Pentair MasterTemp Error Codes: Service Heater Light, ERR & E-Codes

This guide explains every verified Pentair MasterTemp error code and service light — what each one means, the most likely cause, and the checks that are safe to try yourself. We're Perfect Catch in Largo, the Pinellas County company licensed for both the electrical and gas sides of these heater repairs (EC13011994, LI4527, CPC1459998).

How the MasterTemp Tells You What's Wrong

The membrane pad on the front of a MasterTemp 125, 200, 250, 300 or 400 has two service lights — Service System and Service Heater — plus a digital display that can show ERR codes and E-codes. Per Pentair's service documentation, "Service Heater" faults are pinpointed by diagnostic LEDs on the control board behind the access panel — reading those requires removing panels, which is professional territory. Everything you need for a phone call is right on the front display.

Service System — no water flow detected

The heater's water-side safety chain is open, almost always because the water pressure switch isn't detecting enough flow. The heater won't fire until flow is restored. Common triggers: a dirty filter, a closed or misaligned valve, the pump off or running on low speed, or an air lock after cleaning.

Try this first:

• Confirm the pump is running at its normal (high) speed
• Check the filter pressure gauge and clean or backwash a dirty filter
• Empty the skimmer and pump baskets
• Confirm the valves feeding the heater are open
• Check the pool water level and top off if low

Service Heater — combustion/safety chain fault

The control detected a fault in the combustion or safety chain and locked the heater out. A specific board LED or display code identifies which safety tripped — the codes below cover each one.

Try this first:

• Note the exact code on the display (a phone photo helps our tech)
• Power-cycle the heater once at the breaker or timer
• If the light returns, stop and call

ERR PS — water pressure switch open

The pressure switch doesn't see enough water flow for the heater to fire safely, so it refuses to light. It clears itself automatically once flow is restored. Usual suspects: dirty filter, low pump speed, closed valves, clogged baskets, or low water level.

Try this first:

• Clean the filter and watch the pressure gauge come down
• Empty the skimmer and pump baskets
• Verify the pump is on high speed
• Top off the pool water level

ERR HLS — high limit switch tripped (overheat)

Water leaving the first pass of the heat exchanger exceeded roughly 135°F and the overheat safety opened. The cause is usually low flow through the heater, a scaled or sooted heat exchanger, or a stuck thermal regulator.

Try this first:

• Run the same flow checks as ERR PS — filter, baskets, valves
• Allow the heater to cool before it retries

ERR AFS — air flow switch (combustion air not proven)

The blower's vacuum switch didn't prove combustion air, so the heater refuses to release gas — exactly what it's designed to do. Causes include a blocked exhaust vent, debris or an animal nest in the air intake, a weak blower motor, or a cracked vacuum tube. It auto-clears once airflow is restored.

Try this first:

• Look at the top vent from outside and clear leaves or debris off the exterior grille only
• Make sure nothing is stacked around the heater blocking airflow

ERR AGS — automatic gas shutoff (redundant overheat)

Water temperature after the second exchanger pass exceeded roughly 140°F, so the redundant overheat safety cut the gas supply. A tripping AGS suggests the primary safeties may also be compromised — a serious flow restriction, failed thermal regulator, or scaled exchanger is usually behind it.

Try this first:

• Confirm the flow basics only (pump on, filter clean, valves open)
• Power-cycle once. If AGS returns, turn the heater off and call

ERR SFS — stack flue sensor over-temperature

Per Pentair's documentation the front panel is disabled and a power cycle is required, but the underlying causes — a sooted heat exchanger, poor combustion, a blocked flue, or low water flow — all need professional combustion analysis. There are no safe homeowner checks for this one.

ERR IGN — ignition failure lockout

The ignition module tried to light the burner three times and never proved flame, so it locked out. The simplest causes are on the supply side: a closed gas valve to the heater or, on propane systems, an empty LP tank. Beyond that it's a fouled igniter, bad flame sensor, or failed gas valve.

Try this first:

• Confirm the gas supply valve to the heater is on
• On propane, confirm the tank isn't empty
• Press "Heater Off" then back on once to reset

E-Codes: Sensor Circuit Faults

E-codes point at the heater's temperature sensors rather than the safety chain. Sensor and wiring inspection happens inside the cabinet, so homeowner options are limited — but knowing the code before you call speeds up the repair.

E01 — water temperature sensor open

The water thermistor circuit is open — typically a disconnected or broken sensor wire, or a failed thermistor.

Try this first:

• Press "Heater Off" to reset once

E02 / code 126 — water temperature sensor shorted

The water temperature sensor circuit is shorted, usually from water intrusion into the sensor or a failed thermistor. Some displays show this as code "126." There are no safe homeowner checks.

E05 — stack flue sensor open

The flue sensor that reads exhaust temperature has an open circuit — a broken wire or failed sensor. Because this sensor often fails alongside a clogged exhaust, sooted exchanger, or low flow, it deserves a real diagnosis rather than just a part swap.

E06 — stack flue sensor shorted

The flue sensor circuit is shorted, typically a water-damaged or failed sensor element. No safe homeowner checks.

The thermal regulator — the no-code repeat offender

The internal thermal regulator meters water through the heat exchanger to keep it at operating temperature. When it sticks closed, water overheats inside the exchanger and you get recurring HLS and AGS trips — and phantom Service System lights — even with a clean filter. When it sticks open, the heater runs cool, condenses, and soots up, setting the stage for SFS and E05 faults later. It lives inside the heater's water manifold; there's nothing homeowner-serviceable here, but replacing it (often together with the thermistor, HLS and AGS sensors) is a common, permanent fix for heaters that keep throwing service lights.

Why Homeowners Across Pinellas Call Us for Pentair Heaters

• Gas-licensed in-house (LI4527): ignition lockouts, AGS gas shutoffs, and flue-temperature faults are gas-train diagnoses — in Florida that's licensed-gas work, and we don't sub it out.
• Electrical-licensed too (EC13011994): MasterTemps run on 240V with safety circuits throughout — one truck handles the wiring side as well as the burner side.
• Root-cause repairs: we chase the thermal regulator and flow problems behind repeat HLS/AGS trips instead of resetting the same code every month — important with Florida heaters running most of the year.