What is the sensor that is behind the alternator in the engine block (above fuel filter) ?

I was just reading the "zero" mile list and sensors are not in the list.

For instance :

O2 sensor

THere are also two other sensors on the other side of the engine under the intake manifold.

Some of these sensors are cheap (sometimes under 10 dollars).

Should their replacement be part of the "zero" mile ?

How can they be checked ? Volt meter ? Codes from driver side black box ?

Thanks.
--
'89 244DL M47 159K miles