Boiler intermittently shows E19, especially when external sensors or weather compensation are connected.

Safety: Isolate power before working on low-voltage sensor wiring.

• Check sensor location: Ensure the outdoor sensor is mounted correctly (north or northwest wall, away from direct sun and heat sources).
• Inspect cable: Follow the sensor cable route and look for damage, joints exposed to moisture, or rodent damage.
• Measure resistance (engineer level): Disconnect the sensor and measure its resistance. Compare with Thermona's temperature/resistance table. Replace if out of range.
• Bypass test: Temporarily disconnect the external sensor and configure the boiler to run without weather compensation. If E19 disappears, the sensor or its wiring is at fault.