Pool heaters work very straightforwardly: they generate heat to raise the water temperature. As long as they have a reliable water supply, the heater will not overheat. That’s why your unit will display the LO code when there’s something wrong with the water supply.

The LO code signifies that there’s a low water pressure condition. That could be due to a dirty filter, clogged skimmers, or a water valve that isn’t fully open. However, faulty switches within the heater can also cause the same code to appear in some cases.

As you read through this guide, you’ll discover what triggers the LO code to appear. But, more importantly, you’ll learn how to troubleshoot and resolve the issue.

What Does The LO Code Mean?

The LO code on this brand’s pool heaters means it’s experiencing a low water pressure condition.

When the pool heater doesn’t have enough water flowing into it at the correct pressure, it risks damaging itself. More specifically, it could reach dangerous temperatures if insufficient water absorbs the heat it generates.

The pool heater will shut off when the LO code is triggered. Doing so enables the heater to protect itself against damage until you can troubleshoot and resolve the issue.

What Causes The LO Error Code?

As you read earlier, the LO error code is caused by a lack of water pressure. While that’s true most of the time, faulty switches are also known to trigger the code.

Here are the most likely triggers for the LO error code:

  • Dirty Filter Or Skimmers: The LO error code is often caused by something as simple as a dirty filter or skimmer. These will restrict water flow and cause pressure to drop inside the pool heater.
  • Closed valves: Another likely cause is a closed or partially-open water valve. All valves should be fully open for the pool heater to function optimally.
  • Faulty Water Pressure Switch: Besides the two causes above, a faulty water pressure switch could be to blame. This switch regulates the water pressure, so it can cause a low-pressure situation if it’s defective.
  • Problematic Vent Pressure Switch: The vent pressure switch is a safety feature that protects the pool heater. If it’s faulty, it can cause the same error code to appear.
  • Defective Temperature Limit Switch: Similarly, this switch, which acts as a safety feature, can also cause problems when it stops working correctly. One problem it can cause is triggering the LO code.

How Do You Troubleshoot The LO Error Code?

In most cases, the LO error code is straightforward to deal with. Here are the steps you should follow to troubleshoot and resolve the LO error code:

Step 1: Backwash And Clean The Water Filter 

The first and most important step is to backwash and clean the water filter.

Remember: the LO error code is triggered by low water pressure. That typically happens because the dirty filter restricts smooth water flow, which results in low water pressure.

Cleaning the filter will restore pressure and, in most cases, resolve the issue. In more severe cases, you may have to replace the filter entirely. Of course, that’s only if the filter is dirty, worn out, and beyond saving.

Step 2: Empty Skimmers

Once you’re confident the water filter is in optimal condition, you must empty your pool’s skimmers.

Skimmers can also trigger the LO error code when they get clogged and restrict smooth water flow.

Step 3: Ensure All Valves Are Open

Suppose you’ve gone through Steps 1 and 2, but the LO code persists. In that case, the next thing to do is check all the water valves supplying the pool heater.

Those valves are likely closed or only partially opened. Unfortunately, even a partially opened valve can drop the water pressure and trigger the LO code.

So, be sure to open all valves completely to enable optimal water flow.

Step 4: Test Heater Switches

The steps above will resolve the LO code in most cases. However, there are some situations where a failed switch is the problem’s root cause.

More specifically, you’ll want to test these switches:

  • Water pressure switch
  • Vent pressure switch
  • Temperature limit switch

Testing and troubleshooting these switches can be tricky. So if you’re not sure how to do it, you’re better off hiring a qualified professional to do it for you.

Final Thoughts

Overall, the most important thing to remember is that the LO code refers to a low water pressure situation. The pool heater will protect itself automatically by shutting off until the code is resolved.

In most cases, the code is caused by a dirty filter, clogged skimmer, or half-open valve. However, another possibility is a faulty water pressure, vent pressure, or temperature limit switch.

If your getting other codes check out our article: Troubleshooting Your Hayward Pool Heater Error Codes


Re: Means low water pressure through the heater. Automatically shuts down.


Re: Backwash filter, empty skimmers, clean cartridge filter, check valves are open, test switches