Should I fix it?

Head gasket repair cost: what a shop charges, what a test costs, and what Thermalweld costs

Three numbers decide most of these cases. The replacement quote, the price of finding out whether you even need one, and the price of the repair that does not involve taking the engine apart.

What a shop charges to replace a head gasket

$2,000 to $7,000. That is the range for pulling the cylinder head, replacing the gasket and putting it all back, and it climbs with the engine. Gen 3 Prius owners on our videos were quoted $5,000 and up at the dealer. Many shops will not quote it at all. They say they do not do head gaskets, just replace the engine, and an engine swap on an older car often costs more than the car is worth.

What it costs to find out if it is really the head gasket

• Block test, do it yourself: about $10. Most auto parts stores lend the tester; you buy the fluid. Dave walks through it on video: How to do a block test.
• Block test at a shop: around $35 to $50.
• Compression test: near $100 or more. Tells you which cylinder is affected.

If the person who said "blown head gasket" did not do one of these, spend the $10 before you spend anything else. Misdiagnosis is common, and the four questions on how to tell if your head gasket is blown take a minute.

What Thermalweld costs

Every bottle holds two treatments. Pick your engine, or let the qualifier pick it for you.

• Gas, 4, 6 or 8 cylinder: $129 for two treatments, $64.50 each.
• Hybrid (Prius, Camry, CT200h, C-Max): $109 for two treatments, $54.50 each.
• Cadillac Northstar kit, all aluminum V8: $275.
• Diesel, under 10 gallon cooling system: $144. 10 to 20 gallons: $179. Over 20 gallons: $209.
• Extra strength additive, for severe cases on tech support's advice: $15.

Shops get 15% off five or more treatments. Current prices and the kit for your engine are on the price table.

What the guarantee is worth

Fix or free, cracked heads and blocks included. If Thermalweld does not repair your head gasket, cracked head or block in the first 30 days, you get a full refund or a free second treatment, your choice. After 30 days, for as long as you own the car, a fresh treatment ships for $25 shipping and handling if it ever fails. It is honored even if it turns out not to be a head gasket, and tech support reviews the case with you before any second treatment. Original owner, non transferable. Full terms at thermalweld.com/warranty.

When the cheap repair is not on the table

Thermalweld is not for every engine, and the qualifier says so before you pay. An engine that will not start cannot be repaired this way; it has to run for at least 15 minutes. A dipstick reading five inches or more over the full mark points to a cracked block or other damage, and tech support will talk you through it rather than sell you a bottle. An engine that keeps overheating because a thermostat, water pump, radiator or fan has not been fixed needs that part first.

The math most owners end up doing

A $10 test to confirm the diagnosis. Sixty seconds with the qualifier to see whether the engine is a candidate. Then $129 for two treatments against a $2,000 to $7,000 quote, backed by 30 days money back. Customers on the reviews page list the car, the mileage and how far it has gone since, which is the number that matters.