Indirect-fired Weil-McLain water heater does not maintain set temperature; boiler control shows 'Tank Sensor Error' or similar.

Safety first: Turn off power to the boiler and water heater. Hot water in the tank can cause burns; do not remove the sensor well plug while hot.

• Locate the tank sensor: On indirect tanks, the sensor is usually inserted into a dry well on the side of the tank. Verify it is fully inserted and secured.
• Inspect cable and connections: Follow the sensor cable back to the boiler control. Look for cuts, abrasions, or loose terminals. Reseat connectors.
• Check sensor seating: If the sensor has backed out of the well, push it fully in until it bottoms out. Poor contact can cause erratic readings.
• Resistance test: With power off, disconnect the sensor and measure resistance. Compare to the Weil-McLain sensor chart. Replace if out of specification.
• Install correct replacement: Use only the specified Weil-McLain tank sensor model; mixing sensor types can cause incorrect temperature control.