Hayward ProLogic Temperature Sensor Errors (Water & Air)

How ProLogic Temperature Sensors Work

The ProLogic uses NTC (negative temperature coefficient) thermistors for both water temperature and air temperature sensing. As temperature rises, resistance drops — this is the opposite of most resistors. The main PCB supplies 5VDC to the sensor circuit at terminal B. As the sensor's resistance changes with temperature, the voltage measured across it changes proportionally.

Two error types cover sensor faults:

• "Wtr Sensor Short" (or Air Sensor Short): The sensor reads near-zero resistance, which would correspond to an impossibly high temperature. Usually caused by a water-damaged sensor, shorted lead wires, or a wiring fault where the two sensor wires are making contact.
• "Wtr Sensor Open" (or Air Sensor Open): The sensor reads infinite resistance, as if the circuit is broken. Usually caused by a broken sensor element, a broken wire in the lead cable, or a disconnected connector at terminal B.

Either fault will trigger the Check System LED and disable heating. The ProLogic cannot control water temperature without a functioning water sensor.

Step-by-Step Troubleshooting

Owner-Level Checks

1. Confirm the Check System message

• Press Menu until "Default Menu" appears on the display.
• Navigate with (>) until you see the specific fault message — "Wtr Sensor Short," "Wtr Sensor Open," "Air Sensor Short," or "Air Sensor Open."
• Note which sensor is faulting — water and air sensors are separate devices and must be diagnosed independently.

2. Inspect the sensor and wiring

• Locate the water temperature sensor — it is typically installed in a sensor port on the plumbing near the pool equipment pad, in the return line downstream from the filter.
• Locate the air temperature sensor — it is mounted externally to the ProLogic enclosure, sensing ambient air temperature for freeze protection.
• Inspect the sensor bodies for physical damage, corrosion at the connector, or signs of water intrusion into the connector housing.
• Trace the sensor cables back to the terminal B connector on the ProLogic PCB. Confirm the connector is fully seated.

Tech-Level Checks

Method 1: Resistance test (sensor disconnected)

• Turn off the main breaker and disconnect the sensor cable from terminal B on the PCB.
• Set your multimeter to resistance (ohms) and measure across the two sensor wire terminals at the sensor connector.
• Compare the reading to the temperature-resistance chart below. The sensor should read within a reasonable range for the current ambient or water temperature.
• If resistance is near zero (short): the sensor element or wiring is shorted — replace the sensor.
• If resistance is infinite (open): the sensor element or wire is broken — check the wire run first, then replace the sensor if the wire is intact.

Method 2: Voltage test (sensor connected, power on)

• Restore power. With the sensor connected, locate terminal B on the PCB.
• Using a multimeter set to DC voltage, measure across the two sensor terminals at the PCB (pins on terminal B).
• Compare the voltage reading to the voltage column in the chart below.
• If voltage is near 5V (at a normal pool temperature): the sensor is reading very high resistance — sensor is open or disconnected.
• If voltage is near 0V: the sensor is reading near-zero resistance — sensor is shorted.

Sensor Resistance and Voltage Quick Reference

Selected values from the Hayward ProLogic Air/Water Sensor chart (source: TSG-PL447c). Use ambient thermometer or known water temp to find the expected resistance or voltage:

Temperature (°F)	Expected Resistance (Ohms)	Expected Voltage (VDC)
50°F	19,900	3.33
60°F	15,310	3.02
65°F	13,473	2.87
70°F	11,882	2.72
75°F	10,500	2.56
80°F	9,297	2.41
85°F	8,249	2.26
90°F	7,333	2.12
95°F	6,530	1.98
100°F	5,827	1.84
104°F	5,326	1.74

The sensor uses a significant temperature-resistance relationship — at 50°F you should read approximately 19,900 ohms, and at 104°F approximately 5,326 ohms. If your reading is within 10–15% of the expected value, the sensor is likely functional and the wiring or connection is the issue. If it is significantly outside range, replace the sensor.

Testing the sensor terminal block voltage (5VDC supply)

• With the sensor disconnected from terminal B, restore power.
• Measure DC voltage across the two sensor terminal pins at position B on the PCB.
• You should see approximately 5VDC (the sensor supply voltage from the main board).
• If no or significantly low voltage: the sensor supply circuit on the main board has failed. Replace the main board (GLX-PCB-PRO).
• If 5VDC is present and a known-good sensor still generates an error, replace the main board.

Sensor Replacement Notes

Both the water and air sensors are the same thermistor assembly — they are interchangeable from the ProLogic's perspective. The difference is how they are mounted: the water sensor installs in a 1/4" NPT thermowell on the plumbing, while the air sensor mounts in an enclosure outside the panel. Confirm you have the correct sensor housing for each position when ordering replacements.

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