A chemical block test can reveal hidden problems that cause mystery coolant loss, overheating, poor heat output, and even lean codes. Cracked or warped heads along with bad head gaskets can cause those problems long before any serious symptoms are noticed.
Also know as a combustion leak test, the tech uses a special cylinder that fits in the neck of the radiator or coolant reservoir and uses a test fluid (normally deep blue) that changes color to green or yellow on the detection of combustion gasses in the cooling system.
It's good practice to have a chemical block test performed anytime you suspect that an engine has overheated. This can reduce diagnostic time and unnecessary parts replacement. So much so, that some shops perform a chemical block test as the first step in their diagnostic strategy.
It's also a good idea to have a chemical block test performed on vehicles that are being inspected before purchase. Some sellers use tricks to mask cooling system defects, and a block test may reveal an otherwise unseen problem.
Because of the hazards involved with cooling system testing, this type of service should only be performed by trained and qualified individuals. The danger of burns to the operator or bystanders is quite significant. Special precautions must always be used when working near hot cooling systems.