My 1999 Olds Alero's Check engine light stays on. I had it in the shop and was diagnosed EVAP. Many of the fuel lines were replaced, yet light does not go out. Also replaced the gas cap and light is still on.

I'd use a gage and check proper fuel pressure. Also, check for external gas leak, at the fuel pump--fuel lines--fuel injector/s--fuel pressure regulator, and check evap system.

broken evap line/port on fuel pump. Or leaking fuel pump o-ring
check the EVAP canister it is right next to fuel filter could be pcm needs update to fix false codes. Here is a old hillbilly trick that may help put saliva on the seal of gas cap when you fill up every time that may work for you.

CODE PO300 MULTIPLE ENGINE MISFIRE DETECTED CONDITIONS. POSSIBLE CAUSES.WEAK FUEL PUMP.STOPPED UP FUEL FILTER.CHECK FUEL PRESSURE SHOULD BE 41 - 47 PSI. THE MASS AIR FLOW SENSOR CONTAMINATED CAUSE VERY LEAN CONDITIONS.EGR VALVE STUCK OPEN OR PVC VALVE HAS VACUUM LEAK. THE EVAP SYSTEM CANISTER FUEL SOAKED.

LOOK FOR FUEL LEAK UNDER THE HOOD IN THE ENGINE COMPARTMENT.CHECK FUEL LINES.TO FUEL RAILS.AND CHECK FOR FUEL LEAKS AT FUEL PRESSURE REGULATOR.LOOK UNDER CAR FROM FRONT TO REAR TO FUEL FILTER TO FUEL TANK LOOK FOR FUEL DRIPPING.IF YOU FIND NO LEAKS.YOUR EVAP CANISTER NEXT TO FUEL TANK FULL OF GAS OR EVAP CANISTER HOSES DISCONNECTED OR BROKEN.DONT DRIVE VECHICLE UNTIL YOU FIND FUEL LEAK.NOT SAFE.

first check the alternator belt if fine then yes the alternator may be bad.(note) make sure to check the oil level it will also make car die out.

Is your security light on?

HAVE FUEL EVAPORATIVE SYSTEM CHECK OUT.SOUND LIKE PROBLEM IS IN THE EVAP CANISTER PURGE VALVE OR VENT VALVE.YOUR FUEL TANK BUILDING UP WITH GAS VAPORS KEEPING YOU FROM PUMPING GAS IN TANK.

There may be and EVAP solenoid that is leaking. THe OLDS Aurora has this and it is a common failure with this code. Also, the evap lines run to the fuel pump on the top of the gas tank. Some cars have an access panel in the trunk under the carpeting, near the back seat on the right hand side.

have your evaporative purge valve solenoid check. because its controls the gas vapors going to canister purge or vent valve.take it to dealers have the evap system scan because its not venting or controling gas vapor.it suppose vent gas vapor to intake to be burn.if it dont work it causes excess gas vapor pressure built up in tank. make it hard to pump gas into it.

I have a car in the shop right now same thing - it is being caused by a misfire. Need a real technition to diagnose the problem (auto zone - advance auto can't diagnose the problem - only read the code)
mine was caused by a failing fuel injector. good luck