Good news first: your heater is telling you exactly what's wrong. The MasterTemp names its fault on the front LCD — you just need to know what the letters mean. Every code below is taken from Pentair's own installation and user's guide, and the same set applies to the Sta-Rite Max-E-Therm.
Redzone Pools repairs Pentair heaters across Cypress, Tomball, Katy, Spring and Greater Houston — licensed, insured, usually same day.
There's a persistent myth that a MasterTemp only gives you a vague warning and that the real fault is buried on a board inside the cabinet. It isn't. Pentair's guide is explicit: diagnostic errors are displayed on the control panel LCD — the small screen on the front of the heater, the one you already use to set the temperature.
The full set it can show is PS, HLS, SFS, AFS, AGS, IGN or NONE, plus the numbered sensor codes E01, E05, E06 and 126. That's the whole vocabulary. Once you can read it, you know whether you're looking at a five-minute fix, a part, or a stop-and-call situation — before anyone quotes you anything.
Write the code down before you cycle power. Several of these clear themselves when the heater restarts, and a code you've lost is diagnosis time you'll pay for.
Straight from the manual. The reset column matters — some faults clear from the panel, others lock the buttons until power is cycled.
| Code | What it means | How it clears |
|---|---|---|
| ERR PS | Insufficient water flow to the heater. If the pump is running, this usually means the filter and/or skimmers need cleaning. | Clears once flow is restored |
| ERR AFS | Air flow fault. The air flow switch senses pressure drop across the air metering orifice — a blower or restricted-air problem. | Service required |
| ERR HLS | High limit switch. Opens when the heat exchanger outlet temperature runs too high. | Service required |
| ERR AGS | Automatic gas shutoff. Water temperature after the second pass through the heat exchanger exceeded 140°F. | Buttons disabled — cycle power |
| ERR SFS | Excessive stack flue temperature — the flue exceeded 480°F. | Buttons disabled — cycle power |
| ERR IGN | The ignition control module could not light the heater. | Press HEATER OFF |
| E01 | Open water temperature sensor. | Press HEATER OFF |
| 126 | Shorted water temperature sensor. | Service required |
| E05 | Open stack flue sensor. | Buttons disabled — cycle power |
| E06 | Shorted stack flue sensor. | Buttons disabled — cycle power |
Two things to notice. E05 and E06 are sensor faults, not combustion faults — the heater has lost its ability to read the flue, so it refuses to fire rather than run blind. And several codes disable the front panel buttons entirely, which is by design: Pentair wants a human to look before the heater runs again.
This is the code we see most, and it's the one most likely to be fixable without a technician.
The pressure switch exists to confirm the pump is actually moving water before the burner lights. When it reports insufficient flow, people assume the switch has failed. Usually it hasn't — Pentair's own guide says that if the pump is running, ERR PS usually means the filter and/or skimmers need cleaning.
That's a very Houston problem. Between the leaf load in autumn and the general debris a long season generates, filters here foul faster than the manual's baseline assumes. Clean the filter and skimmer baskets, confirm valves are open, and see whether the code clears. If it does, you've fixed it for free. If your filter is genuinely past cleaning, that's a filter problem rather than a heater problem — and the heater is just the thing that noticed first.
The related check: the pressure switch should close with the pump running and open with the pump off. If it's behaving backwards, that's a real fault and a real repair.
These are the codes worth understanding properly, because they point at the same underlying story from two different angles.
ERR AGS trips when water leaving the second pass through the heat exchanger exceeds 140°F. ERR SFS trips when the stack flue exceeds 480°F. In both cases the heater shuts down and locks its buttons until you cycle power.
What usually drives them is heat that isn't leaving fast enough. Too little water moving through the exchanger, or an exchanger whose passages have narrowed, and the same burner output has nowhere to go — so temperatures climb where the safeties are watching.
Which is why these two codes are worth taking seriously rather than resetting repeatedly. A heater that trips AGS, gets power-cycled, runs, and trips again is not a heater with a nuisance fault. It's a heater telling you the same thing twice.
Here's the pattern behind our busiest heater week of the year. In this climate a pool heater typically sits unused from roughly April through September — nobody wants 88°F water in a Houston August. Then the first genuine cold front arrives and thousands of heaters are asked to fire for the first time in months, all in the same week.
Sitting idle in this climate is not neutral. Warm, humid, mostly-outdoor cabinets are attractive to wildlife, and the classic first-fire failures here are pest-related: mud daubers and wasps building in the burner tray and venturi, ants and roaches in control boards, rodents in the vent. Add Gulf Coast humidity working on burner trays and flame sensors, and you get a heater that was fine in March and won't light in November.
Two of the codes above are exactly what that produces. Restricted air from a blocked venturi shows up as ERR AFS. A burner that won't light because the flame sensor is fouled or the igniter is failing shows up as ERR IGN — and a fouled flame sensor looks identical to a dead igniter from the deck, which is why swapping the igniter sometimes changes nothing.
We go through that whole pattern in why Houston heaters won't fire on the first cold morning. The practical move is to fire the heater once in early October rather than waiting for the night you actually want it. If it's going to fail, it fails on a warm afternoon with a technician available — not at 6am on the coldest morning of the year when every pool company in the county is booked.
The other slow killer is scale. Our hard fill water narrows heat exchanger passages over the years, which is precisely the condition that pushes AGS and SFS toward their limits — the reasons are in why Houston's fill water is hard on pool heaters.
Reasonable to try yourself: note the code, clean the filter and skimmer baskets, confirm valves are open and the pump is running properly, check the thermostat is set above current water temperature and in the right mode, and cycle power once to see whether the fault returns.
Stop and call: anything involving gas, anything where the code returns immediately after a reset, any repeated AGS or SFS trip, and anything requiring the cabinet to come apart. A pool heater is a gas appliance — the safeties above exist because the failure modes are serious, and defeating one to keep swimming is not a trade worth making.
If the heater fires but the water never warms, that's a different diagnostic path — our guide on a pool heater that runs but won't heat covers it. Got a Hayward instead? The Hayward H-Series error code guide does the same job for IF, LO, HS and SB. For everything else, pool heater repair is our day job.
E05 is an open stack flue sensor. It's a sensor fault rather than a combustion fault — the heater has lost its ability to read the flue temperature, so it refuses to fire rather than run without that safety input. The related code E06 means the same sensor is shorted instead of open. Both disable the front panel buttons, and the heater is reset by cycling power. Because it's a safety sensor in the flue path, this one needs a technician rather than a reset-and-hope approach.
AGS is the automatic gas shutoff. It trips when the water temperature after the second pass through the heat exchanger exceeds 140°F, cutting gas and disabling the front panel buttons until power is cycled. It nearly always means heat isn't leaving the exchanger fast enough — too little flow, or passages narrowed by scale. If it trips once you can reset it, but a heater that trips AGS repeatedly is reporting a real problem and should be diagnosed rather than reset again.
ERR PS means insufficient water flow to the heater — but Pentair's guide notes that when the pump is running, this usually points to the filter and/or skimmers needing cleaning rather than a failed switch. Clean the filter and baskets, check that valves are fully open, and see whether it clears. It's also worth confirming the pressure switch closes with the pump running and opens with it off; if that's reversed, the switch itself is faulty.
Yes. The Sta-Rite Max-E-Therm uses the identical diagnostic set — its manual lists ERR HLS, ERR AFS, ERR IGN, ERR AGS, ERR SFS, E01, E05, E06 and 126 as the messages that appear on the LCD, with the same meanings and the same reset behaviour. If you have a Max-E-Therm, everything on this page applies to your heater too.
No. That phrasing belongs to other manufacturers and gets attached to Pentair heaters online by mistake. A MasterTemp names its fault directly on the control panel LCD using the codes above — PS, HLS, SFS, AFS, AGS, IGN, or one of the numbered sensor codes. If you're seeing a generic service message rather than one of these, check which brand of heater you actually have, because you may be reading advice written for a different unit.
Once, to see whether the fault was a one-off — that's reasonable. Repeatedly, no. Several of these codes deliberately disable the front panel until power is cycled, which is Pentair's way of forcing a human to look before the burner lights again. AGS and SFS in particular are heat-related safety trips, and a heater that keeps hitting them is telling you the same thing each time. Resetting it repeatedly doesn't fix the cause and removes your early warning.
Because they've usually sat unused since spring. In this climate the heater is idle for roughly five months, and warm, humid cabinets attract pests — mud daubers and wasps in the burner tray and venturi, insects in control boards, rodents in the vent — while Gulf Coast humidity works on burner trays and flame sensors. Then the first cold front asks thousands of heaters to light in the same week. Firing yours once in early October, while technicians are still available, is the cheapest insurance there is.
Read us the code off the LCD and we can usually tell you on the phone whether it's a filter clean, a part, or something that needs the cabinet open — before we come out. Licensed, insured, serving Cypress and Greater Houston.