Air Conditioning Pressure Diagnosis
Reading high and low side pressures to find where an AC system fails.
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Safety notice
- Refrigerant handling requires certification. Never vent refrigerant to atmosphere and never open a pressurised system.
What you will learn
- Read low and high side pressures
- Interpret them
- Distinguish charge, airflow and component faults
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Air conditioning moves heat from the cabin to the outside using refrigerant and a compressor. Manifold gauges show the low-side (suction) and high-side (discharge) pressures; comparing them against ambient temperature and the manufacturer chart reveals the fault.
Both pressures low usually means undercharge or a restriction; both high points to condenser airflow, overcharge or a fan fault; low suction with high discharge suggests a restriction. Check the compressor clutch, pressure switch and the condenser first.
| Symptom | Likely cause |
|---|---|
| Both pressures low | Undercharge, leak, restriction |
| Both pressures high | Airflow, overcharge, fan fault |
| Low suction, high discharge | Restriction (expansion device) |
| No clutch engagement | Electrical, pressure switch, charge |
Key points
- Pressures only mean something with a known ambient temperature.
- Airflow faults mimic charge faults.
- Refrigerant work needs certification and the right equipment.
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Key points
- Read pressures at a known ambient temperature
- Both low = undercharge/restriction on the low side
- Both high = airflow or overcharge
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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