Work through these in order. Each one rules out a whole family of causes, so you avoid replacing a compressor for a fault that was a five euro sensor.
1. Cabin filter and blower
Measure vent temperature and airflow at maximum blower. If airflow is weak, the system is not the problem yet: change the filter and check the blower and its resistor first.
2. Condenser airflow
Confirm the condenser fan runs when the A/C is engaged, and that the matrix is not blocked with flies, leaves or a bent fin block. Head pressure that climbs above 20 bar at idle and falls when a workshop fan is placed at the front is an airflow fault, every time.
3. Static pressure
With the engine off and the system settled, static pressure should roughly correspond to ambient temperature. Very low static pressure means the system is empty and there is a leak to find.
4. Running pressures
- Both sides low: undercharge or a restriction upstream of the gauge.
- Low side high, high side low: compressor not pumping, or an internal valve fault.
- Both sides high: overcharge, air in the system, or condenser airflow loss.
- Low side pulling into vacuum: blocked expansion device or drier.
5. Compressor engagement
Check the clutch coil for supply and earth with the system commanded on. No supply sends you to the pressure switch, relay and climate module. On a variable displacement compressor with no clutch, check the control valve signal before condemning the unit.
6. Leak test
Only after the above. Nitrogen pressure test or an electronic detector at 5 g/yr sensitivity, followed by UV dye if the leak is intermittent. Fit a new drier whenever the system has been open.