DO you have a dmm and know how to use ???
If yes.. you can easily test the O2 sensor .

The car is OBD1 and EFI , so this is a quick test...

Under the passenger side rug is the connector for the O2 sens..
The green wire is a seperate plug and is the sensor wire [ the others are heater]
Unplug the connector and hook up the meter [volts/dc..] to the wire on the sensor side..
Under the hood is the fuel pressure regulator.
Start the car , get it warm, and then disconnect the vac line at the pressure regulator. This should instantly make the engine run rich and the sensor v. will go up to around .8-.9v.
[ keep you finger on the vac line end so you do not create a vac. leak when doing this test or you will be creating a lean condition]

Now , put the line back on and this time disconnect a vac line
[ on this engine , an easy on is the line that goes to the SWOs in front of the engine]
This will make the engine go lean and the meter should drop to low volts [ .2-.3v] .. as you put your thumb on/off this line , you should see the meter respond fairly quickly to these changes..

This is just a primary, easy O2 sensor test.. a scope is best, but for diyer, it works.
All you are doing is manually creating rich/lean conditions to the engine and seeing [ with the meter] if the sensor is responding to your engine mixture changes...
As you have disconnected the sensor from the ECU, it does not
have and closed loop feedback , so the ecu can not correct your manual mix....
You are the ECU...
It is also advisable to see if any gas comes out from the reg vac line when doing this test, as that is also a possible to your condition.[ bad/leaking diaphram causing high fuel pressure and raw gas going back to engine through vac line.- Common problem on EFI 104s]

Check both....for starters