Diagnostic guide

Pressure-temperature troubleshooting basics

Pressure readings become useful when paired with temperatures, airflow, load, refrigerant type and equipment data rather than treated as a single answer.

Updated 2026-09-01 · 6 min read · HVAC/R technicians, apprentices, trainers and service supervisors

Safety and scope note

  • This article is educational and does not replace manufacturer instructions, qualified technician judgement, South African regulatory requirements, electrical or refrigerant safety rules, or a site-specific risk assessment.
  • Do not charge or recover refrigerant based on pressure alone when the equipment requires a weight or manufacturer procedure.
  • High-pressure, electrical and refrigerant hazards must be managed before further diagnosis.

Pressures are clues, not conclusions

A suction or discharge pressure reading by itself can mislead a technician. The same pressure may mean different things depending on refrigerant type, ambient temperature, evaporator load, airflow, humidity, equipment design and metering device. Convert pressures to saturation temperatures and compare them with measured line and air temperatures.

A reliable note set includes refrigerant, outdoor ambient, indoor return air, supply air, suction line temperature, liquid line temperature, suction pressure, discharge or liquid pressure, compressor current and operating mode. Without that context, a pressure chart becomes guesswork.

  • Confirm refrigerant before using any pressure-temperature reference.
  • Measure actual line temperatures with a suitable clamp or probe.
  • Check airflow and coil condition before interpreting charge.
  • Compare readings with manufacturer data when available.

Superheat and subcooling in context

Superheat helps show how fully the evaporator is being fed and whether vapour reaching the compressor is safely above saturation. Subcooling helps show liquid condition leaving the condenser or receiver area, depending on system design. Neither number should be interpreted without knowing the metering device and operating conditions.

A fixed-orifice comfort-cooling system, a TXV system, a cold-room circuit and a heat-pump mode check may all call for different expectations. The safe habit is to use readings as a pattern, not as a one-number rule.

  • High superheat may point to underfeeding, low charge, restriction or high load, depending on context.
  • Low superheat may point to overfeeding, poor airflow, low load or control issues.
  • High head pressure may point to condenser airflow, overcharge, non-condensables or high ambient.
  • Low suction may point to low load, poor airflow, restriction, low charge or control faults.

Build a repeatable diagnostic record

Write readings in a consistent order so another technician can understand what happened. Include before-and-after data when cleaning coils, repairing leaks, adjusting charge or changing controls. This improves customer communication and helps prevent the next callout from starting from zero.

If readings suggest an unsafe condition, stop and resolve the safety issue before chasing performance. High pressure, electrical overheating, icing and suspected leaks all deserve controlled work rather than quick adjustments.

Quick field checklist

  • Identify refrigerant and equipment type before reading a pressure-temperature chart.
  • Measure air temperatures, line temperatures, pressures and compressor current together.
  • Confirm airflow, coil condition and load before adjusting charge.
  • Interpret superheat and subcooling according to metering device and design.
  • Record readings and actions on the job card.

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