Which method is used to detect coolant leaks that are not visible to the naked eye?

• A. Pressure testing only.
• B. Replacing hoses.
• C. Dye or UV tracer.
• D. Monitoring oil color.

Answer: (C)

Dye or UV tracer is used because it makes hidden leaks visible. By adding a fluorescent dye to the coolant, any escaping coolant carries the dye with it. When you shine a UV light on the system, the dye glows, revealing even small seepage through hoses, fittings, gaskets, the radiator, or the heater core that you wouldn’t see with the naked eye.

Pressure testing can show a leak as a drop in pressure but doesn’t always pinpoint tiny or hidden leaks. Replacing hoses is a maintenance action, not a diagnostic method, and monitoring oil color is not a coolant leak test. So the dye/UV tracer approach directly visualizes hidden leaks, making it the best choice here.