Hayward Pool Heater Error Codes: LO, IF, CE & More Explained

Every fault code a Hayward Universal H-Series gas heater (H150, H250, H400) can display — what it means, the safe checks you can do yourself, and where to stop. We're the Pinellas County company licensed for the gas and electrical side of these repairs (CPC1459998 · EC13011994 · LI4527).

One tip before anything else: write down the code before you cycle power. Cycling clears the display, but not the cause — and knowing the exact code cuts your diagnosis (and your service call) in half. And never, ever bypass a safety switch: per Hayward's service documentation, the limit string exists to prevent a cracked heat exchanger and carbon monoxide hazards.

LO — Limit String Open (Not Just "Low Water")

LO is the most misunderstood code Hayward makes. It means one of the safety switches wired in series — the "limit string" of water pressure switch, temperature limit switches, exhaust gas temperature limit, and vent pressure switch — has opened, so the control shuts the heater down. It is not simply a "low water" warning, though low water flow is by far the most common trigger: a dirty filter, a variable-speed pump loafing at low RPM when the heater calls, a misdirected valve, or a low water level can all open that chain.

Try this first:

• Clean or backwash the filter — a clogged filter is the #1 cause of LO.
• Empty the skimmer basket and the pump basket.
• Top up the pool water level so the skimmer isn't gulping air.
• Confirm every valve feeding the heater is open and not diverted (check spa-mode diverters).
• If you have a variable-speed pump, raise the pump speed while the heater is calling for heat and see if the code clears.
• Power-cycle once after fixing the flow problem — not before.

IF — Ignition Failure

The heater tried to light, exceeded its maximum ignition attempts without proving a flame, and locked itself out — it will auto-reset after roughly 60 minutes per Hayward's consumer troubleshooting guide. The usual suspects are a gas supply valve that's off, an empty propane tank or low gas pressure, a dirty flame sensor, ignition wiring, or a failing gas valve.

Try this first:

• Confirm the manual gas shutoff at the heater is fully open.
• Confirm the valve at the meter (or propane tank) is open — and that the propane tank isn't empty.
• Check that other gas appliances in the house work.

IO — Igniter Open / Ignition Lockout

The igniter circuit reads open during the blower run, so the control shuts down and locks out (it can also appear after repeated IF events). The cause is almost always a failed hot-surface igniter, igniter wiring, or the control board itself.

Try this first:

• Power-cycle the heater once. If IO comes back, note it and call.

CE — Communication Error

The ignition control board and the display board have stopped talking to each other — the link either wasn't established within 3 seconds at startup or dropped for more than 30 seconds. It's usually a loose ribbon cable or harness between the boards, moisture or corrosion on the connectors (a real factor in Florida's humid, salt-air coastal climate), or a failing board.

Try this first:

• Power-cycle the heater at the breaker once. If CE returns, write it down and call.

AO — Blower Vacuum Switch Open

The combustion blower started, but its air-prover vacuum switch never closed, so ignition was aborted. Common causes: a blocked exhaust vent, kinked or cracked vacuum tubing, a failed blower, or a failed vacuum switch or relay.

Try this first:

• From outside the heater only, look at the exhaust vent: nothing should be sitting on it or blocking it — leaves, a cover, an animal nest.

AC — Blower Vacuum Switch Closed (When It Shouldn't Be)

The opposite of AO: the vacuum switch reads closed before the blower even starts, so the control refuses to run the blower at all. Per Hayward's official guide this points to a stuck vacuum switch or a welded relay contact. (Some retail guides describe AC as an "AC voltage" fault — Hayward's own definition is the blower vacuum switch error.)

Try this first:

• Power-cycle once. There's nothing else safe to do on this one.

HF — Flame Present With Gas Valve Off

HF means the control is sensing a flame while the gas valve is de-energized — gas is burning when the heater ordered it off. The control locks out and keeps the blower running to purge. The cause is a leaking or failed gas valve or a board fault, and an uncontrolled gas valve is a fire hazard.

Try this first:

• Nothing. Turn the heater's power off at the breaker and call 727-788-4086.

SF — Thermistor / Temperature Sensor Fault

Per Hayward's official guide, SF means the two water thermistors disagree by 5°F or more — a sensor comparison error, not (as some retail guides say) a "stack flue" sensor. Damaged sensor wiring from rodents or corrosion, or a drifted/failed thermistor, are the usual culprits.

Try this first:

• Power-cycle the heater once.

HS — Water Temperature Rising Too Fast

HS trips when inlet temperature exceeds 104°F, or the water temperature rises 6°F or more in 60 seconds while firing — a classic low-flow signature. Three occurrences in an hour puts the heater in a lockout that requires a power cycle. Restricted flow (dirty filter, low pump speed, valve position), a scaled heat exchanger, or an internal bypass problem are the causes.

Try this first:

• Run the same flow checklist as LO: clean the filter, empty both baskets, top up the water level, open all valves.
• Raise your variable-speed pump's speed while the heater runs.
• Then power-cycle once.

bD — Bad Board (Secondary High-Voltage Fault)

The ignition control board failed its own self-diagnostic on the secondary high-voltage circuit. That can mean a blown FC4 fuse, a failed transformer, or a failed fuse board or control module.

Try this first:

• One power cycle. If bD returns, it's a component-level fault.

sB — Stuck Button

A keypad button has been held closed for more than 30 seconds — debris or water on the keypad, a warped bezel, or a failing keypad membrane. The heater keeps operating, and the code clears the moment the button releases. This is the one code that's completely safe to fix yourself.

Try this first:

• Wipe the keypad clean and dry.
• Make sure nothing (a cover, a hose, a toy) is pressing on it.

EE — EEPROM Error

The control module's memory has failed — the board is defective. There's no setting or reset that fixes a corrupt EEPROM.

Try this first:

• One power cycle to rule out a random glitch.

PF — Power / Polarity Fault

The heater is seeing reversed 120V polarity, low supply voltage, or an inadequate ground path. The code clears immediately once the supply problem is corrected — which is exactly the point: the problem is in the wiring feeding the heater, not the heater itself. If PF appeared right after the heater or the pad was serviced or rewired, that's your answer.

Try this first:

• Check that nothing has tripped at your electrical panel.
• If the heater was recently serviced or rewired, don't touch anything — call.

bO — Bypass Operation (Not a Fault)

bO isn't an error at all — it means the heater is being controlled by an external or remote thermostat, usually your automation system. If that's intentional, leave it. If it's not, Hayward's manual gives a safe exit.

Try this first:

• Hold DOWN + MODE for 3 seconds to exit bypass mode and return control to the heater's own keypad.

Why Homeowners Across Pinellas Call Us for Hayward Heaters

• Gas ignition faults are gas-license work. IF, IO, AO, and HF all end at the combustion cabinet — we diagnose and repair gas valves, igniters, and vent systems under our own gas license (LI4527), no subcontractor in the loop.
• PF and board faults are electrical-license work. Reversed polarity, bad grounds, blown transformers, and low supply voltage are panel-and-wiring problems — squarely covered by our electrical license (EC13011994).
• We fix the flow problem behind LO and HS, not just the symptom. As a licensed pool contractor (CPC1459998) we handle the filter, valves, pump speeds, and heater internals as one system — so the code stays gone.

Seeing a code on other equipment? See our guides to Hayward AquaRite salt system codes, Hayward variable-speed pump errors, and OmniLogic troubleshooting.