| Cooling performance | Air from the vents is warm or only slightly cool. | Measure center-vent temperature with the engine running, blower on medium, and the system set to maximum cooling with recirculation selected. | Vent temperature remains close to cabin temperature after several minutes. | The compressor may not be engaging, or the system may have insufficient refrigerant or a restricted flow path. | Check compressor engagement, system pressures, electrical controls, and refrigerant charge using the approved service procedure. | Correct the confirmed cause; do not add refrigerant based on vent temperature alone. | Medium |
| Compressor clutch engagement | The pulley rotates, but the compressor hub does not engage when cooling is requested. | Observe the clutch while checking the control circuit, fuse, relay, pressure sensor, and coil resistance according to service specifications. | No magnetic engagement, or engagement occurs intermittently. | Possible open clutch coil, excessive air gap, low-pressure protection, wiring fault, or control-module inhibition. | Verify the required voltage and ground at the clutch connector. Do not bypass pressure protection for normal operation. | Repair the wiring or control fault, adjust the clutch only when permitted, or replace the clutch or compressor if damaged. | High |
| Unusual compressor noise | Grinding, rattling, squealing, or knocking occurs when the AC operates. | Listen with the hood open and compare the noise with the AC switched off. Inspect the belt, tensioner, pulley bearing, and compressor body. | Noise appears or becomes significantly louder when the compressor is loaded. | Possible internal wear, damaged bearings, clutch or pulley failure, belt misalignment, or excessive system pressure. | Inspect compressor shaft rotation with the system depressurized only by an authorized recovery procedure; check for metal debris in the circuit. | Replace or professionally rebuild the compressor when internal damage is confirmed; flush or replace contaminated components as required. | High |
| Drive belt and pulley | Belt squeal, visible belt dust, pulley wobble, or burning-rubber odor. | Inspect belt ribs, tension, alignment, pulley bearing play, and clutch surface condition with the engine off. | Cracks, glazing, frayed edges, wobble, or rough pulley rotation. | The compressor may be placing excessive load on the belt, or the pulley bearing may be failing independently. | Compare pulley rotation and bearing noise with the compressor unloaded. Check for seized or difficult shaft rotation only under safe workshop conditions. | Replace worn belt or pulley components; replace the compressor if it is seized or mechanically damaged. | High |
| Refrigerant pressure | Cooling is poor, and pressure readings appear abnormal. | Connect approved manifold or digital service equipment and record low-side and high-side pressures at a known ambient temperature. | Readings are outside the vehicle manufacturer's specified range or show an unusual low-side/high-side relationship. | Possible undercharge, overcharge, air or moisture contamination, restriction, condenser airflow problem, or compressor inefficiency. | Recover and weigh the refrigerant before recharging. Compare the recovered amount with the specified charge and inspect for leaks. | Repair leaks, evacuate the system, and recharge by the specified mass. Never diagnose charge level from pressure alone. | High |
| Leak detection | Refrigerant loss is suspected, or the system repeatedly needs recharging. | Inspect hose connections, service ports, condenser, evaporator drain area, compressor shaft seal, and pressure switch using approved leak-detection methods. | Oil residue, fluorescent dye trace, electronic detector response, or bubbles at a connection. | A refrigerant leak is present; compressor replacement may not be necessary unless the compressor itself is the leak source. | Confirm the exact leak location after cleaning the area and repeat the test where necessary. | Replace seals or the leaking component, then evacuate and recharge the system correctly. | High |
| Low-side restriction | Low suction pressure, weak cooling, or frost near a restriction point. | Compare pressure readings and inspect temperature changes across the expansion device, filter-drier, and related lines. | Abnormal pressure drop or a sharp temperature difference across a component. | Possible blocked expansion device, restricted filter-drier, kinked hose, or contamination. | Confirm the restriction with temperature measurements and component-specific testing after refrigerant recovery. | Replace the restricted component and address contamination before recharging. | High |
| High-side pressure and airflow | Cooling decreases at idle, or the engine cooling fan runs abnormally. | Inspect condenser fins, airflow path, fan operation, and high-side pressure under controlled test conditions. | High-side pressure rises excessively, especially at low vehicle speed. | Insufficient condenser airflow, blocked fins, overcharge, non-condensable gas, or compressor displacement problem. | Verify fan command and airflow before condemning the compressor. Confirm refrigerant mass and condenser condition. | Repair fan or airflow faults, clean or replace the condenser when necessary, and correct the refrigerant charge. | High |
| Compressor efficiency | The compressor engages, but the system produces little cooling. | Evaluate the pressure difference between the low and high sides while checking compressor speed, charge level, and condenser airflow. | Pressure separation remains insufficient after correct charge and airflow are confirmed. | Internal valve, piston, scroll, or drive mechanism wear may be preventing effective compression. | Rule out incorrect charge, restrictions, sensor faults, and airflow problems before identifying compressor inefficiency. | Replace or rebuild the compressor using compatible parts and follow the required oil and flushing procedures. | High |
| Compressor oil and contamination | Oil leakage, dark oil, metallic particles, or compressor seizure is found. | Inspect recovered oil and removed components for abnormal color, odor, sludge, or metal fragments. | Metal debris or severe discoloration is present in the oil or connected components. | Internal compressor failure may have contaminated the entire AC circuit. | Inspect the condenser, expansion device, hoses, and filter-drier. Follow the vehicle-specific contamination procedure. | Replace affected components, use the correct oil type and quantity, flush only components approved for flushing, evacuate, and recharge. | High |
| Electrical control system | Compressor operation changes when wiring or connectors are moved. | Scan for HVAC-related diagnostic codes and inspect connectors, grounds, fuses, relays, pressure sensors, and command signals. | Intermittent voltage, corrosion, poor ground, or a stored control-system fault. | The compressor may be mechanically sound but prevented from operating by an electrical or electronic fault. | Test the circuit under load and compare sensor values with actual pressure and temperature conditions. | Repair wiring, connectors, grounds, or sensors before replacing the compressor. | Medium |
| Diagnosis confirmation | A compressor replacement is being considered. | Document symptoms, visual findings, pressure behavior, electrical tests, leak results, refrigerant quantity, and contamination status. | Mechanical failure is supported by multiple independent test results. | Compressor replacement is justified only after other causes have been excluded. | Obtain a stable post-repair result: correct charge by mass, no leaks, normal clutch or control operation, suitable vent temperature, and no abnormal noise. | Complete the repair, perform the required system evacuation and recharge, and verify operation at idle and road speed. | Medium |
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