Where are the O2 sensors for a 99 Chevy Silverado? (5.3L)

Anrez, your 1991 Chevy trucks O2 sensor could be located in the Y pipe just ahead of converter or in the front portion of the converter.
Check the attached links,instruction and guides, Good luck
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SORRY I DONT HAVE A PICTURE BUT BANK 1 & BANK 2 SENSOR 1 REFER TO THE LEFT AND RIGHT SENSORS BEFORE THE CAT CONVERTER ( UPSTREAM) SENSOR 2 REFERS TO THE SENSOR AFTER THE CAT (DOWNSTREAM) YOU TUBE HAS GOOD VIDEOS

mean you replace front O2,codes mean back O2 sensor,spark plugs,fuel filter,could be battery[300].

The P0141 means O2 Sensor Heater Circuit (Bank 1 Sensor 2)
The P0121 means Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem
The P0200 means Injector Circuit
The P0308 means Cylinder 8 Misfire Detected

Hope them can help you! Good luck!
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DTC Number
P0160 DTC Name O2 Sensor Circuit No Activity Detected Bank 2 Sensor DTC Number P0300 DTC Name Random/Multiple Cylinder Misfire Detected

attached to air filter housing it is incorporated into the mass airflow sensor (maf)

That code is for a bad O2 Sensor circuit. If it is just the O2 Sensor that has gone bad, yes it is a very easy repair to do at home.

follow the pipes from the cat forward, you shld be able to find them. from the cat down or back, would be your rear sensors.

codes p0420 and p0430 are both post o2 sensors, the ones after the convertor, both sides if replacing

your o2 sensor is located on your exaust. You have either two or four, I think their is only two on a 99 chevy if I remember correctly, pre catalyst and post catalyst. This means one before your catalytic converter and one after. The pre is usualy the culprit but they will not stop your truck from cranking over. This sounds more like a battery,battery cable, or starter problem. Hope this helps, Clay