Hayward LO means the flow switch did not close because water flow through the heater is below the 20 GPM minimum. A pump not running at heating speed is the most common cause for variable-speed pump owners. A clogged filter is the next most common.
I've seen too many homeowners replace a perfectly good flow switch when the real culprit was a dirty filter or a pump running too slow. The LO code is a safety lockout designed to keep your heat exchanger from melting or cracking. Think of it as a watchdog. If your variable-speed pump's set below 2400 RPM, or your filter pressure is up by 10 PSI, that watchdog is just doing its job.
HaywardPoolheaterSeverity: high85% DIY Success
Time to Fix
15–60 min
Difficulty
beginner
Parts Cost
$20 – $50
Tools Needed
Multimeter with continuity mode, Pool filter pressure gauge
What Does the LO Code Mean?
You usually see this pop up right after a heavy pollen season or when someone messes with the pool valves. Since Hayward heaters need a steady 20 to 30 gallons per minute to stay happy, any restriction in the plumbing triggers this fault. It's rarely a part failure and almost always a maintenance issue you can fix in five minutes without a single tool.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Variable-speed pump running at too low an RPM for heating mode40%
Clogged pool filter creating high resistance24%
Heater bypass valve open or partially open14%
Flow switch failure12%
Pump basket or strainer full of debris10%
Symptoms You May Notice
LO flashing on the control panel, usually within the first 10 seconds of a heat attempt before the igniter even clicks once.
Heater won't fire at all. You hear the board click on, then silence. No ignition sequence, no burner noise, nothing.
Worked perfectly yesterday, now showing LO right after you cleaned the filter or touched a plumbing valve.
Heater fires and warms the water for a few minutes then suddenly cuts out with LO, which usually means your flow is borderline and something is pinching off slightly once the pipes warm up.
Pump sounds totally normal and returns are pushing water into the pool just fine, but the heater still refuses to start.
Can you reset a Hayward poolheater to clear the LO code?
Most Hayward models clear the LO code automatically the moment they detect proper flow again. To force a fresh check, press the Mode button to cycle to Standby, wait 10 seconds, then press Mode again to switch back to Heat. If you've got a variable-speed pump, let it run at full speed for at least a full minute before expecting the code to clear. The heater literally needs that water pressure there to push the flow switch paddle closed before it'll agree to fire.
Tools Required for Diagnosis
Multimeter with continuity modePool filter pressure gaugePhillips #2 screwdriverGarden hose with spray nozzle for rinsing cartridge filterBucket or old towels for water spillage when opening pump basket
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Flow switchCHXFLO002 · $20–$50
CHXFLO002
$20 – $50
Frequently Asked Questions
What does LO mean on a Hayward pool heater?
Think of LO as a low flow warning. Inside your Hayward heater there's a small plastic paddle called a flow switch. When the pump pushes enough water through, it moves that paddle and tells the control board it's safe to fire the burners. If the water flow's too weak to move that paddle, the heater stays off to protect itself. It's not being difficult. It's literally just saying 'I don't have enough water moving through me to run safely right now.'
What is the minimum flow rate for a Hayward H-Series pool heater?
Most H-Series units need at least 20 to 25 gallons per minute. In the field, I look for 2500 RPM or higher on variable-speed models to be safe. If you've got a big 400k BTU unit, you really want closer to 30 GPM. Anything less and the heater might cycle on and off or just throw the LO code right away. When in doubt, run the pump faster than you think you need to. More flow is always better for the heat exchanger anyway.
Why does my Hayward heater show LO only when the filter is dirty?
It comes down to resistance. As your filter packs up with dirt and debris, the pump has to work way harder to push water through it. That extra resistance lowers the actual volume reaching the heater. I tell my customers that if their pressure gauge is 10 PSI over the clean baseline, they're basically begging for a LO code. The pump's still spinning and the returns are still flowing, but the heater's getting just barely enough to trip its own safety lockout.
Can a bad flow switch cause LO on a Hayward heater?
It happens, but honestly I'd say 90% of the time the switch is just doing its job and reporting a real flow problem. Before you spend money on a new switch, try running the pump without the filter cartridge installed for a quick minute. If the LO code clears, your switch is fine and your filter's just dirty. If it still says LO with no filter and the pump cranked all the way up, then it's time to test the switch with a multimeter before you replace it.
Will a LO code damage my Hayward pool heater?
The LO code's actually protecting your wallet right now. Without that flow switch, the heater would basically become a giant tea kettle. The water inside would flash to steam, the copper heat exchanger tubes would warp, and you'd be looking at a three thousand dollar repair or a full heater replacement. So yeah, it's annoying when you just want a warm pool. But it's way cheaper than ignoring it and letting the heater cook itself from the inside out.
How do I know if my flow switch actually needs replacing?
Test it before you buy one. With the pump running hard and all your baskets clean and your filter freshly serviced, pull the two wires off the switch and test for continuity with a multimeter. A closed circuit means the switch is fine and you've got a real flow problem somewhere upstream. An open circuit with everything else working perfectly means the switch is done. They're usually 30 to 60 bucks and take about 20 minutes to swap out. But don't buy one until you've confirmed everything else first.