1999 Alero overheating

Well the light staying on could be a short in the wiring or a failed temp switch. And that could affect the radiator fan but I doubt it.
Have you checked the water pump and thermostat ? Is coolant flowing in the system ?

check for low coolant level.check coolant overflow jug if it is empty, more likely air is in the coolant system.add more coolant to overflow jug until coolant level stay at cold full mark,dont over fill coolant overflow jug.if all is good, to be on the safe side, replace radiator cap,replace thermostat,if your engine overheat at a long slow driving traffic.or long stop idling, if cooling fan dont turn on more likely engine coolant temperature sensor could be faulty.

YOU NEED BLEED AIR OUT THE COOLING SYSTEM.WAIT TO ENGINE COOL DOWN.YOUR VECHICLE YOU HAVE TO POUR COOLANT IN THE COOLANT OVERFLOW JUG UNTIL COOLANT LEVEL STAY AT THE FULL COLD MARK.THEN START ENGINE LET IT IDLE A FEW MINUTES WATCH COOLANT IN THE OVERFLOW JUG WHEN IT DROPS ADD MORE COOLANT UNTIL COOLANT LEVEL STAY AT FULL COLD MARK.WHEN COOLANT LEVEL STOP DROPPING IN OVERFLOW JUG.SCREW RADIATOR CAP BACK ON OVERFLOW JUG WATCH ENGINE COOLANT TEMPERATURE GAUGE AS COOLANT TEMPERATURE START CLIMBING TURN OFF ENGINE LET IT SET A WHILE KEEP FROM GETTING SCALDED.TAKE LARGE RAG PLACE OVER COOLANT OVERFLOW JUG CAP SLOWLY OPEN RADIATOR CAP A LITTLE AT A TIME UNTIL PRESSURE RELEASE.REMOVE OVER FLOW JUG CAP.ADD MORE COOLANT UNTIL COOLANT LEVEL AT THE FUL COLD MARK ON THE OVERFLOW JUG.THEN SCREW CAP BACK ON THE OVERFLOW JUG.START ENGINE LET IT IDLE AGAIN WATCH ENGINE COOLANT TEMPERATURE GAUGE WHEN TEMPERATURE GAUGE START CLIMBING.TURN OFF ENGINE WAIT A WHILE.CHECK COOLANT LEVEL IN THE OVER FLOW JUG.DO THIS PROCEDURE UNTIL THE COOLANT LEVEL IN THE COOLANT OVERFLOW JUG STOP DROPPING AND STAY AT FULL COLD MARK,WHEN THE TOP RADIATOR HOSE GET HOT THERMOSTAT OPENING AIR IS OUT THE COOLANT SYSTEM. WHEN ENGINE COOLS DOWN.CHECK COOLANT OVERFLOW JUG COOLANT MUST BE IN THE COOLANT OVER FLOW JUG AT THE FULL COLD MARK WHEN ENGINE COLD. DONT .DONT OVERFILL COOLANT OVERFLOW JUG.KEEP COOLANT IN THE OVERFLOW JUG IF YOU LET COOLANT OVERFLOW JUG RUN DRY AIR WILL BE DRAW IN COOLANT SYSTEM CAUSE ENGINE TO OVERHEAT.

OVERHEATING CAN BE CAUSE BY MANY THINGS LIKE FAULTY THERMOSTAT AND FAULTY RADIATOR PRESSURE CAP,TOO MUCH WATER IN COOLANT SYSTEM NEED 50 / 50 WATER AND ANTIFREEZE.LOOK FOR WATER PUMP WEEP HOLE LEAK, IF COOLANT LEAKING OUT WEEP HOLE WATER PUMP BAD NEED REPLACING.CHECK FOR LOW COOLANT LEVEL, YOUR RADIATOR OVERFLOW JUG SHOULD HAVE COOLANT AT THE FULL COLD MARK IF NOT KEEP ADDING COOLANT INTO COOLANT OVERFLOW JUG UNTIL COOLANT STOP DROPPING AND FULL COLD MARK, IF OVERFLOW COOLANT JUG WAS EMPTY RADIATOR IS LOW ON COOLANT.WHICH CAUSE AIR IN COOLANT SYSTEM KEEPING THE ENGINE COOLANT TEMPERATURE SENSOR FROM BEING SUBMERGE IN HOT COOLANT WHICH TURN ON THE COOLANT FANS.WHEN ENGINE COOLANT TEMPERATURE GET CERTAIN SET TEMPERATURE LIKE 190 DEGREES THE PCM AND COOLANT SENSOR WILL TURN ON COOLANT FANS, IF VECHICLE OVERHEATING COOLANT FANS NOT WORKING EITHER YOU HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR CHECK COOLANT FANS FUSE AND RELAYS. YOU CAN CODE SCAN CAR TO SEE IF YOU HAVE FAULTY PCM, FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR FAULTY COOLANT FAN RELAY.IF COOLANT FAN AND ENGINE COOLANT TEMPERATURE SENSOR OKAY,YOU HAVE CORRECT COOLANT LEVEL IN COOLANT SYSTEM, THERMOSTAT AND RADIATOR PRESSURE CAP BEEN REPLACED, YOU COULD HAVE BLOWN HEAD GASKET CHECK ENGINE OIL LOOKS LIKE MILK SHAKE HEAD GASKET BLOWED ALSO WHEN HEAD GASKET BLOWED YOU CAN DRIVE CAR SHORT DISTANCE START OVERHEATING AND YOU LOOSE ENGINE POWER SUSPECT BLOWED HEAD GASKET.

BLEED AIR OUT COOLANT SYSTEM.WAIT UNTIL ENGINE COOL DOWN.REMOVE COOLANT OVER FLOW PRESSURE CAP.IF YOU HAVE 3.1 OR 3.4 ENGINE OPEN AIR BLEED SCREW ON WATER PUMP BYPASS HOSE.OPEN BLEED SCREW.WATCH COOLANT IN OVER FLOW AS YOU OPEN BLEED SCREW COOLANT WILL DROP IN OVER FLOW JUG. AS IT DROPS CLOSE BLEED SCREW ADD MORE COOLANT IN OVER FLOW JUG DO THIS UNTIL COOLANT LEVEL STAY AT COLD FULL MARK.WHEN DONE TIGHTEN BLEED SCREW DONT OVER TIGHTEN IT OR IT WILL STRIP OUT BREAK.CRANK CAR UP UNTIL THERMOSTAT OPEN THAT WHEN TOP RADIATOR HOSE GET HOT.WATCH COOLANT GAUGE,IF ENGINE TEMPERATURE RISES TURN OFF ENGINE WAIT UNTIL COOL.REMOVE RADIATOR PRESSURE CAP USING A RAG TO KEEP FROM GETTING HANDS SCALED.ADD MORE COOLANT IF NEEDED WHEN ENGINE COOLANT STOP DROPPING IN OVER FLOW JUG AND ENGINE DONT OVER HEAT AIR IS OUT OF COOLANT SYSTEM.

Sounds like you have a faulty head gasket or maybe a cracked head.This can cause overheating and the check engine light to come on.The code for the o2 sensor is probably caused by coolant getting into the exhaust system and coating the sensor.The coolant fan may work but it cannot remove enough heat from the rad to make the engine cool off.Try some engine coolant stop leak to see if it will help.This maybe only a temperary fix.Let me know if you need more.MOE

TAKE RADIATOR CAP OFF WITH ENGINE COLD AND CHECK WATER ..MAKE SURE RADIATOR NOT BLOCKED,,PLASTIC BAG ETC. THIS SOUNDS LIKE LOW WATER OR RADIATOR ..STOPPED UP INSIDE ..IF YOU HAVE GOOD HEAT IT'S FULL OF WATER. REPLACE T STAT B4 RADIATOR..IT'S CHEAPER. ALSO CHECK FOR RADIATOR COOLING FAN OPPERATION..I,M RULING OUT THE WATER PUMP.

One guess is there is air in the cooling system. The air pockets will cause temporary overheating (fluctuating guage).

It sounds like a bad head gasket is creating these air pockets.

If the temp gauge is reading normal, the lights are flashing, and the coolant level is full, the level sensor in the coolant bottle is the problem. I'm a service consultant for a Saturn dealer and I've run across this many times.

change the thermostat and the temp gauge in the radiator