My au Falcon teperature light comes on about a minute after the engine is started, the temperature gauge shows low temperature, the radiator is not boiling, I have taken the temperature sender unit out of the overflow bottle and cleaned it but the temperature light still comes on.

Check your coolant level, if it's too low it won't come in to contact with the temperature sensor.

you have overheated the engine maybe thermostat or water pump fault

have you checked the fuse and also the relay.

This may be an electrical issue in the dash and a phantom problem. I would suggest measuring the water temperature in the radiator to see if it matches the guage (normal temperature should be around 90decC. Higher revs would increase water flow via the water pump not lower it so I am thinking some kind of restriction - check your radiator fins are not blocked there could be blockages in the radiator or less likely in the block valleys, other things to check are that your hoses are in good condition and they don't flatten with heat. A faulty thermostat is also possible.

your antifreeze is low that means you have to fill the radiator and the overflow bottle.the reason the gauge fluctuates is that you have air in the system.with the radiator cap removed,start the car turn the heater on high and let the engine run till the thermostat opens.you will know this as the level of antifreeze will fall,fill the radiator and replace the cap turn off the engine and heater and fill the over flow bottle. you may have a bleeder on the top of the engine around the themostat, but if you dont no worry cause your problem should be solved.

CHECK COOLANT FAN FUSES AND RELAYS. CHECK COOLANT LEVEL.MAKE SURE ITS NOT TOO LOW BECAUSE IF COOLANT TOO LOW . BY NO COOLANT AROUND OR COOLANT SENSOR NOT SUBMERGED IN COOLANT COOLING FANS WONT TURN ON.MAKE SURE YOU HAVE 50/50 ANTIFREEZE AND WATER. CHECK FOR CLAPSE RADIATOR HOSES.CHECK RADIATOR HOSE AT WATER PUMP.IF HOSE LOOK CLAPSE REPLACE IT AND BLEED COOLANT SYSTEM.IF EVERY THING HAS BEEN DONE YOU NEED ENGINE BLOCK FLUSHED OUT.HEATER CORE ALSO COULD BE STOPPED UP.A VERY HIGH MILEAGE ENGINE WILL OVER HEAT.OLD ENGINE LOSES COMPRESSION AND HORSE POWER.PUTTING MORE LOAD ON THE OLD ENGINE WILL MAKE IT OVER HEAT.IF COOLANT LEVEL IS LOW.BLEED COOLANT SYSTEM.START THE ENGINE LET IT IDLE FEW MINUTES WATCH THE COOLANT TEMPERATURE GAUGE.WHEN TEMPERATURE START RISING.TURN OFF THE ENGINE LET IT SET FOR 20 MINUTES.TAKE A LARGE RAG PLACE OVER RADIATOR CAP ON COOLANT EXPANSION TANK SLOWLY OPEN RADIATOR CAP JUST A LITTLE TO RELEASE A LITTLE PRESSURE AT A TIME.DONT OPEN CAP RAPIDLY OR YOU WILL GET SCALDED.ONCE THE RADIATOR CAP REMOVED.IF COOLANT LOW ADD MORE COOLANT IN THE THE COOLANT EXPANSION UNTIL YOU GET TO THE COLD MARK.REPEAT THIS PROCEDURE START ENGINE LET IT RUN UNTIL TEMP GAUGE START CLIMBING A BIT DONT LET CAR RUN TO OVER HEAT JUST IDLE UNTIL TEMP JUST START TO RISE.THEN YOU TURN OFF ENGINE. WAIT 20 MINUTES.THEN USE LARGE RAG OPEN RADIATOR CAP LITTLE AT A TIME. DO THIS PROCEDURE UNTIL THE COOLANT IN THE EXPANSION TANK STOP DROPPING. THEN ALL THE AIR IS BLEED OUT THE COOLANT.WHEN COOLANT LEVEL STOP DROPPING, WHEN YOU DRIVE CAR AROUND.AND TURN OFF ENGINE.WHEN ENGINE OFF CAR SET FOR A WHILE KEEP CHECK ON THE COOLANT LEVEL IN COOLANT EXPANSION TANK. THE COOLANT LEVEL SHOULD BE AT THE COLD MARK IN THE EXPANSION TANK.MAKE SURE COOLANT IS IN THE EXPANSION TANK.IF NOT AIR WILL DRAW IN COOLANT SYSTEM. CAUSING ENGINE TO OVER HEAT.IF BLEEDING AIR OUT THE COOLANT SYSTEM DONT HELP AND COOLANT FANS DONT TURN ON YOU HAVE PCM PROBLEMS.

You have air pockets in the system. With the engine cold, remove the radiator cap. Then run the engine until the thermostat opens as you top off the radiator and keep an eye on the temperature gauge. If you think the car is going to overheat, replace the radiator cap, shut off the engine and wait 15 to 20 minutes to let the engine cool. Once the thermostat opens, lightly grab the upper radiator hose to feel for air pockets. You will feel the bubbles as they pass through the hose. Then have someone else rev the engine to 1800 - 2000 rpm and hold it there while you top off the radiator completely and then install and tighten the radiator cap. DO NOT DROP THE RPM BEFORE THE CAP IS SECURED OR YOU MAY BE SERIOUSLY BURNED!! Finally, let the engine run and monitor the temperature and road test. You may have to repeat the steps 2 or more times to remove all of the air but usually it will clear the first time.

The sensor for the fans is in the rad at the bottom. T-stat may be bad or the rad may be clogged. Cooling system needs attention.