Jandy JXi Water Sensor Failure: Shorted and Open Sensor Troubleshooting

What Water Sensor Faults Mean

The JXi uses a thermistor (temperature-dependent resistor) to measure water temperature. The control board reads the sensor resistance and converts it to a temperature value. This reading controls when the heater fires and shuts off based on the thermostat set point.

A shorted sensor reads near-zero resistance, which the board interprets as an impossibly high temperature. A shorted condition can come from the sensor itself failing, water getting into the sensor housing, or wiring that has become corroded and is creating a short circuit.

An open sensor reads infinite resistance, which means the circuit is broken. This can happen when the sensor lead wire is cut, a connector has come loose, the sensor plug at the PIB (Power Interface Board) has backed out, or the thermistor element inside the sensor has failed open.

In either case, the heater cannot determine water temperature and will not fire.

Step-by-Step Troubleshooting

Owner-Level Checks

Check for obvious wiring issues

1. Open the heater control panel and look for any disconnected wires or connectors that have come loose.
2. Check for signs of water intrusion inside the control compartment: corrosion on terminals, water stains, or green/white deposits on connectors.
3. If you see obvious damage or disconnected wires, do not attempt to reconnect them yourself. Call a technician.

Tech-Level Checks

Test the water temperature sensor

1. Locate the water temperature sensor. It is typically a probe inserted into the heat exchanger header with two lead wires running to the PIB.
2. Disconnect the sensor leads at the PIB connector.
3. Measure resistance across the two sensor leads with a multimeter.
4. The water temperature sensor is a thermistor whose resistance changes with temperature. Resistance decreases as temperature rises. A qualified technician can compare the sensor reading to the manufacturer's resistance chart to determine if the sensor is within spec.
5. If the reading is near zero ohms, the sensor is shorted. If the reading is infinite (OL), the sensor is open. Replace the sensor in either case.

Check the wiring between sensor and PIB

1. If the sensor itself tests good (correct resistance for current water temperature), the problem is in the wiring.
2. Inspect the wire run from the sensor to the PIB for damage: chewed by rodents, pinched by panels, corroded connections.
3. Check the connector at the PIB for corrosion, bent pins, or a loose plug. Clean contacts with electrical contact cleaner and reseat.
4. Test continuity of each wire from the sensor end to the PIB end to identify any break in the wire.

Check the PIB (Power Interface Board)

1. If the sensor and wiring both test good, the fault may be on the PIB itself.
2. Inspect the PIB for visible damage: burnt components, corroded traces, or water damage.
3. If the PIB is damaged, it will need replacement.

Common Parts That Fix This Problem

• Water temperature sensor (thermistor)
• Sensor wiring harness
• PIB connector or plug
• PIB (Power Interface Board) if damaged

How to Prevent Water Sensor Failures

• Keep the heater control compartment sealed against water intrusion. Replace any damaged gaskets or seals.
• Inspect wiring connections at each pre-season startup for signs of corrosion.
• Avoid pressure washing the heater control panel area.
• Keep rodents away from the heater with appropriate pest control around the equipment pad.