To fix a coolant leak under the engine, first, identify the leak’s source by inspecting the hoses, the radiator, the water pump, the coolant reservoir, and the engine block Tighten any loose clamps and replace damaged hoses. If the radiator is leaking, it may need to be repaired or replaced. For a leaky water pump, replacement is necessary. If the coolant reservoir is cracked, replace it. Use a coolant system pressure tester to pinpoint the leak if it’s not visible. Once the leak source is fixed, refill the coolant and bleed the system to remove air pockets. If the leak persists or if you are unsure about performing these steps, consult a professional mechanic to avoid further engine damage.