The oxygen sensor on the 1999 Oldsmobile Cutlass Sierra is located on the lower exhaust. If you need to replace it, they are available for about $20 online.
In the fuel tank
Replacing a thermostat on a 1999 Oldsmobile Cutlass is very easy to do. First you will find the thermostat by following the upper radiator hose. It is under the exhaust crossover. The thermostat has two 13 mm bolts holding it in place. Take the bolts off to remove the thermostat and replace with a new one.
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dextron / mercon type 2
The 1999 Oldsmobile Cutlass OBD 2 port is under driver side dash left of steering column
Remove the wiper arm from your 1999 Oldsmobile Cutlass wiper motor. Remove the wiper motor wiring harness. Remove the wiper motor retaining bolts. Reverse the process to install the new wiper motor.
I'd call a locksmith. They will be able to tell you the exact problem and how to fix it.