Q: Losing coolant but no leaks are found in the system and the radiator fan is not coming on.

Hey there. Let’s start with the fan. The fan is controlled by the engine ECU and the fan relay. When the engine coolant reaches a certain temperature, the ECU tells the relay to turn on the fan. Likewise, the ECU may turn on the fan if the air conditioning system develops a certain amount of pressure. If the car is staying cool, and the AC is off, the fan may never come on. If the car is heating up and the fan does not come on, there is likely a fault in the fan relay or a temperature sensor.

Either of these problems should show up on a scanner. There may be no connection at all with the coolant leak.

Coolant leaks can be pretty sneaky. Most of the time, when you ask a mechanic to check for a coolant leak, they will pressurize the system to try and cause the system to leak. But some leaks will occur under low pressure and seal themselves off at high pressure. Leaks like this can escape the pressure test. Place a large sheet of paper or cardboard under the car overnight and look for spots the next day. If there are any coolant drips, it will help you and your mechanic locate them. To get some help with this, you can contact YourMechanic and they will send a technician to your home to diagnose the leak and tell you what you need to do.