When an LO code appears on your Hayward pool heater, you are not alone. The LO code is one of the most common problems faced by the owner. The LO error commonly prevents the igniter from turning on, hence disrupting the entire pool schedule. The good news? LO is typically a safety or flow-related warning, and most causes can be fixed yourself—no technician is required.
This is a comprehensive guide that explains what the LO code means, the specific problems that trigger it, and provides step-by-step fixes for Hayward H-Series, Universal H-Series, and other Hayward models.
What is the LO code on a Hayward heater?
Hayward LO code denotes Low Water Flow or Low Pressure. Your heater utilizes a pressure switch and internal sensors that monitor the water flow through the system. When the flow drops below a safe threshold, the heater will lock out the ignition, preventing overheating or internal damage.
In other words:
LO =The heater is not getting enough water flow.
Result → Heater turns off or won’t ignite.
Low water flow could be a problem caused by dirty filters, clogged pump baskets, closed valves, blocked plumbing, or a failing pump.
Also read: Gree Mini Split Troubleshooting.
Symptoms that come with the Hayward LO code
You may notice one or more of the following:
- Heater turns on but will not ignite
- Heater switches off after a few seconds.
- Pump speed drops too low
- Water temperature does not rise
- You hear clicking, but no burner will light
- System keeps rebooting
- Pressure reading is lower than normal
These symptoms confirm your system has a flow or pressure problem.
Top 12 Causes of the Hayward Heater LO Code (With Fixes)
Below are the most common reasons your Hayward heater displays the LO error—and exactly how to fix each one:
1. Dirty Or Clogged Pool Filter
The #1 cause of low water flow is a dirty filter.
Signs
- Pressure gauge reading higher than usual
- Pump loses prime
- Heater turns on/off intermittently
Fix
- Backwash your sand filter
- Clean or replace the cartridge.
- Deep clean DE grids
Restart your system after cleaning and check if LO code clears.
2. Low Pump Speed (Variable-Speed Pumps)
If you have a variable-speed pump and it is running too slowly, the heater will not get enough water flow.
Fix
- Increase RPM to at least 2500–3000 RPM (this level is required for most heaters)
- Create a dedicated “Heater Mode” speed setting.
Heaters need a good, solid flow to operate safely.
3. The pump basket is clogged.
Leaves, grass, and other debris in the pump basket cause reduced flow and initiate the LO code.
Fix
- Turn off the pump
- Open the pump lid
- Take off the basket and wash it well
- Grease the O-ring and then reassemble
This often resolves the error instantly.
4. The Skimmer Basket Is Clogged
Water cannot reach the pump if the skimmer is clogged.
Fix
Clean the leaves, bugs and debris out of the skimmer and make certain that the weir door does not stick.
5. Air Leaks in the Suction Line
Even minor air leaks reduce water flow and cause the heater to display LO.
Common leak locations
- Pump lid O-ring
- Cracked pump lid
- Suction-side plumbing fittings
- Skimmer connection
Fix
- Replace worn O-rings
- Re-lubricate pump lid
- Check and tighten unions
- Seal small leaks with pool putty
If you notice air bubbles in the pump, this is likely the cause.
6. Wrong Valve Position
Multiport or diverter valves may be turned inadvertently, sending water to circuits other than the heater.
Correct
- Ensure that valves are in the “Pool”, “Filter”, or “Heater ON” position.
- Improperly installed valve orientation will immediately cause flow issues.
7. Heater Inlet or Outlet is blocked
Debris may collect inside the heater manifold or plumbing.
Fix
- Power off
- Open heater unions
- Flush your heater with a hose
- Make sure that nothing obstructs
The LO error should clear if the water flow improves.
8. Faulty Pressure Switch
The water flow is sensed by the heater’s pressure switch. If this fails, the heater may show LO even if the flow is normal.
Fix
- Check wiring to the pressure switch
- Check for corrosion or loose connectors
- Replace the pressure switch if defective
This part is inexpensive and easy to replace.
9. Internal Scale Buildup
Hard water produces calcium scale inside the heat exchanger, which restricts flow.
Fix
10. Pump Losing Prime
Water will not reach the heater in case the pump is sucking air or running dry.
Fix
- Fill pump with water
- Re-seal the pump lid
- Eliminate air leaks
- Ensure the water level in the pool is high enough.
Water level should be mid-skimmer opening.
11. Clogged Pipes or Suction Lines
Underground pipes are often blocked by debris, leaves, or even small animals.
Fix
- Use a drain king/bladder to flush the lines
- Backwash plumbing
- Call a technician if blockage is deep underground
12. Inadequate or Failed Pool Pump
If the pump motor is dying, water flow will be too weak for the heater.
Correct
- Check for humming or overheating motor
- Replace pump if faulty
- Heaters require strong and consistent flow, and a weak pump cannot support them.
How to Reset Hayward LO Code: A Simple Procedure
After addressing the root cause, restart your heater:
- Turn off the heater
- Shut off the pump
- Wait 1 minute
- Turn on the heater.
If the LO code clears, your problem is fixed. If not, proceed with troubleshooting
Advanced Troubleshooting for Persistent LO Errors
If the above fixes didn’t help, here are deeper checks:
1. Test the Pressure Switch
Check for continuity with a multimeter when the pump is operating.
No continuity = bad switch.
2. Heater Bypass Valve Check
Some systems may have a bypass which is stuck partially closed.
3. Search for collapsed or damaged pipes
Underground suction pipes can collapse over time, which reduces flow.
4. Check Heater’s Internal Filters (If Equipped)
Some Hayward models do have internal screens that clog.
5.The digital flow sensor can fail and cause faulty LO codes.
How to Avoid Hayward LO Code in the Future
To avoid recurring LO issues:
- Clean filters regularly
- Run pump at proper RPM
- Keep baskets clear
- Maintain appropriate water level
- Check for leaks monthly
- Descale once a year.
- Schedule annual pool equipment servicing
Simple maintenance will keep your heater running efficiently.
FAQs About Hayward Heater LO Code
1. What does LO mean on a Hayward heater?
It means Low water flow, caused by blockages, low pump speed, or pressure switch issues.
2. Can I use my heater with the LO code?
No, the heater will lock out ignition for safety.
3. Why does my heater click but not ignite?
Poor flow prevents ignition, and thus the heater keeps trying but shuts down.
4. How do I know whether my pressure switch is bad?
If the flow is strong and yet you are still seeing LO, test or replace the switch. 6. Does a dirty filter cause the LO code? Yes- it’s one of the most common triggers.
Final Thoughts
When your Hayward heater gives the LO code, the problem is almost always either low water flow or failure of a pressure switch. Most problems can be resolved following the steps in this guide: cleaning filters, increasing pump speed, clearing baskets, checking valves, and inspecting the heater-without having to call a technician. If the problem continues after troubleshooting, you may be dealing with an internal heater issue or bad sensor that requires professional service.
