An apparatus and method are developed to improve accuracy of diagnostic tests for A/C systems. In particular, a refrigerant recovery unit may have its hoses pre-charged with a refrigerant to a predetermined pressure or amount before performing the diagnostic tests on the A/C system of the vehicle. For example, a predetermine amount of refrigerant may be injected into the hoses of the refrigerant recovery unit to pre-charge them. The pre-charged hoses of the refrigerant recovery unit may prevent a refrigerant flow from the A/C system to the hoses connected thereto before the diagnostic tests are implemented and thus, improve the accuracy of the diagnostic tests.