Because your car is Electronic FI, there is an easy O2 test that a DIY can do W/a DVM..

Unplug the sens and hook up meter to sig wire from sens. Warm engine and let run...
Watching meter, unplug the vac line to the fuel regulator. [I hold my finger over the line to stop any vac leak while I do this part]
This should immediately [ less than 300 m/sec] show a rich reading of a min of .9 volts. You can put the vac line off/on several times and watch the O2 sens output change...
This vac line to the reg is what keeps the fuel pressure in check...by disconnecting it, the fuel pressure increases causing the rich condition...
Now for the lean [ low voltage ] O2 test...
Same deal, but this time you disconnect a vac line [ on the 104,
the line to the SOVs is a good one] , Now, the O2 should drop to less than .2 volts in less than 300 m/s. Do this several times and you will see the sens speed/reading..
A scope is much better as you can chart the process and get exact cross counts/speeds/volts, but a DMV and a little experience
and one can tell a bad sensor without too much trouble..
In other words , you are manually Causing Rich/Lean conditions
and watching the o2 sensors response to those conditions.
Because you have unplugged the sensor from the system, the ECU has no control over your manual conditions, so this is a direct O2 sensor test ... EFI cars make this test possible as they are designed to run at certain fuel pressures..., whereas on others you have to ad fuel/propane to do the enrichment part
of the test.... a little more work...but same idea..
I personally never change an O2 sensor until I test that
sensor/ circuit and I never change one just because a CE lamp comes on....