Pentair ERR HLS: High-Limit Switch Open
ERR HLS on a Pentair MasterTemp or MiniMax NT heater means the high-limit switch tripped because water in the heat exchanger got hotter than about 135°F.
high severityApplies to: Pentair MasterTemp and MiniMax NT residential gas pool/spa heaters
What it means
A heater that was running fine a minute ago suddenly stops mid-cycle, and ERR HLS appears where the set temperature used to be. Unlike ERR PS, this isn't about whether water is moving at all — it's about how hot that water got while it was inside the heat exchanger. The high-limit switch is a physical safety device wired into the control circuit, and once water passing across it exceeds roughly 135°F, the switch pops open and cuts power to the gas valve immediately, mid-fire if it has to. On many units that switch also has to be reset by hand — a small button inside the control compartment — once whatever caused the overheating is actually resolved, which is worth knowing before assuming there's a display button that clears it on its own.
Likely causes
All three of these come down to the same thing: too much heat building up relative to how fast water is carrying it away.
- Restricted or inadequate water flow through the heater
- Scale buildup narrowing the heat exchanger's internal passages
- A miscalibrated or failed high-limit switch
What you can check yourself
- Check and restore normal flow (clean filter, open valves, adequate pump speed)
- Let the heater cool, then clear the fault and retry
If flow is clearly adequate and the fault repeats, have a technician check for scale buildup in the heat exchanger and test the high-limit switch — this switch is a real overheat safety device and shouldn't be bypassed.
Sources
Related reading
- All Pentair error codes
- Pentair ERR PS — water pressure switch open
- Pentair ERR AGS — automatic gas shutoff
- Pentair E01 — open water temperature sensor
- Symptom: Heater Runs But Pool Isn't Getting Warmer