Jandy LXi Water Temperature Sensor Failure: FAULT-SHORTED/OPEN H2O SENSOR

What the Water Sensor Faults Mean

The LXi uses a thermistor (temperature-sensitive resistor) to measure water temperature. The electronic controller reads the thermistor resistance to determine current water temperature and decide whether to call for heat based on the set point. If the thermistor circuit is broken or shorted, the controller cannot determine water temperature and displays the appropriate fault.

FAULT-OPEN WATER SENSOR indicates infinite resistance — the thermistor wire is broken, disconnected, or the sensor element itself has failed open. FAULT-SHORTED H2O SENSOR indicates near-zero resistance — the thermistor wires are touching each other or the sensor element has failed shorted.

Step-by-Step Troubleshooting

Owner-Level Checks

Power cycle the heater

1. Turn the heater control to OFF, wait 30 seconds, and restart. Occasionally a transient electrical issue can trigger the fault.
2. If the fault clears and the heater operates normally, monitor for recurrence.

Tech-Level Checks

Inspect the sensor wiring

1. Locate where the temperature sensor connects to the Power Interface Board inside the heater.
2. Ensure the sensor connector is firmly seated in the board. A loose connection is one of the most common causes of an open sensor fault.
3. Inspect the sensor wires for damage, chafing, or pinching, especially where they route near hot components.
4. Look for corroded or burnt terminals at the connector.

Test the thermistor

1. Disconnect the sensor from the Power Interface Board.
2. Measure resistance across the sensor leads with a multimeter. At typical pool water temperature (75–85°F), a working thermistor should read in the range of several thousand ohms.
3. For an open sensor fault: if the meter reads OL (infinite), the sensor or wiring has failed. For a shorted sensor fault: if the meter reads near zero ohms, the sensor or wiring is shorted.
4. If the sensor reads correctly when disconnected, the Power Interface Board itself or the wire harness may be faulty.

Check for water intrusion

1. Water leaking into the sensor well or onto the wiring can cause short circuits. Inspect the sensor well seal.
2. Condensation inside the heater cabinet during cold startups can cause intermittent shorts on exposed terminals.

Common Parts That Fix This Problem

• Water temperature sensor (thermistor)
• Sensor wire harness
• Power Interface Board (if sensor tests good but fault persists)

Model-Specific Notes

• Both the LXi 250 and LXi 400 use the same temperature sensor and controller.
• If you replace the sensor and the fault persists, the Power Interface Board is the next suspect.

How to Prevent Sensor Failures

• During annual service, inspect the sensor connection at the Power Interface Board for corrosion or looseness.
• Keep the heater cabinet dry. Fix any water leaks around pipe connections promptly.
• Route sensor wires away from heat sources inside the cabinet.
• Maintain proper water chemistry to prevent corrosion of the sensor well and seal.