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74 Discharge temperature lockout

High severity Carrier Infinity 18VS 25VNA8

What the control is telling you

Amber STATUS light flashes 74. Discharge temperature kept going over the limit (elevated from code 59). The unit locks out for 2 hours. Type: System malfunction; reset: 2 hours.

Applies to: Carrier Infinity 18VS variable speed heat pump 25VNA8 (AOC control + inverter)

Likely causes

  • Cooling: High load or overcharge
  • Heating: Low charge or loss of charge at low outdoor temperature
  • Heating: EXV restriction or EXV / TXV problem
  • Discharge sensor unplugged, wiring broken or sensor failed
  • Indoor airflow too low or off
  • Outdoor airflow too low or off
  • Heating: Reversing valve bypassing or stuck halfway
  • Failed AOC board

Test and repair steps

  1. Cooling check subcooling in Charging Mode (or the subcooling chart on the control box door) and adjust charge.
  2. Heating compare pressures with the heating check chart; if they do not match, recover, evacuate and weigh in the rating plate charge.
  3. Heating troubleshoot the EXV (coil, harness); power cycle and listen for the EXV moving on power-up; check the TXV.
  4. Check the discharge thermistor (50K about 50 kohm at 77°F) and its harness.
  5. Check indoor and outdoor fans and coils.
  6. Check the AOC fuse and that the reversing valve gets 24 VDC in cooling; check harness and connectors.
  7. Nuisance trips when the unit is not running: see TIC 2015-0017.

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Source: Carrier 25VNA8 / 24VNA9 Service Manual, Table 6 Fault Code Actions, p.22. Summarized in our own words; check the original manual for your model.

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