Pentair Pool Heater Error Code 18: How to Fix It

Q: What does error code 18 mean on a Pentair pool heater?

Error code 18 on a Pentair pool heater indicates a water temperature sensor (thermistor) fault. The heater's control board is receiving a signal from the sensor that is outside the acceptable range — either an open circuit, a short circuit, or a reading that suggests the sensor has failed. The heater shuts down to prevent unsafe operation until the issue is resolved.

Q: Can I still use my pool while error code 18 is displayed?

Yes, your pool is safe to swim in, but your heater will not operate while displaying error code 18. The pool water will not be heated until the fault is cleared. If you need warm water, you will need to diagnose and resolve the error before the heater will run.

Q: How much does a Pentair pool heater thermistor cost?

A replacement water temperature sensor (thermistor) for a Pentair pool heater typically costs between $15 and $50 for the part alone, depending on the specific model. If you hire a pool technician, expect to pay an additional $75–$150 or more in labor. Replacing the sensor yourself is the most cost-effective approach if you are comfortable with basic repairs.

Q: Why does my Pentair heater keep showing error code 18 after replacing the sensor?

If error code 18 returns after replacing the thermistor, the issue is likely elsewhere. Common causes include a damaged wiring harness between the sensor and the control board, a corroded connector, low water flow through the heater, or a faulty main control board. Check all wiring carefully, confirm adequate pump flow and clean filters, and if all else fails, have a technician evaluate the control board.

Q: How do I test a Pentair pool heater thermistor with a multimeter?

Disconnect the thermistor from the control board completely. Set your multimeter to measure resistance in ohms (Ω). Touch one probe to each of the two sensor terminals. At room temperature (around 77°F / 25°C), a functional Pentair thermistor should read approximately 10,000 ohms (10 kΩ). An OL reading (open loop) or a reading near zero ohms means the sensor has failed and needs to be replaced.

Abstract

Error code 18 on a Pentair pool heater indicates a fault with the water temperature sensor, also known as the thermistor. This sensor monitors the water temperature entering and exiting the heater, and when it fails or sends an out-of-range signal, the heater shuts down as a safety precaution. The fix typically involves inspecting the sensor wiring, testing the sensor with a multimeter, and replacing the thermistor if it has failed.