After reassembling the system

• A. Leak test with nitrogen
• B. Vacuum test with air
• C. Pressure test with oxygen
• D. Run the system without oil

Answer: (A)

The main idea here is to verify system integrity by confirming there are no leaks after reassembly. Using nitrogen for a leak test is the best choice because nitrogen is dry and inert, so it won’t react with oil or refrigerant and it won’t pose a fire risk if a leak exists. You pressurize the sealed system to a safe test pressure and watch for a loss of pressure or for bubbles at joints with a leak detector or soapy solution. If the pressure holds, the joints are considered leak-free; if it doesn’t, you’ve found a leak that needs repair before charging.

A vacuum test with air isn’t ideal for leak detection because air, especially humid air, can complicate results and moisture can be drawn in, masking leaks or causing damage during a test. A pressure test with oxygen is hazardous and unnecessary—oxygen can react with oils and refrigerants and creates safety risks. Running the system without oil isn’t a leak test at all and can severely damage the compressor, so it doesn’t serve the purpose of verifying system integrity after reassembly.