Before deciding what the system trouble is, which of the following variables should be checked by a technician?

It is essential for a technician to check all the listed variables—pressure readings, temperature readings, and system history—before determining what the system trouble might be.

Pressure readings provide critical information about the system's operation, as abnormal pressures can indicate issues such as blockages, leaks, or component failures. Analyzing temperature readings is equally important because they can reveal problems related to the efficiency of heat exchange, overheating components, or coolant issues.

Additionally, examining the system history offers insight into past occurrences, maintenance performed, and any previous troubles that could be affecting the current functionality. By having a comprehensive understanding of these factors, a technician can form a more accurate diagnosis, leading to effective troubleshooting and resolution of the issue. Thus, checking all these variables is a fundamental step in the troubleshooting process.