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

It is important for a technician to check all relevant variables before diagnosing system trouble. Each of these factors—pressure readings, temperature readings, and system history—provides critical insights into the system's operational state.

Pressure readings are vital as they can indicate whether the system is functioning within its design specifications or if there are anomalies that may point to issues such as blockages or leaks. Similarly, temperature readings help in understanding the thermal performance of the system. Abnormal temperatures can signal problems such as overheating, insufficient cooling, or component failure.

System history offers context that can be crucial for troubleshooting. By examining past performance data and any previous issues or repairs, a technician can identify patterns that may suggest a recurring problem or an emerging trend.

In summary, by evaluating all these aspects together, a technician can establish a more accurate understanding of the system's current condition, allowing for a more effective and targeted approach to identifying and resolving any issues.