How to read the display
The MasterTemp shows one of two kinds of code on its LCD. The ERR codes (PS, HLS, AFS, IGN, AGS, SFS) are safety interlocks: a switch or sensor in the chain between "call for heat" and "burner on" has opened, and the heater refuses to fire until it closes. The E codes and 126 are sensor faults: a thermistor or the stack flue sensor is reading open or shorted, so the control board no longer trusts what it sees.
Pentair's own manual is blunt about the ERR codes and the sensor codes: if any of them is displayed, shut the heater down and have a qualified technician check the system. The one exception in practice is ERR PS, which is a flow problem most owners can clear themselves. Everything below comes from the MasterTemp Installation and User's Guide (Pentair P/N 472592, Rev. Q, July 2023), Table 8, with the Fox Valley context added.
ERR PS: no water flow through the heater
The most common code by far. The water pressure switch has not closed, which means the heater does not see enough flow to fire safely. The manual's note: if water flow is established, the error clears and normal operation resumes on its own. The "Service System" light on the panel points at the same switch.
In the Fox Valley this shows up at spring openings, when a filter that sat all winter is loaded, and in midsummer when skimmer baskets fill with cottonwood. Check that the pump is running and primed, empty the skimmer and pump baskets, clean or backwash the filter, and confirm every valve on the way to the heater is open. If the code stays on after that, the switch itself or its setting is the question, and that is a technician visit.
ERR HLS: water too hot after the first pass
The high limit switch opens when water leaving the heat exchanger on its first pass goes above 135°F. The error clears once the temperature falls back under 135°F. The usual cause is not a broken heater but water moving too slowly through it: a dirty filter, a pump on a low speed, a partly closed valve, or a heat exchanger scaled up from hard Fox Valley well and municipal water. One trip on a hot afternoon is worth noting. Repeated trips mean the flow or the exchanger needs to be looked at.
ERR AFS: the blower cannot prove airflow
The air flow switch senses the pressure drop across the blower. ERR AFS means the blower is not running or the air through it is restricted. The manual gives the heater 60 seconds to establish airflow; if it does, normal operation resumes, and if it does not, the heater has to be turned off with the HEATER OFF button. Look at the intake and the exhaust for leaves, mulch, a wasp nest, or a cover that blew over the top. A blower that hums but does not spin, or a switch that will not close with a clear path, is a repair.
ERR IGN: the heater tried to light and could not
The ignition control module could not light the burner. Pressing HEATER OFF clears the code. Before calling anyone, confirm the gas valve on the supply line is open and that other gas appliances in the house are working. If gas is on and ERR IGN comes back on the next try, the igniter, the gas valve, or the flame sensor is the likely fault, and gas work is not a homeowner job.
ERR AGS and ERR SFS: the hard stops
These two disable the front panel buttons. ERR AGS is the automatic gas shutoff: water after the second pass through the heat exchanger went above 140°F. ERR SFS is excessive stack flue temperature: the flue passed 480°F. Both point to heat that is not leaving the heater the way it should, from low flow, a fouled exchanger, or a venting problem. The only way to reset either is to cycle power to the heater. Do not keep resetting and refiring. One reset to see if it was a fluke, then a technician.
E01, 126, E05, E06: sensor faults
E01 is an open water temperature sensor and 126 is the same sensor shorted. The manual's fix is to verify the thermistor wiring and replace the thermistor if the wiring is fine. E01 clears with the HEATER OFF button once the sensor reads again.
E05 is an open stack flue sensor and E06 is a shorted one. E06 disables the front panel and needs a power cycle to reset. These sensors sit in the hottest part of the heater and do fail with age; water intrusion after a wet spring is another cause. Either way the fix is a sensor, not a setting.
Reset a MasterTemp after a code
A safe sequence for the codes a homeowner can act on. If the same code returns after this, stop and call.
- Write down the exact code on the display before touching anything
- Press HEATER OFF on the control panel
- With the pump running, check filter pressure, empty the skimmer and pump baskets, and confirm every valve to the heater is open
- Look at the heater's intake and exhaust for debris, nests, or a cover
- For ERR AGS, ERR SFS, or E06, turn the heater's breaker off, wait 30 seconds, and turn it back on
- Turn the heater back on and watch the first heating cycle; if the code comes back, shut it down and call a technician
When to call
Call for any code that comes back after one reset, for ERR IGN when the gas supply is confirmed on, and for ERR AGS, ERR SFS, or ERR HLS more than once in a season, because those are the heater telling you it is running hotter than it should. A MasterTemp or ETi that has been fed dirty water for a few seasons often has a scaled exchanger behind the codes, and that is cheaper to address before the exchanger fails.