Display shows E4; water temperature fluctuates or unit shuts down intermittently.

Safety first: Turn OFF electrical power and close water supply before removing covers.

Inspection steps:

• Locate temperature sensors: Remove the front cover. Identify the inlet and outlet NTC sensors threaded into the water path or clipped to the heat exchanger.
• Check wiring: Inspect the sensor harnesses for broken insulation, loose plugs, or corrosion at connectors. Reseat each connector firmly.
• Look for scale: If the outlet sensor is mounted on the heat exchanger, heavy scale can cause localized overheating. Consider descaling the heat exchanger if you see white or chalky buildup.
• Test sensors (if you have a meter): Disconnect the sensor and measure resistance at room temperature. Compare to Marey specifications. A sensor reading open (infinite) or short (near 0 ohms) must be replaced.
• Replace faulty sensor: Shut off water, relieve pressure, and unscrew the defective NTC. Install the new sensor with appropriate sealing (O-ring or thread seal per manual). Reconnect wiring.

When to call a technician: If you cannot access or identify the sensors, or if the control board appears to misread temperatures even with new sensors.