What actions should be taken before starting a compressor?

Before starting a compressor, it's essential to check oil levels and confirm there are no blockages. This step is crucial as oil plays a vital role in lubricating the compressor's moving parts, reducing friction and preventing wear and tear. Insufficient oil levels can lead to overheating, inefficiency, and potential damage to the compressor.

Additionally, blockages in the system can restrict the flow of refrigerant, leading to improper functioning or even failure of the compressor. Ensuring that both oil levels are adequate and that there are no blockages helps to maintain the health of the refrigeration system and promotes optimal performance.

The other choices do not directly relate to the immediate actions required before starting a compressor. Monitoring refrigerant levels is important, but it is not the primary step taken just before starting the compressor. System upgrades and replacing the compressor are significant actions that don't need to occur before every startup, making them irrelevant in this context.