Already tested for leaks. Is it normal for coolant reservoir to go from high to low every 2-3 weeks?

Already tested for leaks. Is it normal for coolant reservoir to go from high to low every 2-3 weeks? The dealership says yes its normal. Also the check engine light is on. They got to go off but I went and picked it up the check engine light came on. I left it there. My 2015 ford fiesta engine is still under warranty for 7000 more miles.

The reservoir will normally go up & down, depending on how much you drive the car, but ok, just so it stays somewhere between the COLD & HOT lines.

If it does not need coolant added it is normal, but if coolant is disappearing it has a problem that must be corrected. The check engine light still has to be addressed - they don't have it fixed.

The coolant level in your car's radiator overflow tank CAN fluctuate up and down a bit, but as long as the fluid level stays between the full mark and the add mark, you're good to go. If you have to ADD coolant frequently, then you have a problem. The Check Engine light indicates SOME sort of problem with the engine. If the dealer can't fix it, then READ your owner's manual and find out how to contact the manufacturer's customer service department.

Get it fixed BEFORE your warranty runs out.

The cooling system is SEALED and no fluid should leak out. Ford might have a better way and not seal the cooling system and let some leak into the combustion chamber and the environment through their design.

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