[Tink]

The sensor at the t-stat housing goes to the computer. The one at the head goes to the gauge.

Have you ever replaced the fuel pressure regulator or lines in the intake on this Blazer? Common systems of these failing are bad idle, hard to start, shaky revs, low gas mileage... certainly could have fouled your O2 sensor.

If you remove the upper intake manifold (10 bolts, 10mm deep
socket), a leaking fuel system will have washing on either side where you can
see the 'gold'. This is where the fuel has leaked into the intake and cleaned
the inside. Your able to determine which component is leaking by which side
has the washing. If the drivers side of the intake is washed the fuel lines are
leaking. If the passenger side has washing the fuel pressure regulator itself is leaking. Its not common, but it is possible for both items to fail and leak.