HVAC Toolkit

HVAC Fault Diagnosis Guide

Identify common problems by symptoms and readings

Quick Diagnosis by Symptoms

High Head Pressure

Discharge pressure above normal

  • • Dirty condenser coil - Clean coil, check airflow
  • • Condenser fan failure - Check motor, capacitor, blade
  • • Overcharge - Recover excess refrigerant
  • • Non-condensables (air) - Recover, evacuate, recharge
  • • Restriction after condenser - Check liquid line

Low Head Pressure

Discharge pressure below normal

  • • Low refrigerant charge - Check for leaks, add charge
  • • Compressor valve leak - Check discharge valve
  • • Restriction before evaporator - Check TXV/filter-drier
  • • Low ambient temperature - Normal in cool weather

High Suction Pressure

Suction pressure above normal

  • • Overcharge - Recover excess refrigerant
  • • TXV stuck open - Check superheat, replace valve
  • • High heat load - Check space conditions
  • • Compressor suction valve leak - Check compressor

Low Suction Pressure

Suction pressure below normal, possible icing

  • • Low refrigerant charge - Check for leaks
  • • Restricted metering device - Check TXV/cap tube
  • • Low/no evaporator airflow - Check filter, blower, ductwork
  • • Iced evaporator - Check defrost, airflow
  • • Restricted filter-drier - Check temperature drop across

Compressor Not Running

Compressor won't start or short cycles

  • • No power - Check disconnect, breaker, wiring
  • • Thermostat issue - Check settings and wiring
  • • Capacitor failure - Test run/start capacitors
  • • Contactor failure - Check coil and contacts
  • • Safety lockout - Check high/low pressure switches
  • • Compressor internal failure - Check windings, mechanical

Short Cycling

Compressor starts and stops frequently

  • • Low refrigerant - Low pressure cutout
  • • Dirty condenser - High pressure cutout
  • • Oversized system - Quick temperature satisfaction
  • • Thermostat location - Near supply vent or heat source
  • • Electrical issue - Loose connections, bad capacitor

Pressure & Temperature Diagnostic Matrix

Suction Press. Head Press. Superheat Subcooling Likely Problem Action
Low Low High Low Low charge / Leak Leak test, repair, recharge
Low Normal High Normal/High Restriction (liquid line) Check filter-drier, TXV
Low Normal Low Normal Low evaporator airflow Check filter, blower, ducts
Normal High Normal High Overcharge Recover excess refrigerant
Normal High Normal Normal/Low Dirty condenser / Poor airflow Clean coil, check fan
High High Low High Overcharge + High load Check charge, reduce load
High Normal Low Normal TXV stuck open / Flooded evap Check TXV, sensing bulb
High Low Low Low Compressor valve failure Check compression ratio, replace

Electrical Testing Quick Reference

Compressor Winding Test

Single Phase (CSR)

Measure between terminals:

  • • C to S = Start winding (higher ohms)
  • • C to R = Run winding (lower ohms)
  • • S to R = Start + Run (highest ohms)

Rule: C-S + C-R = S-R (approximately)

Ground Test

Measure from each terminal to ground (compressor body). Should read infinite (OL on meter). Any reading = grounded winding = compressor failure.

Capacitor Testing

Capacitance Test

Use multimeter in capacitance mode:

  • • Should read within ±6% of rated value
  • • If low = capacitor weak, replace
  • • If shorted = 0 reading, replace
  • • If open = no reading, replace

Safety Warning

Always discharge capacitors before testing. Use a resistor (20kΩ, 5W) across terminals, not a screwdriver.

Common Electrical Symptoms

Symptom Possible Cause Test/Action
Compressor hums but won't start Bad start capacitor, stuck compressor Test capacitor, check amp draw
Compressor trips breaker immediately Grounded winding, locked rotor Ground test, check mechanical
Compressor runs but weak cooling Bad run capacitor, valve failure Test capacitor, check amp draw
Fan motor won't start Bad capacitor, winding failure Test capacitor, check windings
Contactor chatters Low voltage, weak coil Check supply voltage, coil resistance

Charge Verification Guide

TXV Systems - Use Subcooling

Target Subcooling: 10-15°F (5-8°C)

To measure subcooling:

  1. Read liquid line temperature (near condenser outlet)
  2. Read high side pressure
  3. Convert pressure to saturation temperature (P-T chart)
  4. Subcooling = Saturation temp - Liquid line temp

Low subcooling = Low charge or restriction before condenser

High subcooling = Overcharge or restriction after condenser

Fixed Orifice - Use Superheat

Target Superheat: 10-15°F (5-8°C)

To measure superheat:

  1. Read suction line temperature (near compressor inlet)
  2. Read low side pressure
  3. Convert pressure to saturation temperature (P-T chart)
  4. Superheat = Suction line temp - Saturation temp

High superheat = Low charge, restriction, or low airflow

Low superheat = Overcharge or flooded evaporator