Hayward Pool Heater BO Error Code: Blower Overheat

Quick Answer

The Hayward BO code indicates a blower or combustion overheat fault. The most common fix is clearing debris from the air intake and exhaust vents to restore proper airflow. If vents are clear, check for a failing blower motor or a tripped thermal limit switch.

A BO fault often signals that your Hayward heater is struggling to breathe or the blower motor is nearing the end of its life. Beyond simple vent obstructions, this code can point to internal soot buildup or failing motor bearings that increase operating temperatures. Addressing these thermal issues promptly prevents permanent damage to the combustion chamber and ensures safe operation throughout the swimming season.

HaywardPoolheaterSeverity: highDifficulty: intermediate73% DIY Success

Time to Fix

30–120 min

Difficulty

intermediate

Parts Cost

$15 – $220

Tools Needed

Phillips #2 screwdriver, Flathead screwdriver

What Does the BO Code Mean?

Think of the BO code as your heater's way of gasping for air. While the manual points to the blower motor, I often find that the thermal limit switch itself has just become hypersensitive after years of heat cycles. Before you drop hundreds on a new motor, we need to determine if the unit is actually getting too hot or if the safety sensor is just crying wolf.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Blocked air intake louvers reducing combustion air volume40%

Blocked exhaust vent trapping heat in combustion chamber24%

Blower motor bearing failure reducing fan speed14%

BO thermal limit switch failure (stuck closed or faulty)12%

Insufficient clearance around heater cabinet10%

Symptoms You May Notice

The BO code locks the panel and the heater shuts down mid-cycle, sometimes only 10-15 minutes into a heating run, and it'll keep doing it every time you reset without fixing the cause.
Blower makes a high-pitched whine or rhythmic grinding sound during startup or while running.
The side panels or top of the cabinet feel noticeably hotter than usual when you put your hand near them.
You can smell something hot or scorched near the heater even after it shuts down and the blower stops.
Heater fires up and runs fine for a stretch, then cuts out with BO every single time you try to restart it.

Can you reset a Hayward poolheater to clear the BO code?

Clear the code by cycling the Mode button to Standby, then back to your desired setting. However, if the thermal limit switch is still physically too hot, the board will reject the reset. I recommend letting the unit sit for thirty minutes with the power off. This gives the metal housing time to contract and the switch a chance to click back into its closed position.

Tools Required for Diagnosis

Phillips #2 screwdriverFlathead screwdriverDigital multimeter (set to ohms/continuity)Flashlight or headlampShop vacuum with narrow crevice attachmentSoft bristle brushNeedle-nose pliers

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test

Expected Range0–2 ohms

ConditionIf Open (OL) or infinite, replace component.

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
Combustion blower motorCHXBLW001 · $100–$220	CHXBLW001	$100 – $220
BO thermal limit switchCHXTLS003 · $15–$35	CHXTLS003	$15 – $35

Frequently Asked Questions

What does BO mean on a Hayward pool heater?

In the field, we call this the suffocation code. It means the internal safety sensor detected temperatures high enough to melt components or cause a fire. The system shuts down the gas valve and cuts the blower to prevent further heat buildup. It's a critical safety feature that protects the heat exchanger and control board from thermal damage. Ignore it and keep resetting without fixing the cause, and you're looking at a warped heat exchanger, which runs $400-800 to fix at minimum.

Why does my Hayward heater show BO on hot days?

Heat is cumulative. On a 95-100 degree day, the air entering the heater is already hot, so it's way less effective at cooling the motor. If your motor's aging or your vents are even 20% blocked, that extra ambient heat is often enough to push the internal sensor past its trip point. Keeping the area around the heater clear is even more important during peak summer months. Some techs recommend shortening run times on extremely hot days if your unit's got some age on it.

Can I restart my Hayward heater after a BO code?

You can try, but it's usually just a temporary fix. The BO code is a hard lockout for a reason. If the underlying airflow issue isn't solved, the heater will trip again within ten or fifteen minutes. Worse, repeatedly resetting a BO code without fixing the cause can eventually warp the heat exchanger or fry the ignition control module. Those are $400-800 repairs. Fix the cause first, then reset.

How far from a wall does a Hayward H-Series heater need to be?

Hayward's pretty strict about this. You need at least 12 inches of clear space on the sides and back, and 24 inches in front for service access. I see people build beautiful wooden equipment enclosures all the time that look great but basically choke the heater. Without fresh air constantly coming in, the unit re-circulates its own exhaust heat and you'll get BO codes constantly. Check your installation manual for your specific model since clearances can vary slightly between units.

How do I know if my Hayward blower motor needs replacement?

Aside from the noise, check the coast-down time. When you shut the heater off, a good blower should spin freely for several seconds before stopping. If it stops almost instantly, the bearings are seized. Also look for bubbling paint or discolored metal on the motor housing itself. That's a clear sign it's been running way too hot for way too long. Another thing I check: touch the motor housing right after a run cycle. If it's too hot to keep your hand on, that motor's on its way out.

Related Hayward Poolheater Error Codes

Code: HUBHayward Pool Heater Error Codes Guide Code: AOHayward Pool Heater AO Error Code Code: RESETHow to Reset a Hayward Pool Heater

Same Fix on Other Brands

Pentair poolheaterERR AFS: Pentair Pool Heater ERR AFS Error

Models Known to Experience BO Errors

This repair applies to most Hayward poolheaters with this error code. Common model numbers include:

H400FDN, H350FDN, H300FDN, H250FDN, H200FDN, H150FDN, H400FDP, H250FDP

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026