The term "co test kühlwasser" refers to a CO2 (carbon dioxide) test for coolant (Kühlwasser in German) in a vehicle. This test is used to detect leaks in the engine's cooling system, specifically to check if combustion gases (CO2) are escaping into the coolant. Such leaks may indicate problems like a blown head gasket, cracks in the cylinder head, or engine block damage. Here's how the CO2 test for coolant works:

• The test device is fitted onto the radiator or coolant reservoir opening.
• It contains a test fluid that changes color (usually from blue to yellow) if CO2 gas is present in the coolant.
• The presence of CO2 indicates combustion gases are leaking, which can lead to overheating and coolant loss.
• The engine is usually warmed up for the test, and sometimes the test involves creating pressure in the cooling system.
• A positive CO2 test suggests a leak that needs immediate repair.

This test is widely used because it provides a quick, reliable, and inexpensive way to diagnose head gasket leaks or other critical engine problems that cause mixing of combustion gases with coolant. It is especially useful when the coolant level drops without visible external leaks and before more invasive engine repairs are undertaken. In summary, a CO2 test on coolant (Kühlwasser) is a diagnostic procedure to detect combustion gas leaks in the cooling system, signaling potential engine damage requiring attention.