Ask for a combustion leak test before you buy a used car that runs hot, loses coolant or blows white smoke. A block tester checks for exhaust gas in the cooling system, and a yellow result points to a head gasket problem.
For this November 5, 2026 guide I read MotorTrend's block-tester walk-through and Rick Muscoplat's step-by-step head gasket test. Both describe the same chemical test. So, use them to ask the inspector for it, and leave the radiator cap alone on a hot engine.
Know the warning signs
MotorTrend says a head gasket problem often shows as steam, water or white smoke from the exhaust, with coolant in the oil or the other way around. Look at the oil cap and dipstick. Check the coolant level against the marks. Note any sweet smell at the tailpipe.
Understand how the tester works
MotorTrend describes a glass tube and bulb that pull air from the top of the radiator through a blue chemical. If combustion gas is present, it turns from blue to yellow. The test is cheap and quick. It still needs to be done right.
Test warm and circulating
MotorTrend says the engine should be warm and idling with coolant circulating. Rick's guide says the thermostat must be open first. A cold test can miss a leak. Ask the inspector to note the temperature on the sheet.
Avoid a false positive
Both sources say to lower the coolant 2 to 3 inches below the neck so the tester pulls air, not coolant. MotorTrend says coolant in the fluid can give a false positive. Rick's guide says to start over with fresh fluid if that happens. Ask how the shop handled it.
Ask about a gas analyzer
MotorTrend says it prefers a four-gas analyzer at the radiator for an accurate reading of hydrocarbons. Ask whether the shop has one. Two methods agreeing is stronger than one. Our compression and leak-down guide covers the next test if the result is positive.
Check the fluid is fresh
Rick's guide says the test fluid has a shelf life and should be used within a year. Old fluid can mislead. It is a small thing, but ask. Write down the colour result.
Price the repair before you negotiate
A head gasket job is a big repair, and the cause matters too. Ask the shop whether overheating warped anything. Reported cooling work on a history report does not prove today's gasket is sealed.
Ask the seller the right questions
Before the appointment, ask the seller when this part was last checked or repaired, by which shop, and whether there is an invoice. Ask if any warning light has come on recently, and whether the car has been cleaned underneath. A freshly washed underside or a battery that was just disconnected can hide clues. Write the answers down so you can compare them with what the inspector finds.
Know what you can check yourself
You can look, listen, smell and take notes without tools. Leave lifting the car, opening fluid systems and pulling wheels to the shop, which has the stands, gauges and insurance for that work. A careful note of when a symptom shows up, at what speed and temperature, is worth more to the inspector than a guess. It also keeps the conversation with the seller calm and factual.
Use history reports for a different question
A clean history report cannot tell you the present condition of the part you just checked. Reported repairs and present wear are separate questions; our head gasket report guide explains how that coverage works. The NYC consumer-agency guidance suggests getting a history report through NMVTIS and having an independent mechanic check the car. Our VIN verification guide helps make sure every document describes the same vehicle.
Turn the check into a written finding
Ask for the block-test colour, the engine temperature and the coolant level in writing. Honestly, a car that eats coolant with no visible leak deserves this test. Put this item on the work order using our pre-purchase inspection checklist, and ask for a repair scope with a local estimate rather than a verbal promise that the fix is cheap.
We are a licensed CARFAX and AutoCheck reseller, not affiliated with CARFAX, AutoCheck or Experian. Guest reports have been about $5.50, member reports about $4.50 and a two-report bundle about $6.75; confirm the price at checkout because it can change. A report can miss unreported work, so keep the inspection appointment even when the history looks quiet.
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