Why do my cooling fans not work sometimes?

2003 Ford Focus. The cooling fans usually work when they need to, but sometimes they don't which causes the car overheats. I've replaced the entire cooling fan assembly including the resister and a resister connected that was burned up. I'm still having the same issue. It happens when I drive around in the heat for a while. I've also replaces the thermostat and the temperature gauge with no luck.

Replace the sender which will be in or near the thermostat housing. If the sender works and the fan(s) work the problem will go away.

If the resistor burned up, you may need a new fan or a higher resistance.

How's the coolant level in the radiator?

Replace the fan relay. That works hard and wears out or sticks.

Check trouble codes with a scan tool. Cylinder head temp sensor could have issues and is not providing the correct input to the PCM. If there are no codes, you will need a scanner and check live data on the cylinder head temp sensor when the engine is running. Cylinder head temp sensor could be showing false data when the engine is running hot.

You can also connect jumper wires from the cooling fan motor power and ground pins and locate the other ends near the battery (Do not let them touch!). When the cooling fan stops working, connect the jumper wire connected to the power pin of the motor to battery positive. Cooling fan works, there's an issue with the power side of the fan motor circuit. Cooling fan does not work, connect the other jumper wire (connected to the ground pin of the motor) to battery negative. Cooling fan works, there's a ground issue. Cooling fan does not work, connect both jumper wires to the battery (power wire to battery positive, ground wire to battery negative). Cooling fan does not work, the motor is bad.

Check the fan circuit here:

https://www.focusfanatics.com/forum/general-technical-chat/597617-2003-2-0-zetec-cooling-fans-dont-run.html

I'm curious about "It happens when I drive around in the heat for a while." That suggests the radiator is plugged; not many things care about how long you drive around or how hot it is, but the radiator is responsible for handling all that. If it is just the fans the temperature will drop well within a minute of getting rolling again. With the Ford engine temperature management system you only have two indications - okay (center of gauge) and hot (into the red). It is an idiot light with a needle.

The last Focus I worked on with those symptoms (a 2000 from Phoenix) had a radiator so badly plugged I could barely blow air through it. It had obviously had at least two, probably more, coolant changes using Phoenix tap water and the tubes were plugged solid with limestone. How to know? Drain the coolant and remove both hoses from the radiator, then put a garden hose in the top fitting and turn the water on full. Water should gush out the bottom and only splash a little out the top. The plugged one gushed out the top and dribbled out the bottom. If the radiator is bad replace it - at 15 years old it has had a good run.

Let a mechanic fix it because you tried once and FAILED. The sensors are not reading correctly FOR SOME REASON. UNLESS YOU WANT TO BUY ANOTHER MOTOR? Could be as simple as an air pocket(instead of coolant around the sensor) so it overheats and does not know it is hot yet. Tell the mechanic ALL THE STUFF YOU DID. HE CAN FIND THE PROBLEM FASTER WITHOUT HAVING TO DO THE SAME WORK TWICE. SAVING HIM TIME AND YOU MONEY.

The thermostat and the temperature gauge are not directly related to the fans operating. Each fan may have its own relay, which may be intermittent (defective).

Check to make sure (if equipped) that the active speed sense solenoid operated lower grill air intake vanes are opening properly.
Look at the high amp connectors for a melted wire, including the relay socket. It's usually a bad wire crimp connector / melted wire.
This is why you don't throw parts at electrical problems, it almost always causes more problems messing with it guessing at it.

Your car probably has a variable speed active fan control module setup, look for the melted wires at the fan module connector.

Hi so time to give it to an expert inside of poke and no hope.