Pentair MasterTemp E06: Shorted Stack Flue Sensor

What E06 Actually Means

Where E05 means the stack flue sensor circuit is open (infinite resistance), E06 means it is shorted (near-zero resistance). The control board interprets a short as the sensor reporting an impossibly high temperature, because the sensor's resistance decreases as temperature rises. Near-zero resistance would imply temperatures far beyond the sensor's range.

The control board treats this as a critical fault and performs a hard lockout, disabling the front panel entirely. Only a power cycle at the breaker will restore operation after the sensor is repaired or replaced.

A shorted sensor is often caused by heat damage to the wiring insulation. When the insulation melts or burns away near the sensor mounting point, the bare wires can contact each other, creating a dead short. Less commonly, the sensor element itself can fail internally in a shorted state.

Step-by-Step Troubleshooting

Owner-Level Checks

Note the lockout state

1. The front panel will be completely unresponsive. This is expected for E06.
2. Do not cycle the breaker until you have a plan to investigate the sensor.

Tech-Level Checks

Inspect the sensor wiring for damage

1. Power off at the breaker. Open the heater and trace the stack flue sensor wire from the control board to the sensor in the exhaust path.
2. Look for melted insulation, bare wire, or places where the two leads contact each other. This is the most common cause of E06.
3. If you find damaged wiring, the repair may be as simple as replacing the sensor assembly (which typically includes new leads).

Test the sensor

1. Disconnect the sensor leads from the board. Measure resistance across the leads.
2. At room temperature, the sensor should show a specific resistance per the manual's chart. If it reads near zero ohms, the sensor is shorted.
3. If the sensor itself reads correctly when disconnected from the harness, the short is in the wiring, not the sensor.

Check for water or condensation in the connector

1. Moisture in the control board connector can create a low-resistance path between pins, mimicking a shorted sensor.
2. Dry the connector thoroughly and apply dielectric grease to prevent recurrence.

Replace and test

1. Install the new sensor, route the wire carefully away from direct heat, and secure it.
2. Cycle the breaker to restore the front panel.
3. Call for heat and verify the heater fires and runs without error.

Common Parts That Fix This Problem

• Stack flue sensor assembly (includes sensor and leads)
• High-temperature wire if repairing the harness
• Dielectric grease for the connector
• Control board (rare, only if the board-side connector is damaged)

Model-Specific Notes

• E06 requires a power cycle to reset, unlike E05 which may clear with HEATER OFF. This is because the board interprets a short as an extreme temperature reading and applies the same hard lockout as ERR SFS and ERR AGS.
• On HD (Heavy Duty) and ASME models, the sensor and wiring are identical to standard models. The diagnostic process is the same.
• When routing the new sensor wire, keep it secured away from the heat exchanger fins and burner flame path. Use the same routing as the original installation.
• If both E05 and E06 have occurred on the same unit within a short period, suspect chronic heat damage to the wiring rather than coincidental sensor failures.

How to Prevent E06 From Coming Back

• Inspect the stack flue sensor wiring at every annual service. Heat damage is progressive and visible before it causes a failure.
• Ensure the sensor wire is routed per the factory installation and is not resting on hot surfaces.
• Replace the sensor proactively if the insulation shows discoloration, cracking, or brittleness.
• Apply dielectric grease to the board connector to protect against moisture intrusion.