Hayward Heater SF & bO Code: (Causes & Fixes)

An SF Hayward heater code is a sensor failure code that indicates problems with the sensor communicating with the control board. Typically, Your Hayward heater will have one main sensor with two thermistors inside.

The two thermistors are for data reliability and help the system to easily throw out errors if the readings don’t match or if the readings are “out-of-bounds”.

The SF code on your Hayward heater will typically be triggered when there is an excessive temperature difference between the two thermistors (5 °F or more). The code can also be caused by an ‘out of bounds’ condition on both thermistors (less than 10°F or greater than 180°F.)

Below is a chart for SF code possible causes and fixes

Check for faulty wiringInspect wiring on your sensor
Sensor is defectiveReplace temperature sensor
Check control boardReplace the ICB.
Leaking waterCheck for cracks in sensor coercing
Checking piping on pressure assemblyReplace copper corroded
Check thermistorsReplace defective thermistor


How to diagnose Hayward Heater SF code

1.       Check for faulty wiring in your sensor connection. Corroded sensor wiring could trigger the SF code.  Inspect Temperature Sensor (thermistor) wire. Make sure the sensor is plugged into the ICB securely. Otherwise, if the sensor wiring connections seem to have no problem, go to the next step.

2.       Measure resistance on the temperature sensor

3.       Check the control board and replace it if necessary

4.       Find out if there is leaking water to sensor wiring

5.       Check in the pressure assembly for corroded copper piping

6.       Ensure that you test your thermistors for any issues

SF Code Fixes

  1. Check for faulty wiring on the sensor. Ensure that wires are not broken or eaten up by rodents. Also, make sure that the sensor is securely connected to the back of the control panel.
  2. For a defective sensor: You can find your sensor on the left side of your plumbing manifold.
  3. But it’s always important to ensure that all the other wirings and the connections at the temperature control are all good before you consider making a replacement. A lot of times, it’s just loose wire that could be causing the SF error. Measure resistance between the black wire and each red wire, the Temperature Sensor resistance should be the same (10k ohms at 77° F). If readings are significantly different from each other, replace the temperature sensor.
  4. Bad control Board: This is usually if you have the SF code, but your heater is relatively new.
  5. Leaking water: In some models, the sensor is made of plastic coverings. When these crack, they can allow water to get inside and when this happens, they are going to read as a short. The good news however is that when the leaked water dries up, the sensor should get back to running normally.
  6. Over time the copper piping to the water pressure switch can get corroded. When this happens, it will end up leaking the water which will eventually corrode the wiring of the thermistor. To fix this problem, you’ll need to replace both the copper piping (of the pressure assembly) and the thermistor. It should be noted that copper piping and the thermistor are only accessible if you take 2 panels off the manifold.
  7. For a thermistor that is defective, replacement is the solution. Here are a few steps to help you replace your thermistor.

i)         Turn pump, gas supply, and heater power “OFF”.

ii)       Drain off the heat exchanger of all the water.

iii)     Remove the front access panel. (If it’s not accessible take off the 2nd panel)

iv)     Unplug the thermistor connector from the ignition control board.

v)       From outside the heater, pull the thermistor leads out of the control box and through the hole in the

Having looked at the SF code, it is also worth looking at another common Hayward Heater code, which is the BO code.

What causes the BO code on a Hayward Heater?

A BO code on your pool heater indicates a bypass operation. When you are using a remote thermostat to control temperature, a BO code will normally display on your heater, and this is normal. However, a BO can also be triggered if the dip switch is flipped to the “on” position even if you are not using an external controller.

Chart for Hayward heater BO code causes and possible fixes

Control module is in bypass operationPress and hold the down button for 3s  
Flipped dip switchPut it in the off position

Before you start diagnosing your heater, take a look at the back of the control module.  There are some “dip switches”. These switches have an on or off position.

Check switch number 2 and make sure it is in the “off” position. The “on” position is for a remote thermostat. On the “on” position, your heater will display the BO code.

How to diagnose and fix the bO code

It should be noted that the Bo code in most cases is a normal display when you have an external thermostat for controlling your heater.

In that case, you don’t need to worry about fixing the code. In short, no service is required when you are using an external thermostat. But If your heater is not being controlled by a remote thermostat, then you can try to remove this code by changing the setting by using the MODE key to put the heater into STANDBY.

This is how you remove your heater from bypass mode

1)      Press and hold the DOWN key and

2)      then press and hold the MODE key.

3)      Hold down both keys for 3 seconds until the indication ‘‘bO’’ is removed from the display.

How to put your heater in Bo Mode

1.       Press the Mode button to place the heater in Standby Mode

2.       Hold the Down Arrow and Mode buttons simultaneously for 3 seconds for Bypass operation. “bo” will be displayed on the screen.

3.       Heater must then be in the Spa or Pool Mode for operation. The heater will fire when instructed by the external control.


What does a high limit switch do on a pool heater?

The high limit switch on a Hayward pool heater is a safety device that is wired in series with the gas valve and the thermostat. If the water temperature going into the pool heater exceeds the maximum set, the gas valve will close, shutting off gas to the burners.

Unusual high-limit operations are a result of problems with the flow of water. Reduced water flow may be caused by

1.       Excessive water flow through the external bypass (if one is used)

2.       Dirty filters

3.       Limescale accumulation in the heat exchanger.

What does a thermistor do in a pool heater?

A thermistor in your Hayward pool heater checks the inlet water temperature. If the return water temperature is less than what is set on the operation control, a thermistor will send a message to the control module to start a heating cycle through a series of safety interlocks.

Other Hayward Heater Error Codes: