Pentair MasterTemp E05 Versus E06 Ignition Fault Tree

Why this matters

The Pentair MasterTemp (125, 175, 200, 250, 300, 400 BTU) throws E05 and E06 as its two most common ignition faults, and they sit one step apart in the firing sequence. E05 is "ignition lockout after three trials" - the heater tried to light and never proved flame. E06 is "stack flue sensor / flame current fault after a successful light" - the heater lit, then the integrated firing control either lost flame current or saw a stack-temperature problem. Misreading one for the other sends a tech swapping an ignition control board when the real fault is a fouled flame rod, a partially blocked exhaust, or a gas-supply pressure drop. This tree walks the MasterTemp ignition sequence in firing order and splits E05 from E06 by where the heater stops.

Symptom presentation

On the two-line LCD you see either:

• E05 - "Ignition Lockout." Heater purges, the hot surface igniter glows, the gas valve opens, but flame never establishes (or never holds the trial-for-ignition window). After three failed trials the control locks out for one hour or until power-cycled.
• E06 - "Stack Flue Sensor Open" on older firmware, or a flame-current / stack-temperature fault on newer boards. The heater lights and runs, then drops out, often after several minutes, and may relight repeatedly before locking.

Pull the heater's run history if the board supports it. A pattern of E05 every cold start points at ignition; an E06 that appears only after the heater has been firing several minutes points at flame sensing or exhaust.

Quick checks

Before opening the cabinet:

• Filter pressure and pump speed. MasterTemp needs the water-pressure switch closed (roughly 2-3 PSI rise across the heater bypass). Low flow on a VS pump set too slow will not throw E05/E06 directly but causes nuisance high-limit and stack faults that mimic them.
• Gas supply. Confirm the gas is on and any other gas appliance fires. A nearly-empty LP tank or a regulator iced over in winter is a classic E05.
• Air filter / louver. The MasterTemp pulls combustion air through a side air filter. A leaf-packed filter starves the burner and produces E05 (no light) or E06 (lights, then sooty flameout).

Isolation tree - E05 (lockout, no flame)

1. Igniter glow. Watch through the sight glass on a call for heat. The hot surface igniter (P/N 472100) should glow bright orange. No glow: test it cold for continuity (should read roughly 40-75 ohms; open = replace), and confirm 120 VAC at the igniter leads during the trial.
2. Gas valve opens. With the igniter glowing, listen and meter the valve. The ignition control sends 24 VAC to the valve at the end of igniter warmup. Igniter glows, valve gets signal, but no gas: valve fault or no gas at the valve.
3. Gas pressure under fire. Manometer on the inlet tap during the trial. Natural gas needs 4.5-10.5 inch w.c.; LP needs 11-13 inch w.c. The single most common E05 on a new install is undersized gas line - a half-inch line where the run length demands three-quarter inch - so inlet pressure collapses the instant the valve opens.
4. Flame rod position. If gas lights briefly but the trial fails, the flame rod may be out of the flame envelope (bent after a swap) or carbon-bridged. This blurs into E06 territory - if it lights at all, suspect flame sensing next.

Isolation tree - E06 (lit, then dropped)

1. Flame current. E06 most often means the flame rod is not rectifying enough signal. Meter microamps DC in series with the flame sense lead. MasterTemp wants a stable reading above roughly 1.5-2 microamps DC. Low or drifting: clean the flame rod with fine emery cloth (never steel wool), confirm a tight burner-to-cabinet ground, and verify the rod sits in the flame.
2. Stack flue sensor. Older firmware labels E06 "Stack Flue Sensor Open." The stack sensor (thermistor in the exhaust) reads open or out-of-range when the connector corrodes, the sensor fails, or exhaust is partially blocked. Meter the sensor resistance against the Pentair stack-sensor temperature table; an open circuit at the connector is a frequent find.
3. Exhaust restriction / downdraft. A heater that lights clean then trips E06 after minutes is often backdrafting or running rich from a restricted exhaust or a sooted heat exchanger. Inspect the top of the heat exchanger and the exhaust path. Soot on a heater that should burn clean equals air starvation or downdraft.
4. Heat exchanger sooting from low flow. Chronic low water flow lets the exchanger overheat locally, soots the tubes, and degrades both flame sensing and stack temperature - producing intermittent E06. Confirm adequate GPM and clean the exchanger before condemning the board.

Confirming the diagnosis

E05 confirmed when, with the igniter glowing and the valve signaled, you prove gas does not establish a stable flame within the trial - and you have a specific upstream cause (low gas pressure under fire, dead igniter, no valve actuation, flame rod out of envelope).

E06 confirmed when the heater lights and holds a visible flame but the control still faults - and you measure either low flame microamps, an open/out-of-range stack sensor, or evidence of exhaust restriction. A board that reports E06 with a clean strong flame and a good stack sensor reading is the rare case where the firing control itself (P/N 42002-0007S) is suspect.

Remediation

• E05 from gas supply: correct line sizing, regulator, or LP fill; re-manometer under fire to confirm pressure holds within spec.
• E05 from igniter: replace the hot surface igniter, verify glow and continuity.
• E06 from flame rod: clean or replace the flame sensor, restore ground, re-seat in the flame; recheck microamps.
• E06 from stack sensor: replace the stack flue sensor and repair the corroded connector.
• E06 from exhaust/soot: clear the restriction, clean the heat exchanger, correct the downdraft or low-flow root cause before relighting. Never reset E06 repeatedly without finding why it sooted.

References

• Pentair MasterTemp Installation and User's Guide, document P/N 472592 Rev L
• Pentair MasterTemp Service and Troubleshooting Guide (fault-code table E01-E27)
• ANSI Z21.56-2022 / CSA 4.7-2022, Gas-Fired Pool Heaters
• NFPA 54 / ANSI Z223.1-2024, National Fuel Gas Code, Sections 7 and 9
• Pentair Technical Bulletin on MasterTemp flame-sense microamp threshold and stack sensor resistance table