Posted on 1/30/2026
If your coolant reservoir keeps getting low, your car is telling you something. Coolant doesn’t get used up like fuel. In a healthy cooling system, the level stays pretty steady between services. So when the reservoir level keeps dropping, it usually means one of two things: the system is leaking, or the engine is pushing coolant out of the system under certain conditions. Either way, it’s worth taking seriously because running low on coolant can lead to overheating, poor heater performance, and expensive engine damage if it goes far enough. What The Reservoir Level Is Actually Showing You The reservoir is a buffer tank. As the coolant heats up, it expands and moves into the reservoir. As it cools, it contracts and gets pulled back into the radiator or pressurized part of the system. That’s why the reservoir has a cold-fill line and a hot-fill line. If the reservoir keeps dropping when you check it cold, the system is losing coolant somewhere or n ... read more