An A/C manifold gauge set is the primary diagnostic tool for automotive climate control systems. It features two pressure gauges mounted to a brass block: a blue low-pressure gauge (suction side) and a red high-pressure gauge (discharge side). Three hoses connect the manifold to the vehicle's service ports. Reading these two pressures side-by-side lets you diagnose low refrigerant charge, compressor failures, blockages, or cooling fan problems.
The low-side gauge also measures vacuum in inches of mercury. This is useful when pulling a vacuum on the system to check for leaks before recharging. Always close the manual manifold valves before starting the engine. This keeps the high-pressure refrigerant from backing up into the low-pressure can or hose.