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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
| SF CODE POSSIBLE CAUSE | SF MAIN FIXES |
| Check for faulty wiring | Inspect wiring on your sensor |
| Sensor is defective | Replace temperature sensor |
| Check control board | Replace the ICB. |
| Leaking water | Check for cracks in sensor coercing |
| Checking piping on pressure assembly | Replace copper corroded |
| Check thermistors | Replace defective thermistor |
Time For a New Hayward Pool Heater?
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 the 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
- Check for faulty wiring on the sensor. Ensure that wires are not broken or eaten up by rodents. Also, ensure the sensor is securely connected to the back of the control panel.
- For a defective sensor: You can find your sensor on the left side of your plumbing manifold.
- But it’s always essential to ensure that all the other wirings and the connections at the temperature control are all good before you consider making a replacement. Often, it’s just loose wire that could be causing the SF error. Measure the 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.
- Bad control Board: This can produce the SF code, if your heater is relatively new.
- Leaking water: In some models, the sensor has plastic coverings. When these crack, they can allow water to get inside; when this happens, they will read as a short. The good news, however, is that when the leaked water dries up, the sensor should get back to running normally.
- Over time the copper piping to the water pressure switch can get corroded. When this happens, it will leak water, eventually, corrode the thermistor wiring. To fix this problem, you must replace 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.
- For a defective thermistor, replacement is the solution. Here are a few steps to help you replace your thermistor.
- Replacement fd thermistor
- Fits hayward universal h-series low nox pool heater models h150fdn, h150fdp, h200fdn, h200fdp, h250fdn, h250fdp
- Also fits hayward universal h-series low nox pool heater models h300fdn, h300fdp, h350fdn, h350fdp, h400fdn, h400fdp
- Weight 4 pounds
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 standard 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
| BO CODE CAUSES | BO MAIN FIXES |
| Control module is in bypass operation | Press and hold the down button for 3s |
| Flipped dip switch | Put 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 b0 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 a remote thermostat is not controlling your heater, 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 b0 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. Suppose the return water temperature is less than what is set on the operation control. In that case, 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:
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.