My car is heating up but only when I am driving at 40 or 45, if I keep driving under 35 it does not heat up. What could be wrong?

The fan is working.
I have changed the Thermostat and did the coolant flush.

"Engine Coolant sensor is bad" do you mean, car is not heating up but the sensor is bad? The fan is working. I have changed the Thermostat and did the coolant flush. "Engine Coolant sensor is bad" do you mean, car is not heating up but the sensor is bad?

If your fans are coming on then the sensor is working. You said you did acoolant flush. Did you just replace the coolant? Some time s the radiator needs cleaned . this takes removing all thee coolant put in water and add a radiator flush compound that cleans the system. you have to drive on it for a few days, read the instructions. To check the radiator you can pull the top and bottom hoses off the engine and run a garden hose to it. turn it on about half to three quarters on. the water should run out the bottom as fast as you put it in. It shouldn't back up threw the top hose. if it does, the the radiator is getting clogged up. don't have it professionally cleaned , it wont make it. trust me. Just get a new one, shop and tell everyone the cheapest price, they will try to beat it. If the radiator checks out, then go to the water pump.. Dodge/Mitsubishi Are good pumps, but I have replaced a few that the impellars are worn out or they are spinning on the shaft. If your fans are coming on then the sensor is working. You said you did acoolant flush. Did you just replace the coolant? Some time s the radiator needs cleaned . this takes removing all thee coolant put in water and add a radiator flush compound that cleans the system. you have to drive on it for a few days, read the instructions. To check the radiator you can pull the top and bottom hoses off the engine and run a garden hose to it. turn it on about half to three quarters on. the water should run out the bottom as fast as you put it in. It shouldn't back up threw the top hose. if it does, the the radiator is getting clogged up. don't have it professionally cleaned , it wont make it. trust me. Just get a new one, shop and tell everyone the cheapest price, they will try to beat it. If the radiator checks out, then go to the water pump.. Dodge/Mitsubishi Are good pumps, but I have replaced a few that the impellars are worn out or they are spinning on the shaft.