Among the following, which is NOT a function of a test light?

A test light is primarily used to identify the presence of voltage in electrical circuits. Its fundamental function is to illuminate when it comes into contact with a live electrical source, effectively indicating whether or not voltage is present. This is valuable for diagnosing electrical issues in heavy-duty vehicles and equipment, where ensuring safe operation is paramount.

Testing electrical resistance, on the other hand, is a function suited for multimeters or ohmmeters, not test lights. Test lights do not provide information about how much resistance is in a circuit; they only confirm if there is voltage available.

Additionally, while a test light can help identify grounded circuits and detect open circuits by showing the presence or absence of voltage, it does not measure resistance. Consequently, the role of a test light is clearly not to test electrical resistance, making it the correct answer to the question regarding which is not a function of a test light.