ASTG12LVCC - Error 99 Thermistor Failure / Water Ingress

This article summarises the factory investigation into repeated Error 99 faults reported on ASTG12LVCC indoor units, including the thermistor analysis, suspected failure mechanism and production countermeasure.

Applies To

Issue Summary

• Error 99 occurred on 42 ASTG12LVCC units at the same site.
• The reported faults were rectified by replacing the PCB and thermistor.
• Six returned thermistor samples were analysed by SEMITEC, the thermistor manufacturer.
• The investigation identified evidence consistent with water ingress into the thermistor assembly, resulting in increased resistance and incorrect fault detection.

Thermistor Resistance Analysis

Resistance was measured on six returned thermistor samples at the stated R55 condition. The specified acceptable range was 13.629 to 14.472.

Samples No. 1 and No. 5 were outside the stated resistance range.

Detailed Analysis Findings

Disassembly Inspection

The samples with abnormal resistance were disassembled and inspected. No electrolyte peeling or obvious failure of the thermistor element itself was identified.

Adhesion / Contamination Analysis

• Sample No. 1: No adhesion material was identified that was considered likely to affect thermistor characteristics.
• Sample No. 5: Metallic adhesion material was identified.
• Qualitative analysis detected copper (Cu).
• The factory investigation considered this consistent with copper originating from the electrode / electrical terminal area.

Suspected Root Cause

The investigation also noted that some thermistor samples returned to a normal resistance value after internal moisture evaporated. This supports the possibility of an intermittent moisture-related failure.

Water Ingress Investigation

Simulation Testing

Comparison of returned sample and new thermistor:

• Boiling cycle test - 100 cycles: no difference identified.
• PTC test - 2 atmosphere pressure for 120 hours: no difference identified.

Thermal shock test:

Test cycle: 80°C for 5 minutes / normal temperature for 30 seconds / 20°C for 5 minutes.

The factory also checked whether the thermistor was subjected to excessive deformation during installation. The fitted condition was not considered to impose extreme stress.

Appearance Check

Some returned samples showed separation between the heat-shrink tube and PVC tube at the bonding surface. The original heat-shrink tube was intended to provide waterproofing, however the effective adhesion area was small and the materials were flexible, making consistent sealing difficult.

Countermeasure

• The PVC tube is extended to cover the copper tube.
• The heat-shrink tube length is increased from 10 mm to 15 mm or more.
• Countermeasure parts passed thermal shock testing through 500 cycles without failure.
• The revised PVC tube covering specification was confirmed as manufacturable.
• Installation / attachment checks were also completed with no issue identified.

Product and Spare Parts Action

Service Guidance

• For ASTG12LVCC units presenting Error 99, inspect the thermistor and associated wiring before replacing major components.
• Where the thermistor is suspected, check for signs of moisture ingress, deterioration or separation of the protective tubing.
• If replacing the thermistor, ensure the replacement is the countermeasure version of spare part 9900563011.
• If the fault persists after thermistor replacement, continue normal Error 99 diagnostic checks including the PCB and associated sensor circuit.