A Hayward LO code is a limit string open error that occurs when any safety circuits on your Hayward pool heater opens. This forces the heater to shut down and go into lockdown to protect your whole equipment from damage.
The LO code on a Hayward heater is a safety feature that indicates a low water flow issue. This is a protective measure to prevent the heater from overheating or sustaining damage. The LO Code issued could stem from various factors such as obstructions in the pool skimmers, a dirty pump basket, a faulty ignition, a clogged heat exchanger, or low gas pressure. It also means Limit Open, where one of the limit switches is not closing to complete the circuit.
To clear the LO code, fix or replace the wiring on the limit switches and any other issues with your heater’s limit switch, and you may need to do a backwash to clear out any obstructions. Besides that, you’ll need to set the correct water pressure setting. That is if your heater is installed below or above the water levels of your pool or spa.
What Causes the Hayward Heater LO Code?
1. There Is A Fault With The Water Pressure Switch
Chart: Water pressure switch fault possible causes and main fixes.
| FAULTY PRESSURE SWITCH POSSIBLE CAUSE | FIXES |
| PUMP NOT RUNNING | TURN ON THE PUMP |
| LOW WATER FLOW | ENSURE THAT THE WATER FLOW RATE TO HEATER IS ABOVE MINIMUM THRESHOLD. |
| A FAULTY WIRING OR CONNECTION | ENSURE NOT LOSE OR BROKEN WIRES ON PRESSURE SWITCH |
| WATER PRESSURE SWITCH CONTACTS OPEN | MEASURE FOR CONTINUITY ACROSS THE WATER PRESSURE SWITCH |
| THERE IS LOW PUMP PRESSURE | CLEAN UP FILTERS OR CLEAR BLOCKAGES |
| INCORRECT WATER PRESSURE SWITCH SETTING | ADJUST WATER PRESSURE SETTING ACCORDINGLY |
| A DEFECTIVE WATER PRESSURE SWITCH | REPLACE WATER PRESSURE SWITCH |
How to diagnose if the water pressure switch has a fault
A faulty water pressure switch is among the most common causes of the LO code on a Hayward heater. You can take the following steps to diagnose a faulty water pressure switch.
I. Verify that the water pump is running
II. Verify also that the water flow rate is above the minimum threshold
III. If the above steps do not show any problems, check for faulty wiring or connection in the pressure switch wiring
IV. Verify the status of water pressure switch contacts
V. Ensure there is no low water pressure from pump
VI. Ensure there is a correct water pressure switch setting
VII. If all the above diagnostic steps look good, ensure the water pressure switch is not defective.
Replacement parts are discussed below:
- Water pressure switch assembly replacement
- Fits hayward millivolt electronic h-series ed1 and ed2 style pool heater
- Weighs 23-pound
- Water pressure switch
- Fits hayward universal h-series low nox pool heater models h150fdn, h150fdp, h200fdn, h200fdp, h250fdn, h250fdp
- Also fits hayward universal h-series low nox pool heater models h300fdn, h300fdp, h350fdn, h350fdp, h400fdn, h400fdp
- Weighs 20 pounds
- Replacement Water Pressure Switch For C Spa Series And H Series Hayward Heaters
- Replacement Pressure Switch For Hayward Above Ground H Series Pool Heater
- Replacement Pressure Switch For Hayward C Spa Series Spa Heaters
- Country Of Origin: United States
LO CODE FIXES
Each diagnostic step above has possible fixes for each of them; let’s look at them.
I. It’s normal for your heater to display the LO code when the pump is not running. So, ensure that the pump is turned on and is running; this should clear the error code.
If the LO error does not clear despite the pump running, go to the next troubleshooting step.
II. A low water flow to the heater often results in the LO code. To fix this problem, verify that the water flow rate to the heater is above the minimum threshold. That is (20 GPM for H150FD and H200FD) and, (25 GPM for H250FD and H300FD) and, (30 GPM for H350FD and H400FD).
If LO does not clear despite the water flow rate being above the minimum threshold, something else could be causing this problem. At this stage, proceed to the next step.
III. A fault in the water pressure switch wiring could also be the culprit for LO code. You need to inspect the wiring at your water pressure switch at this stage.
If possible, verify that the wire harness terminals are securely fastened to spade terminals on the water pressure switch. Otherwise, if everything at this stage seems fine, let’s move to the next step of our troubleshooting.
IV. If all the above troubleshooting steps do not reveal a possible problem, your pressure switch contacts could be causing the LO code. You’ll need to remove the wire leads from the water pressure switch and measure them for continuity.
If open, the problem is probably with the contacts, but if closed, the LO code is not caused by a water pressure switch, so you’ll need to investigate further the cause of the problem.
V. Clogged filters or blockages in your water flow system can cause the LO error. Clogged filters and blockages lower the pump pressure.
Verify that filters are not clogged, and clean them if necessary. Also, check the position of valves in the plumbing system.
VI. Is your heater installed above or below the water level? The pressure switch is the default factory setting for most typical deck installations.
However, you’ll need to correctly set the pressure switch for a heater installed above or below the water level.
VII. If none of the above steps clears the LO code, you might be dealing with a defective water pressure switch. The pressure switch will be replaced to clear the error code.
2. Vent pressure switch fault
Vent pressure switch fault causes and main fixes
| FAULTY VENT PRESSURE POSSIBLE CAUSE | FIXES |
| A FAULTY WIRING OR CONNECTION | INSPECT WIRING ON VENT PRESSURE SWITCH |
| VENT PRESSURE SWITCH OPEN | MEASURE CONTINUITY ACROSS VENT PRESSURE SWITCH |
| RESTRICTED OR BLOCKED FLUE | ENSURE FLUE IS NOT BLOCKED OR RESTRICTED |
| VENT PRESSURE SWITCH IS DEFECTIVE | REPLACE A DEFECTIVE PRESSURE SWITCH |
How to diagnose a faulty vent pressure switch.
A faulty vent pressure switch will typically trigger the LO code on your Hayward heater. However, you can take a few steps to diagnose and eventually fix a faulty vent pressure switch.
Below are the steps to take to diagnose a faulty pressure switch.
I. Check for faulty wiring or connection
II. Check whether the vent pressure switch contacts are working and can conduct voltage
III. Verify that the flue is not blocked or restricted.
IV. Check whether the vent pressure switch is defective or not.
Vent Pressure Switch FIXES
For each diagnostic step above, the following are possible fixes to each of them.
I. A faulty wiring in the vent pressure switch can cause the LO code. To solve the problem, inspect the wiring on your vent pressure switch and if possible, verify that the wire harness terminals are securely fastened to spade terminals on the vent pressure switch.
Otherwise, if things are ok at this stage, move on to the next step.
II. Verify the status of vent pressure switch contacts. Remove the wire leads from the vent pressure switch and jumper leads to do this. Then turn on the heater and measure for continuity across the vent pressure switch.
If closed, the LO error code is probably not caused by a vent pressure switch fault. Reconnect the wire leads to the vent pressure switch and go to the next step.
III. Ensure that the flue is not blocked or restricted. Check the user manual for the correct vent sizing requirements.
IV. A defective vent pressure switch can cause the LO code. To solve this problem, you must replace the faulty pressure switch.
- Indoor vent pressure switch kit
- Fits hayward universal h-series low nox pool heater models h150fdn, h150fdp, h200fdn, h200fdp, h250fdn, h250fdp
- Also fits hayward universal h-series low nox pool heater models h300fdn, h300fdp, h350fdn, h350fdp, h400fdn, h400fdp
- Weight 3.4 pounds
- 0 to 2000-feet
- Vent pressure switch replacement
- Fits hayward universal h-series low nox induced draft heater
- Weighs 8.3-pound
3. Temperature limit switch Fault
| TEMPERATURE LIMIT SWITCH FAULT POSSIBLE CAUSE | MAIN FIXES |
| FAULTY WIRING | ENSURE THERE ARE NO LOOSE WIRES |
| TEMPERATURE LIMITS CONTACTS ARE OPEN | MEASURE CONTINUITY ACROSS THE SWITCH |
| LOW WATER FLOW | VERIFY WATER FLOW RATE IS ABOVE MINIMUM THRESHOLD |
| DEFECTIVE TEMPERATURE LIMIT SWITCH | REPLACE TEMPERATURE LIMIT SWITCH |
Steps on how to diagnose a temperature limit switch fault
I. Check for faulty wiring or connection
II. Verify the status of temperature limit switch contacts
III. Ensure that the water flow rate is above the minimum requirement
IV. Is the temperature limit switch defective?
FIXES
I. The first thing you want to do is check whether or not the wiring on the temperature limit switch is OK. To do this, look closely at the temperature limit switch wiring.
Ensure that there are no breakages or loose wires. Also, ensure that wire harness terminals are securely fastened to spade terminals on temperature limit switches. If the wiring on the temperature limit switch is ok, go to the next step.
II. Verify that the temperature limits’ contacts can conduct voltage. Remove the wire leads from the limit switch and jumper switch. Turn on the heater and measure continuity across the temperature limit switches.
If closed, the LO is probably not caused by the temperature limit switch fault. Don’t forget to remove the jumper from the leads and reconnect the leads to temperature limits.
III. Verify that there is adequate water flow to the heater. The water flow rate needs to be above the minimum requirement. If the water flow is sufficient, go to the next step.
IV. If nothing works from above, you might be dealing with a defective temperature limit switch. To fix this, you’ll need to replace it with a non-defective one.
- Item Package Weight: 4.9 lb
- Country Of Origin: United States
- Temperature limiters kit replacement
- Package Dimensions: 14.223 L x 0.762H x10.16W (centimeters)
4. Exhaust gas temperature limit fault
Exhaust gas temperature limit fault causes and main fixes
| FAULT POSSIBLE CAUSE | FIXES |
| FAULTY CONNECTION | INSPECT TEMPERATURE LIMIT SWITCH WIRING |
| TEMPERATURE LIMIT CONTACTS ARE OPEN | MEASURE CONTINUITY ACROSSLIMIT SWITCH CONTACTS. |
| DAMAGES HEAT EXCHANGER | REPLACE THE HEAT EXCHANGER |
How to diagnose a faulty Exhaust gas temperature limit:
I. Check for faulty wiring or connection
II. Check the status of the temperature limit contacts
III. Is the heat exchanger damaged?
FIXES
I. Take a look at the temperature limit switch wiring. Ensure wire harness terminals are securely fastened to spade terminals on temperature limit switches. This is located in the flue collector panel above the center of the header.
If this is OK, go to the next step.
II. Test the temperature limits contact for continuity. Remove the wire leads from the limit switch and measure continuity across contacts. If closed, it’s unlikely that the exhaust gas switch fault causes the LO code.
Reconnect the wire leads and go to the next step.
III. If nothing works at this point, you will likely have a damaged heat exchanger and need a replacement to fix this problem. However, there are a few things you need to take note of before replacing your heat exchanger.
A damaged heat exchanger is generally due to low water flow, poor water chemistry, or other factors. So, it’s essential to first rectify all issues with the water flow and the water chemistry to the heater.
Then replace the heat exchanger and the gas exhaust limit; otherwise, replacing the heat exchanger without fixing these other issues could damage the entire equipment.
- Heat exchanger assembly
- Fits hayward universal h-series low nox pool heater model h250fd
- Weight 48.5 pounds. Low NOx Emissions – Meets Clean Air Quality Standards
- 2 inch x 2 1/2 inch CPVC Union Plumbing Connections
- 250 mv heat exchanger assembly replacement
- Fits hayward h-series ed2 style pool heater
- Genuine hayward replacement part
Final Thought
As we have seen, there could be several causes for the Hayward heater LO code. As outlined in this guide, it’s vital to begin troubleshooting with the commonest and easiest ones.
That said, we hope that you have found a solution after reading this troubleshooting guide. If not, then you may want to consider a newer heater, I’ve listed a few below:
- BEST FOR: In-ground pools and spas with a surface area up to 1,200 ft. where natural gas is available. Its low environmental impact makes it ideal for pools and spas located where low-NOx emissions are required.
- PREMIUM PERFORMANCE: The Hayward Universal H-Series 400,000 BTU Natural Gas Heater delivers fast premium heating performance for years of reliable, long-lasting comfort.
- Durable cupro nickel heat exchanger
- Industry-leading hydraulic performance with faster speed-to-heat capability
- BEST FOR: In-ground pools that are used regularly and benefit from maintaining an ideal consistent temperature. Non-corrosive design makes it durable for coastal climates.
- ECONOMICAL HEATING: Using the surrounding air to heat the pool, HeatPro is more economical than propane, natural gas and electric heaters for maintaining the ideal water temperature.
Other Hayward Heater Error Codes:
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