Hayward Pool Heater LO Code [Reasons + Solutions]

Faulty water temperature switch, malfunctioning vent pressure switch, and defective temperature limit switch are accountable for displaying the Hayward pool heater LO code.

Also, the damaged flow switch diaphragm and clogged filter are responsible for showing this code on your pool heater. 

However, going through the article, you will learn all about reasons behind LO code and how to fix them by yourself. So, let’s start…

What Does LO Code Mean On The Pool Heater?

The limit string open error code in the pool heater is called the LO code. This code gives the signal that the heater’s limit string is open.

The open limit string forces the pool heater to go into lockout mode by turning it by itself. In simple words, The Lo code on the pool heater indicates the heater itself is facing low water pressure. 

Hayward Pool Heater LO Code [Reasons + Solutions]

When your pool heater fails to sense the proper water flow, it shows the LO error code. The main factors responsible for displaying this Hayward pool heater error codes and their easy fixing ways are the following: 

Note: Also read how to fix CE code on Hayward heater by yourself.

1. Water Temperature Switch Malfunction

One of the major culprits accountable for the Hayward pool heater code lo is the defective water temperature switch. The following factors are responsible for creating this problem:

  • The water pressure switch’s incorrect settings 
  • The malfunctioning water pressure switch itself
  • The pump is not running, or the faulty pump 
  • Defective electrical connections 
  • Poorly positioned valves 
  • Filthy filters 
  • The water temperature switch fails to work due to the inadequate water flow 
  • The water pressure switch contacts are not working 
  • Low pump pressure 
  • The water pressure switch’s bad connection 


First, check whether the malfunctioning water temperature switch is responsible for displaying the Hayward pool heater error code e03 or not.

If so, replace the faulty water temperature switch. And follow the fixing ways below to resolve the Hayward pool heater error codes lo issue. 

  • Ensure the correct settings of your pool heater’s water pressure switch 
  • Inspect whether the water pump is perfectly running or not 
  • Check the heater’s electrical connections and repair or replace the corroded power connection 
  • Ensure the valve’s proper position 
  • Properly clean the filters 
  • Ensure adequate water flow so that the water temperature switch functions perfectly 
  • Check the water pressure switch’s contacts and ensure that they are working correctly 
  • Make sure the pump pressure is adequate or perfect 
  • Ensure the water pressure switch’s perfect connection 

2. Defective Vent Pressure Switch

The vent pressure switch in your Hayward pool heater ensures and determines the incoming water flow proper amount.

Thus if the vent pressure switch is faulty, you will see the Hayward h400 pool heater lo code on the heater’s display. The main concerning causes of it are:  

  • Faulty connection and wiring issue 
  • Vent pressure switch’s risky contacts 
  • Restricted or blocked flue 
  • The malfunctioning vent pressure switch itself 
  • Vent pressure switch’s harness terminals and spade terminals have an incorrect and insecure connection 


Inspect your Hayward pool heater’s vent pressure switch and replace the malfunctioning switch. The other general fixes to the above troubles are: 

  • Locate your heater’s vent pressure switch and ensure its perfect connection and wiring 
  • Verify the vent switch’s contact integrity 
  • Ensure the appropriate size of your heater’s vent and remove the flue’s blockages
  • Replace the defective vent pressure switch as its repairing is not possible 
  • Ensure correct and secure connection between the vent pressure switch’s harness terminals and spade terminals

3. Flawed Temperature Limit Switch

The temperature limit switch malfunction is another reason for which you see the Hayward pool heater lo failure code.

The temperature limit switch ensures the safe limit of your pool heater’s temperature. The concerning factors responsible for this issue are: 

  • Temperature limit switch’s faulty wiring 
  • Limit switch’s Fragile contacts  
  • The temperature limit switch itself is not working 
  • Inadequate water flow 
  • Loose connection of the temperature limit switches’ spade terminals and harness terminals 


Replace your pool heater’s faulty temperature limit switch. And in general, the Hayward pool heater lo code fix requires performing the following:

  • Check the switch’s wiring and connection issue and correct them 
  • From the limit switch and jumper, leads remove the wire leads and verify the temperature limit switch’s contact 
  • Ensure adequate water flow of 20, 25, and 30 GPM 
  • Tighten the connection of the temperature limit switches’ spade terminals and harness terminals

4. Failed Flow Switch Diaphragm

The pressure switch’s inside contains a diaphragm. After reaching a particular system pressure, the diaphragm stops the motor by pushing against the mechanism. If the flow switch diaphragm is defective, you will see the Hayward heater lo code on display. The factors responsible for this are:

  • The open pressure relief valve 
  • Defective check valve 
  • Improper air line pressure and size 
  • Fluid line restrictions 
  • Frequent opening and closing of pressure valves 


Replace your Hayward pool heater’s malfunctioning flow switch diaphragm. And follow the other solutions below: 

  • Make sure the pressure relief valve is close 
  • Replace the faulty check valve 
  • Ensure proper air line pressure and size
  • Make sure there are no fluid line restrictions in the pump
  • Avoid the pressure valve’s frequent opening and closing 

5. Clogged Filter

A dirty or clogged filter is the most common reason for which you may see the Hayward gas pool heater lo code.

The dirty cartridges, DE filter’s breakdown, or the filthy sand filter is responsible for this error code. The dirty filter causes a low flow of water. 


Properly clean your pool heater’s water filter. Also, thoroughly clean the cartridges and backwash the heater’s sand filter several times.

Then run the heater for at least 45 minutes, and the LO code will disappear. A clean filter will surely increase the water flow and remedy the problem. 


What does AO mean on a Hayward Pool heater?

The AO means the blower vacuum switch open error in the Hayward pool heater. After starting the blower, if the blower prover switch remains open, the control will go into lockout after stopping the ignition trial. After closing the button, the blower will run again after an automatic reset. 

Why does my Hayward pool heater keep shutting off?

Your Hayward pool heater keeps shutting off mainly because of the low water pressure, dirty filter, heater’s defective pressure switch, or inappropriate temperature setting. 

How do I unlock my Hayward pool heater?

First, reset your Hayward pool heater’s electrical circuit breaker. Turn off the breaker switch and then flip the switch again in the “ON” position. Remove the digital control panel’s cover. Now push the button “Mode” and select the heater’s “Standby” setting. 

Why does my Hayward pool heater keep turning on and off?

Your Hayward pool heater keeps turning on and off mainly due to the faulty exchanger, blocked vent, improper combustion, excess flames or heat, or defective heater itself. Also, dirty filters, insufficient water flow, and closed valves cause this trouble. 

Why won’t my pool heater fire up?

Your pool heater won’t fire up due to the dirty pool filter, restricted or inadequate water flow, faulty pump basket, etc. Also, insufficient air supply or inadequate venting can hinder your pool heater’s fire. 


Now you know all the reasons behind the Hayward pool heater lo code and their solutions. So, whenever you notice the LO code on the heater display, simply follow our guide and fix it yourself.

If you have any more queries, you can let us know in the comment box. We will be happy to help you.

About William

William is the founder of Fireplacehubs.com. He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

Leave a Comment