Temperature sensor
Common error-code part. Confirm resistance and connector style.
ClimateMaster Trilogy Geothermal Error Codes
ClimateMaster Trilogy Q units use the ClimateTalk communicating system with a color touchscreen thermostat for diagnostics. Fault codes display directly on the Trilogy thermostat. Non-communicating installations use LED flash codes on the control board.
Trilogy Fault Code Table
| Code | Fault Description | Common Cause | Severity |
|---|---|---|---|
| LO | Low-pressure fault | Low charge, low loop flow | Lockout |
| HI | High-pressure fault | Dirty coil, high EWT, overcharge | Lockout |
| FP | Freeze protection | Low EWT or low refrigerant | Lockout |
| CO | High discharge temp | Refrigerant problem | Lockout |
| OA | Over-amperage | Compressor overloading | Lockout |
| CC | Contactor check | Contactor failure | Lockout |
| EE | EEPROM error | Control board corruption | Lockout |
| FL | Flow fault | Low loop GPM | Warning |
| ST | Sensor fault | Temperature sensor failure | Warning |
| RS | Reversing valve stuck | RV or solenoid failure | Lockout |
| CM | Communication fault | ClimateTalk wiring | Warning |
Most Common Trilogy Faults
FP - Freeze Protection
Trilogy units monitor entering water temperature (EWT). FP activates when EWT approaches 32 degrees F. Check loop pump, pressure, and antifreeze mix. Propylene glycol mix should be tested annually with a refractometer.
LO - Low-Pressure Fault
Check loop flow first: Trilogy requires 1.5-3.0 GPM/ton depending on model. Low refrigerant charge also triggers LO - check subcooling and superheat before adding refrigerant.
HI - High-Pressure Fault
On closed-loop systems: EWT above 90 degrees F in cooling causes elevated discharge pressures. Check loop sizing and ground loop heat rejection. On water-to-water units: check heat exchanger for scaling.
FL - Flow Fault
ClimateMaster uses a flow switch that opens on low GPM. Verify loop pump operation, check for closed isolation valves, and purge air from system. Measure differential pressure across flow center.
Parts Commonly Needed
| Part | Notes |
|---|---|
| ClimateTalk thermostat | Required for fault code display |
| Control board | Match unit model and revision |
| Flow center pump | Match GPM requirement |
| TXV | Match refrigerant and model |
| Reversing valve | Match model spec |
| EWT sensor | Check resistance vs. temperature |
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Pro tip: ClimateMaster Trilogy logs operational data continuously. Use the ClimateTalk thermostat trend screen to review EWT, LWT, refrigerant pressures, and compressor amps over the past 24 hours before diagnosing.