The coolant expansion tank (reservoir) on your 2019 Audi Q5 can develop leaks from cracks, cap seal failure, or connection issues. As the cooling system's pressure relief and expansion point, the tank handles thermal stress that can cause failures.
Expansion Tank Function
The expansion tank accommodates coolant expansion as it heats up and provides a fill point for the cooling system. It's pressurized along with the rest of the cooling system, containing hot coolant under pressure.
Plastic Tank Failures
Modern expansion tanks are typically plastic, which can crack from heat cycling, age, and pressure stress. Cracks often develop at high-stress points like mounting tabs, fill necks, or hose connections.
Pressure Cap Issues
The pressure cap seals the tank and maintains system pressure. A failing cap seal allows coolant to escape, particularly when the system is hot and pressurized. The cap gasket can deteriorate or the cap spring can weaken.
Hose Connections
Where hoses connect to the tank, the plastic fittings can crack or connections can loosen. These leaks may appear intermittent as they're worse when the system is pressurized.
Level Sensor Seals
The coolant level sensor mounted in the tank has a seal that can leak. These leaks appear around the sensor housing area.
Code P0128 Relevance
Code P0128 indicates the engine doesn't reach proper operating temperature. Coolant loss from tank leaks can affect thermostat operation and system function, triggering temperature regulation codes.