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Trane A5AHC004A1B3 MCB 3 Flash troubleshooting

Active refrigerant leak or sensor-failure alarm. Use this page to document the issue and prepare for service. GreenFlow should handle diagnosis for electrical, refrigerant, lockout, or recurring conditions.

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What the code means

MCB 3 Flash: Active refrigerant leak or sensor-failure alarm

RunTru by Trane AHR-SVX005C-EN MCB diagnostic-code table for A5AHC convertible air handlers identifies 3 Flash as an active refrigerant leak alarm, sensor communication error, or sensor failure depending on software version.

System area
Refrigerant mitigation control board
Service level
Call GreenFlow

Safe checks before calling

  • Confirm the thermostat is on, set correctly, and calling for heating or cooling.
  • Check the air filter and visible return-air path for obvious airflow restrictions.
  • Look around the outdoor unit for leaves, snow, ice, or other visible obstructions without opening the cabinet.
  • Take photos of the thermostat, outdoor unit, nameplate, and any visible flash pattern before scheduling service.

When to stop and call GreenFlow

Call GreenFlow if the code repeats, the system is locked out, heating or cooling is not restored, there is ice buildup, electrical odor, unusual noise, or any issue involving pressure switches, sensors, wiring, refrigerant, reversing-valve operation, or warranty coverage.

Share the visible code, model and serial number if available, photos, current symptoms, and whether the issue affects heating, cooling, or both.

MCB 3 Flash questions

What does MCB 3 Flash mean on a Trane A5AHC004A1B3?

RunTru by Trane AHR-SVX005C-EN MCB diagnostic-code table for A5AHC convertible air handlers identifies 3 Flash as an active refrigerant leak alarm, sensor communication error, or sensor failure depending on software version.

What should I safely check before scheduling service?

Check thermostat status, filter condition, visible outdoor-unit obstructions, and take photos of the flash pattern and nameplate. Do not open electrical panels or refrigerant-side components.

When should GreenFlow handle this code?

Call GreenFlow if the code repeats, the system is locked out, heating or cooling is not restored, there is ice buildup, electrical odor, unusual noise, or any issue involving pressure switches, sensors, wiring, refrigerant, reversing-valve operation, or warranty coverage.

Need help with this code?

GreenFlow can review the code, photos, equipment access, and symptoms before scheduling the right next step.

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