A stop leak earns its place in one situation: a small, slow loss of coolant from somewhere that would be awkward or expensive to replace this week. Forte describe theirs as an emergency repair that prevents coolant loss and keeps the vehicle safely on the road until the replacement part is available, and that lets an extended trip be finished without a roadside bill. That is the honest job description. It is a bridge to the proper repair, not a substitute for it, and it is strictly for minor leaks at early stages.

• Seeping and weeping coolant leaks From radiators, hoses, gaskets and water pumps: the slow damp patch, the crust at a seam, the level that needs topping up every few weeks.
• Hairline cracks and pinholes The natural fibres in Forte Radiator Stop Leak physically plug hairline cracks and pinholes in the cooling system.
• Corrosion and limescale prevention It conditions the system by removing oxygen from the coolant, which prevents the internal build-up of limescale and corrosion.
• Head gasket failure A failed head gasket lets combustion gas into the coolant or coolant into the cylinders. No stop leak fixes that, and Forte say so plainly.
• Major cracks and severe physical damage A split radiator tank, a burst hose or a cracked head or block need the part replaced.
• White exhaust smoke, overheating, or a significant and rapid drop in coolant Any of these means the additive will not suffice and professional mechanical repair is required.

If you are unsure which side of that line your leak is on, the section further down on telling a minor leak from a head gasket will settle it in a few minutes with no tools. How to add it, how long it takes and what it will not fix are on the Forte Radiator Stop Leak & Cooling System Conditioner page.

There are broadly three types on the shelf. Fibre-based conditioners carry fine natural or synthetic fibres that lodge in the hole and build up a plug where coolant is escaping. Chemical or polymer sealants are designed to cure at the point of the leak where the coolant meets air. Powders, pellets and tablets are an older category that dissolves into the coolant. Each maker's label states what its product is designed for and what it must not go into; for anything not on this page, read that label.

Forte's product is the first type, and the criteria below are the ones that matter when you are choosing. The Forte column is taken from Forte's published product information. The other columns describe the category only, because it would not be honest to attribute figures to products whose data sheets are not in front of us.

The row that should decide it for most people is the blockage guarantee. A heater matrix is a narrow, expensive, awkward-to-reach part, and the worst outcome of a stop leak is not that it fails to seal the leak but that it seals the heater. Forte's guarantee on that point, when the product is used as directed, is why Phil stocks this one and not another. The full product page is at Forte Radiator Stop Leak & Cooling System Conditioner. Dose, price and the honest limits are on the Forte Radiator Stop Leak & Cooling System Conditioner product page.

This is the check that matters most, because the wrong call wastes the £20 and, worse, delays a repair that gets more expensive the longer an engine runs hot. None of it needs tools.

• Points to a minor leak: a slow drop The level falls over weeks, not days. You top up a little each time and it holds for a while.
• Points to a minor leak: you can find it A damp hose joint, a crusty white or coloured deposit at a radiator seam or around the water pump, a sweet smell after a drive, a small puddle under the front after parking.
• Points to a minor leak: everything else is normal The temperature gauge sits where it always has, the heater blows hot, the exhaust looks normal and the oil on the dipstick is clean.
• Points to something bigger: white exhaust smoke Persistent white smoke that smells sweet once the engine is warm is coolant burning in the cylinders. Stop leak will not touch it.
• Points to something bigger: overheating or a rapid drop Coolant that disappears in a day or two, or a gauge that climbs, means the loss is too fast for a minor-leak product and the engine is at risk.
• Points to something bigger: bubbles, pressure or mayonnaise Bubbling in the expansion tank with the engine running, a hose that goes rock hard, or a creamy deposit under the oil filler cap and on the dipstick all point at a head gasket. Get it pressure-tested.

If the ticks describe your car, the Bio Degreaser flush followed by the Stop Leak is a sensible £40 and will very likely keep you on the road until you replace the weeping part. If any of the crosses do, save the money for the diagnosis. Phil would rather tell you that by email than sell you a bottle that cannot work; his address is on the product page.