O2 sensor how do i get to the o2 sensor bank 1 sensor1 on 1999 oldsmobile aurora

One of the O2 sensors has malfunctioned (probably Bank 1 Sensor 3).

The car must be on a lift as the sensors are under the car in the exhaust pipe. You want the cylinder bank 2 sensor but be advised it is not recommended that just one be replaced, as they may send confusing signals from old and new senor to the cars computer this may turn the check engine light back on. The sensors are "upstream" (before the catalytic converters) and "downstream" (after the catalytic converters) In order to know what you need I need to know the fault code you received.

Which crank sensor ? yours has two .
Crankshaft Position (CKP) Sensor A
Center front of the engine, near the base of the engine oil level indicator
Engine Controls Component Views
Engine Controls Connector End Views
Crankshaft Position (CKP) Sensor B
Center front of the engine, near the base of the engine oil level indicator
Engine Oil Level Switch
Center front of the engine, in the engine oil pan
Front Heated Oxygen Sensor (HO2S) (Bank 2 Sensor 1)
Center rear of the engine, in the right hand exhaust manifold

O2 (Oxygen) sensors are in the exhaust system.

LOCATIONS There are different locations in the exhaust system where O2S sensors are positioned. The locations have been given numbers and will be used in the accompanying charts to identify the positions of O2S sensors in most vehicles. Due to mid-year production changes or factory inconsistencies, all models may not be covered. If a vehicle being serviced is not covered in the charts, inspect the exhaust system (while cold!) in the general locations to find the applicable O2S sensors. NOTE: If equipped with dual exhaust systems, there may be up to 4 or 5 O2S sensors in the exhaust system. Be sure to locate all of them before commencing with any testing or service. Fig. 1: Location No. 1 — down pipe or exhaust manifold
Fig. 2: Location No. 1 — typical O2S sensor located in the exhaust manifold
Fig. 3: Location No. 2 — left and right banks of a V-type engine
Fig. 4: Location No. 3 — exhaust collector (where more than one pipe joins together)
Fig. 5: Location No. 4 — outlet of the catalytic converter
Fig. 6: Location No. 5 — inlet and outlet of the catalytic converter

Check for a vacuum leak. You can do this acouple ways the easyest way is too spray some brake clean around engine and if engine picks up you have a leak in that area.Thats the most common fault.

The bank 1 sensor should be the one before the CAT on the side of the engine that includes cylinder #1. If you're not sure which bank that is, look across the heads from the side of the engine (front of the car) The bank with #1 on it will have the head a little (1"-2") more towards the front of the engine. The bank that has the head set back a bit will be #2. When there are sensors after the CAT, they're not oxygen sensors, they're CAT efficiency sensors.

P0174 Basically this means that an O2 sensor in bank 2 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 2 is generally the side of the engine that doesn't have cylinder #1.
P0140 Basically this means the an O2 sensor in Bank 1 detected a lean condition, same as mentioned above. Bank 1 is the side where cylinder #1 is.
In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling.

P1133 and P1153 indicate that the HO25 Sensor has gone bad. There the O2 sensor to the left and right bank at the exhaust manifold. When the engine first start's the PCM runs on a open loop and ignores the O2 readings till it reaches it's operating temperature. Once the operating temperature has been reached the diagnostic will only run once per ignition cycle and if the PCM detects a lesser then specified value, a DTC of P1133 or P1153 will come up.
Once the problem has been repaired, after 3 start cycle will reset the MIL light. If the O2 is just dirty and slow to heat up try cycling the ignition start 3 times and the light should go off if the O2 sensor does not fail.
Try cleaning the MAF first and see if the light comes off after a few ignition cycle, If the service light is still on you can always have Auto Zone clear the DTC codes and see if it comes back which then check the left and right bank wire harness to the O2 sensors. 
Good luck and hope this helps, keep me posted.

"Bank one" is the bank of cylinders that starts with the #1 cylinder. In this case this will be the bank of three cylinders that is closest to the firewall. Sensor #1 refers to the O2 sensor that is between the engine and the catalytic converter. So the Bank 1, sensor 1, will be the O2 sensor that is mounted in the rear exhaust manifold or tubing just before the cat.

bank 1 sensor 1 will be in exhaust manifold by firewall in the middle. bank 1 sensor 2 will be behind convertor .bank 2 sensor 1 will be in the front manifold down at the end of it