1999buick century 3.1 my fans dont turn on unless i turn a/c on

Has he replaced the thermostat in the cooling system? That is usually a very common reason for cars overheating.

Check the coils and see if they good.Or a bad spark plug. Very few times is the O2 sensor but dont hurt to test it

if engine at long stand still,the engine temperature will rise about 190 degrees the cooling fan suppose to automaticaly turn on when coolant temperature sensor get hot it cause its resistance decrease causing pcm to send power to coolant fan relays and cooling fans, when you turn on AC the cooling fans suppose to turn on to cool down AC CONDENSOR, IF NOT AC WONT WORK COOLING FAN COOLS DOWN REFRIGERANT SO IT CAN RECYCLE THROUGH THE SYSTEM AGAIN.your cooling is suppose to turn on when engine at a sit still if vechicle not moving as coolant temperature rises at a set point the cooling fans will turn on cool down engine.cooling fans dont turn on when you first cold stsrt vechicle unless you have AC SWITCH ON.if cooling fans dont turn on when vechicle at a long stand still or moving along slow traffic.if coolant temperature sensor been replaced and fuses and relays okay have dealership check see if pcm driver circuit working faulty pcm will cause cooling fans not to turn on.

I just replaced a set of cooling fan motors for this problem, and the connectors on the motors were even melted, had to replace connector pigtails as well, th fans also cools the a/c condenser so if the fans quit then your a/c will not be very cool unless your driving and the air flow will aid like a fan would..

First try replacing the Relay for the system. If that doesnt work check/replace the pressure and temp switch on the drier. The 124 body style have issues with A/C overall so dont be too frustrated. Also in the short term try turning the temp wheel to max cold (blue dot) and restarting the car. Sometimes it will force the relay and compressor to lock on while driving. Also be aware if your Aux Electric fans dont come on the system doesnt get cooled properly and will not stay on.
Good Luck.

Fans will not turn on unless the ac is on or the engine is at 225 degrees . Might be a fan relay or sensor.

You need to change the "Coolant Temperature Sensor". That usually controls the fan. If the fan does not run, you will overheat.

REMOVE FAN SHROUD REMOVE FOUR BOLTS RETAINING FAN TO WATERPUMP IF YOUR PUMP AND FAN HAS THE SCREW ON SET-UP WHERE THE FAN SCREWS ON THE PUMP STANDING IN FRONT OF IT STRIKE THE NUT ON THE LEFT HAND SIDE WITH PUNCH AND HAMMER THIS WILL TURN IT COUNTERCLOCK WISE ONCE BROKEN LOOSE IT WILL SCREW RIGHT OFF UNLESS YOU HAVE ACCESS TO A FAN WRENCH SET THESE CAN BE GOTTEN AT AUTOZONE TOOL LOANER PROGRAM

u need to buy the special coolant to your car i have the same car whit the same problem and is cause to the incorrent cooland the cooland to your car is the  DEX-COOL  if u dont find that u need some hit       (silicate-free) 

The right turn signal will only work if its hit manually instead of just being turned on once. I have checked the fuses and changed the bulbs, all are good. I even cleaned the connection to the light but have had no luck getting it to flash intermittently once I have turnedd the signal on.