
Changed the fuel filter
-Previously
Flushed and refilled the radiator/cooling system
Replaced the thermostat
Replaced the plastic radiator drain plug
Replaced the spark plugs and cables (even braided the cables!)

[richtazz]

Your van only has one oxygen sensor. They didn't go to two sensors until OBD-II emissions in late 95-on.

[jaysonsperling]

Originally Posted by richtazz

Your van only has one oxygen sensor. They didn't go to two sensors until OBD-II emissions in late 95-on.

Ah, it all makes sense now! Thanks for clearing that one up for me.