Hayward HeatPro PO/OP Error: Open Water Temperature Sensor
Quick Summary
- PO (HeatPro) and OP (Summit) indicate an open water temperature sensor — the sensor circuit reads infinite or very high resistance.
- HeatPro uses a 10k ohm sensor; Summit uses a 4.8k ohm sensor — use the correct resistance chart for the model you are servicing.
- First check: inspect the sensor wiring for breaks before condemning the sensor itself.
- If sensor resistance matches the temperature/resistance chart, the control board is faulty — not the sensor.
What PO/OP Means
PO on a HeatPro display (or OP on Summit display) means the control board reads an open circuit from the water temperature sensor. The water temperature sensor is a thermistor — a resistor whose resistance changes with temperature. When the sensor circuit is open (infinite or very high resistance), the board cannot determine pool water temperature and shuts down operation to prevent overheating an unmonitored system.
It is important to distinguish between a sensor problem and a board problem before ordering parts. Both failures produce the same error code, but the fix is very different in cost and time.
HeatPro 10k Ohm Sensor Reference Values
Use a VOM set to at least 20k ohms. Measure resistance wire-to-wire at the sensor. Compare against these values:
| Water Temp (°F) | Expected Resistance (kΩ) |
|---|---|
| 90°F | 7.333 |
| 85°F | 8.249 |
| 80°F | 9.297 |
| 75°F | 10.5 |
| 70°F | 11.882 |
| 65°F | 13.473 |
| 60°F | 15.31 |
| 55°F | 17.435 |
| 50°F | 19.9 |
Use an accurate thermometer to measure actual water temperature at the point where the sensor contacts the water. Then look up the expected resistance. The sensor is good if your reading falls within approximately 10% of the table value.
Step-by-Step Troubleshooting
Step 1: Inspect the Sensor Wiring
Before testing the sensor itself, trace the sensor wiring from the sensor housing to the control board connector.
- Look for any pinched, chafed, abraded, or rodent-damaged sections of wire.
- Check that the connector at the control board is fully seated and the pins are not corroded or bent.
- A broken wire anywhere in the circuit will read as an open sensor. This is the easiest fix and the first thing to rule out.
Step 2: Measure Sensor Resistance
- Disconnect the sensor wiring from the control board.
- Set your VOM to the 20k ohm range or higher.
- Measure resistance wire-to-wire at the sensor end of the harness.
- If resistance reads infinite (OL, overload), the sensor is open — replace the sensor.
- If resistance is very high but not infinite, compare to the 10k ohm chart for the actual pool water temperature. A reading dramatically higher than the chart value for the current temperature indicates a failing sensor — replace it.
- If resistance matches the chart within 10%, the sensor is good and the problem is in the control board.
Step 3: Replace the Water Temperature Sensor
The water temperature sensor on HeatPro units is located in the water path — typically installed in a fitting on the water manifold. To replace:
- Turn off the heat pump and pool pump.
- Close isolation valves on either side of the heat pump if installed, or prepare to manage water flow.
- Unscrew the sensor from its fitting. Wrap the threads of the new sensor with PTFE tape and install snugly.
- Reconnect the wiring, restore water flow, and test operation.
Step 4: Replace the Control Board
If the sensor tests correctly against the resistance chart but PO/OP persists, the control board is misreading the sensor signal. Replace the HeatPro interface board (P/N 1102350901 or current equivalent — confirm with the model number on the data plate before ordering).
Frequently Asked Questions
What is the difference between PO/OP and Pc/SH?
PO/OP means the sensor circuit is open — very high or infinite resistance. Pc/SH means the sensor circuit is shorted — near-zero resistance. Both involve the water temperature sensor, but an open and a short require different diagnoses. Always check resistance first to determine which failure mode you have.
Can I confirm a bad sensor without a resistance chart?
Partially. If the VOM reads OL (overload/infinite) at the sensor terminals with wiring verified intact, the sensor is definitively open and should be replaced regardless of temperature. A resistance chart is necessary to catch a degraded sensor that reads high but not infinite.
Is there a way to verify the board is the problem without buying a new one?
You can temporarily substitute a known-good sensor with a matching resistance value (a resistor of the correct value for the current temperature) to confirm the board reads it correctly. If the board clears the PO/OP error with the substitute, you have confirmed the original sensor is bad. If the board still shows PO/OP with a known-good sensor in circuit, the board is the problem.
The HeatPro is on a Summit platform (HP50TA, HP21404T). Which sensor do I use?
The HP50TA and HP21404T are built on the Summit platform despite carrying the HeatPro name. These models use the Summit 4.8k ohm sensor, not the HeatPro 10k ohm sensor. Always confirm the platform before ordering parts or reading resistance charts.
Should I seal the sensor threads with anything other than PTFE tape?
PTFE tape is the standard for temperature sensor fittings. Do not use pipe dope or thread sealant compounds that can harden and make future removal difficult. Two or three wraps of PTFE tape applied in the thread direction is sufficient for a water-tight seal on sensor ports.