What happens
An incorrect temperature input or control failure prevents the required frost response.
Typical symptoms
Outdoor ice persists beyond the expected sequence, or repeated defrost-related limits occur.
Checks that distinguish this cause
Record operating mode, weather and sequence history. Evaluate the named sensor and control inputs with model data before using a prescribed defrost test; do not force a test through arbitrary jumpers.
What can look similar
Low charge, outdoor airflow restriction or poor drainage can produce icing with intact defrost logic.
Repair direction
Correct the established input or control fault using approved parts and setup.
Verify the repair
Verify the documented defrost sequence and subsequent heating recovery.
Equipment-specific limits
Defrost sequence, fan delays, heaters and low-ambient limits are model-specific. Steam, temporary indoor fan pause or outdoor frost can be normal; persistent ice and repeated faults need evidence.
Practice this diagnosis
This cause is a reference topic; it does not yet have a dedicated simulated call. Browse the practice board.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Daikin: 17 Series service manualModel-specific sensor and control examples; terminal values and code definitions vary.
- Haier: Arctic Next Gen service manualAW/1U EH2VHD model examples of inverter, motor, sensor and circuit diagnostics; obtain the installed model manual.
Editorial review: 2026-09-29. Verify the current manual for the installed model.