HL stands for High Limit.
It means the heater has detected overheating and has shut down to protect the unit.

If HL shows immediately from cold, the most common cause is a faulty temperature probe. Replacing the probe usually resolves this.

If the water heats very fast before HL appears, this usually indicates poor water circulation, often caused by:

• Faulty impeller

• Weak or failing water pump

Restricted flow causes heat to build up inside the heater.

Accordion ConteIf the main body of water overheats and the control area becomes affected, this usually indicates a faulty control panel/main board. In this case, the control panel will need replacing, as internal overheating can cause repeated HL errors and system shutdown.nt

Most commonly this is caused by a failed heater element or a faulty water pressure switch (flow/pressure safety switch). If your pump runs but the water won’t heat, these are the first parts to replace.

This usually points to a failing heater element (internal short to earth). Replace the heater element.

If you’ve ruled out the socket/extension lead, replace the RCD power plug (RCD unit).