How to fix: Low coolant level: how do you go about it?

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The diagnosis is confirmed: an insufficient coolant level, which stops the engine from shedding heat properly and leads to overheating. This is one of the simplest causes to fix, but it does call for one important verification step: understanding why the level was low in the first place. The fix, in principle: top up, with the engine cold, using a coolant that matches spec, then watch the level to catch any leak or internal path it might be taking. Coolant is a mix of demineralized water and glycol-based antifreeze with corrosion inhibitors; never use plain water on its own (it freezes in winter, promotes corrosion, and has too low a boiling point), and never mix in an incompatible fluid with what's already there. Major safety point: never open the radiator cap on a hot engine -- the system is pressurized and scalding fluid will spray out, causing serious burns; always work with the engine cold, and preferably top up at the expansion tank. Risk level: low as long as you stick to the cold-engine rule. On budget: a bottle of ready-to-use coolant runs roughly 8 to 20 euros. Key point: if the level drops again after topping up, don't just keep topping it up indefinitely -- there's a leak to track down (hose, clamp, radiator, pump seal) or, more seriously, the fluid is disappearing into the combustion chamber (head gasket). While you're at it, check how old the fluid is: it needs a full replacement at regular intervals (often every two to four years, check your manual). This tool helps you understand what's going on -- it doesn't replace a certified mechanic.