Daikin McQuay AGZ Scroll Chiller Alarm LOC: Loss of Charge — Leak Detection & Compressor Protection
Published: 2026-06-11
🔍 Symptoms Checklist
- ⚠️ One refrigerant circuit locks out; chiller runs on remaining circuit(s) at reduced capacity
- ⚠️ MicroTech controller displays 'LOC — Circuit 1 Loss of Charge'
- ⚠️ Suction pressure reads below 25 psig on R-410A systems during operation
- ⚠️ Bubble in sight glass during operation (not just startup)
🛠️ OEM Replacement Parts
| Part Name | OEM Part Number | Est. Price |
|---|---|---|
| Low Pressure Transducer (0-200 psig, 0-5V output) | DMQ-SEN06610 |
$210-280 |
| Schrader Core Removal Tool + Replacement Cores (kit of 5) | DMQ-TL00012 |
$60 |
| R-410A Refrigerant (25 lb cylinder) | N/A (supply house) |
$180-220 |
📋 Interactive Diagnostic Procedure
Click each step to expand detailed diagnostic instructions. Follow in sequence — each step builds on the previous one.
Frequently Asked Questions
How do I confirm it's actually low on charge vs a bad transducer?
Attach a calibrated gauge to the suction Schrader port and compare to the MicroTech displayed reading. If the gauge reads 65 psig but MicroTech shows 25 psig, the transducer has drifted and the circuit is fine — replace the transducer. If both read 25 psig, the circuit is genuinely low. Check subcooling: less than 5°F subcooling confirms low charge. More than 15°F subcooling suggests a restriction, not a leak.
The LOC alarm keeps coming back every few weeks but I can't find the leak. What am I missing?
Three possibilities in order of likelihood: (1) microchannel coil pinhole from impact — use UV dye, not soap bubbles; (2) Schrader core leaking at the service port — a $2 core causes thousands in refrigerant loss; replace all cores during any service call; (3) evaporator tube leak — test the water side with a refrigerant sniffer after the chiller has been off for 30 minutes (refrigerant migrates to water side when static).
🔗 Related Error Codes — Scroll Chillers
Other diagnostic guides in the same equipment category. Common failure patterns often share root causes — cross-referencing speeds up diagnosis.
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⚡ Fault-Induced Energy Waste Estimator
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References & Industry Standards
- ASHRAE 15 — Safety Standard for Refrigeration Systems
- ASHRAE 34 — Refrigerant Designation & Safety Classification
- AHRI 550/590 — Water-Chilling & Heat Pump Packages
- SMACNA — HVAC Duct Construction Standards
- Manufacturer Service Manuals — Carrier, Trane, York, Daikin, Lennox