Ask for a chemical combustion-leak test before you buy a used car that overheats, loses coolant or shows milkiness on the dipstick. The dye test can show exhaust gas in the cooling system even when a basic pressure test still looks borderline.
For this November 5, 2026 guide I read Underhood Service on head-gasket service and on compression testing. A chemical block tester draws vapours from the radiator neck through a fluid that changes colour when combustion gases are present. So it is a focused check when coolant loss and overheating stories disagree.
Know what the colour change means
Underhood Service describes chemical testers that change colour when exhaust gas is present in the cooling system. Ask the shop which kit it uses and to show you the fluid before and after. A clear colour change is easier to argue about than a vague maybe.
Run it with the engine at operating temperature
The test needs combustion pressure in the cooling system. A cold engine can give a weak result. Let the car reach temperature and follow the kit instructions the shop uses.
Pair it with compression or leak-down numbers
Underhood Service treats compression and leak-down as core skills next to gasket diagnosis. Our compression and leak-down guide explains those results. A chemical fail plus a weak cylinder points the repair more clearly.
Do not skip a cooling-system pressure test
A pressure test still finds external leaks and weak caps. The chemical test answers a different question: are combustion gases in the coolant. Ask for both when the seller mentions overheating.
Watch for recent coolant top-offs
A seller who keeps adding water may dilute clues. Ask how often coolant is added and whether the overflow bottle loses level while parked. Our combustion-leak test guide covers a related combustion-leak angle from an earlier batch.
Pair the chemical test with compression numbers
Underhood Service treats combustion testing as part of a wider head-gasket diagnosis alongside compression or leak-down work. Our compression and leak-down guide explains those numbers. A single colour change without a follow-up plan is not a complete answer.
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 a written note of the chemical-test result, compression or leak-down figures and any cooling-system pressure findings. 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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