Pentair MasterTemp E05: Open Stack Flue Sensor

What E05 Actually Means

The stack flue sensor is mounted in the exhaust path of the MasterTemp. It monitors combustion exhaust temperature and triggers ERR SFS if the temperature exceeds 480 degrees F. When the control board detects an open circuit on this sensor (infinite resistance), it displays E05 and prevents the heater from firing.

This makes sense from a safety perspective. Without the ability to monitor exhaust temperature, the heater cannot protect against a runaway stack temperature condition. The board locks out until the sensor circuit is restored.

E05 is a sensor failure code, not a temperature alarm. It means the sensor itself or its wiring is broken, not that the exhaust was too hot. Compare this to ERR SFS, which means the sensor actually detected excessive temperature.

Step-by-Step Troubleshooting

Owner-Level Checks

Check the sensor connector

1. With power off, locate the stack flue sensor wire at the control board. It connects to a designated terminal or plug.
2. Ensure the connector is fully seated and not corroded. Exhaust moisture can migrate along wires and corrode connections.

Tech-Level Checks

Test the stack flue sensor

1. Power off. Disconnect the stack flue sensor leads from the board.
2. Measure resistance across the sensor leads. At room temperature, the sensor should read a specific resistance per the manual's chart. If it reads OL (open), the sensor has failed.
3. If the sensor is accessible, you can also heat it gently with a heat gun and watch the resistance change to confirm it responds to temperature.

Inspect the wiring

1. The stack flue sensor sits in the exhaust path where temperatures approach 400 degrees F during normal operation (the manual specifies vent gas temperature is less than 400 degrees F). This environment degrades wire insulation over time.
2. Trace the wire from the sensor to the board. Look for melted insulation, brittle wire, or breaks near the sensor mounting point.
3. Check continuity on each wire independently. An open wire mimics a failed sensor.

Follow the manual's diagnostic diagram

1. The MasterTemp manual includes a specific diagnostic flow for stack flue sensor codes. Follow it step by step.
2. The diagram will walk you through verifying the sensor, the wiring, and the board connector in sequence.

Common Parts That Fix This Problem

• Stack flue sensor
• Sensor wiring harness (often sold as part of the sensor assembly)
• High-temperature wire and connectors if repairing the harness
• Control board (rare, only if the board connector is damaged)

Model-Specific Notes

• All MasterTemp models use the same stack flue sensor type and mounting location in the exhaust path.
• The stack flue sensor is exposed to the harshest thermal environment in the heater. It has a finite lifespan and should be considered a wear item.
• E05 (open sensor) and E06 (shorted sensor) both disable the heater. E06 requires a power cycle to reset. Refer to the manual's diagnostic flow chart for E05 reset procedures specific to your model revision.
• When replacing the sensor, route the new wire away from direct flame contact and secure it with high-temperature fasteners.

How to Prevent E05 From Coming Back

• Inspect the stack flue sensor and its wiring during every annual heater service. Look for insulation degradation.
• Replace the sensor proactively if the wire insulation shows heat damage, even if the sensor still reads correctly.
• Ensure the sensor is properly mounted and not hanging loose in the exhaust stream where vibration could break the leads.
• Keep the combustion chamber clean so flames do not impinge directly on sensor wiring.