The MasterTemp is the heater we repair more than any other, and the reason is simple: for well over a decade it was the unit going into Orange County backyards, so it is the one filling equipment pads now. The good news is that it tells you something when it faults. The catch is that a code points you in a direction, not at a cause, and the difference between those two is most of the money.
Find your code, read across. The last column is the honest answer about whether this is a homeowner job.
| Code | What it means | Most likely cause | Yours to fix? |
|---|---|---|---|
| ERR PS | Water pressure switch did not detect enough flow before ignition. | Dirty filter, pump speed too low, a valve part-closed. | Often yes |
| ERR HLS | High limit switch opened. Water leaving the exchanger got too hot. | Low flow, or scale inside the heat exchanger. | Find the cause first |
| ERR IGN | Tried to light three times and never detected a flame. | Gas off or low, failed ignitor, gas valve, spark gap. | Gas is on |
| ERR AFS | Air flow switch saw no combustion air during the pre-purge. | Blocked vent, weakening blower, cracked vacuum tube. | Technician |
| ERR AGS | Automatic gas shutoff tripped. The last safety in the chain. | Excessive heat in the unit. Something upstream is wrong. | Stop, do not reset |
| ERR SFS | Stack flue sensor fault. It watches exhaust temperature. | Sensor or wiring, sometimes a genuinely hot flue. | Technician |
| E05 | Stack flue sensor circuit reading open. | Failed sensor or a broken wire to it. | Technician |
| E06 | Stack flue sensor circuit reading shorted. | Damaged sensor, often after it has been run hot. | Technician |
| E01 | Water temperature sensor fault. | Failed sensor, loose wiring, or scale on the sensor. | Technician |
These match the current MasterTemp and Max-E-Therm control boards. Older units and earlier board revisions label things differently, so check your model and serial against the Pentair manual before acting on a code. If yours is not here, photograph the display and send it over.
ERR AGS. The automatic gas shutoff is the last safety between a problem and a bad afternoon. It does not trip for fun. Resetting it repeatedly, or letting anyone bypass it, removes the one thing standing between an overheating heater and your house.
Flame where there should not be one. If you can see the burner lit when the heater is supposed to be off, shut the gas off at the heater and call us on (909) 577-5274. That is not a code to look up.
This is the most common MasterTemp code we get called about, and it is the one most likely to have a five-minute answer. The heater is not broken. It checked for water flow, did not find enough, and refused to light. That refusal is a feature.
Work through it in this order:
If the code comes back after all of that, stop guessing. The restriction is somewhere you cannot see — commonly scale inside the heat exchanger, sometimes a failing pressure switch, occasionally a plumbing problem. That needs gauges on the system, which is what a diagnostic is for.
The high limit switch opens when water coming out of the heat exchanger is hotter than it should be. Same root cause as ERR PS most of the time — water was not moving fast enough to carry the heat away — but it has gone a step further.
The important difference: unlike the pressure switch, the high limit does not always clear on its own. There is a physical reset button inside the cabinet. People find it on a forum, press it, the heater runs, and they think it is fixed.
Pressing the high limit reset without fixing why it tripped just queues up the same shutdown, and repeated high limit trips are genuinely hard on a heat exchanger.
If a MasterTemp has been high-limiting for a while before anyone called, we are often looking at a heat exchanger that has already taken damage. That is the difference between a switch and a replacement, and it is decided by how long it was left.
Three attempts, no flame detected, so the board gives up and tells you. Before assuming the worst, confirm the boring things: is the gas valve at the heater open, and is other gas equipment in the house working? A shut valve and an empty propane tank both read as ignition failure.
Past that, the usual suspects are the ignitor itself, the gas valve, or the spark gap being out of spec. All of those sit behind the panel, in among gas and high voltage, which is where the homeowner list ends and ours begins.
These get less attention than the flow codes and deserve more, because they are about the heater breathing.
| What is happening | What we look at |
|---|---|
| ERR AFS — no combustion air detected during pre-purge | Vent terminal blocked, blower motor weakening, the vacuum tube cracked or off. Also, on more occasions than you would expect, a nest. |
| ERR SFS / E05 / E06 — stack flue sensor | The sensor and its wiring first, then the reason the flue was running hot, which usually comes back to flow or scale. |
| ERR AGS — automatic gas shutoff | Everything above, plus the venting and the heat exchanger. This code is a consequence, not a cause. |
A recurring theme: rodents. A heater cabinet is warm, dry and undisturbed for months, and nesting material against a blower or packed into a vent produces exactly these codes. We cover that in more detail in what a heater that will not run is telling you.
Two things make MasterTemp codes such a common call in this county. The first is simple volume: it was the default install for years, so there are more of them out there than anything else, and most are now old enough to need something.
The second is our water. Orange County water is hard, and a pool concentrates it, because every gallon that evaporates leaves its minerals behind. Those minerals build up inside the heat exchanger where nobody can see them, narrow the passages, and quietly reduce flow and heat transfer. That is why the same model that runs for years in a soft-water state starts throwing flow and high limit codes here.
It also means a code is sometimes a water chemistry problem wearing a costume. If we replace a heater and never mention your water, we have only reset the clock.
This is the part the national troubleshooting pages skip, and it is the part that decides what you spend.
| The code says | What it still will not tell you |
|---|---|
| Flow is too low | Whether that is a filter, a pump setting, a valve, or scale inside the exchanger. Four very different bills. |
| The high limit opened | Whether the exchanger survived the episodes before you noticed. |
| Ignition failed | Whether it is an ignitor, a gas valve, or gas supply that was never right. |
| A sensor is open or shorted | Whether the sensor failed on its own or was cooked by something upstream. |
Which is why we would rather put a technician on it than talk anyone through a parts order.
Real numbers for both repair and replacement are on our pool heater repair cost page, and the service itself is described on pool heater repair & installation. If you are choosing between technologies rather than fixing one, there is heat pump vs. gas.
Photograph the display and the equipment pad, tell us what the heater does, and we will tell you what it points to before anyone drives out. If it is a filter, we will say so.
Get a Free EstimateACE Pool Repair & Remodel is a licensed California pool contractor, CSLB #1075061, based at 251 E Imperial Hwy, Fullerton, CA 92835, serving Orange County since 2014. We are an authorized Jandy warranty center and we repair Pentair, Hayward and Raypak equipment as well. We do not run after-hours emergency calls. Related reading: pool heater not working, Jandy JXi error codes, heater repair costs, all equipment services.
