Machine displays ALARM 1 and will not heat; brew boiler temperature reading is missing or implausible.

Safety first: Turn the machine OFF, unplug it from mains, and allow it to cool completely before opening any panels. Hot boilers and live wiring can cause severe burns or electric shock.

Steps to check and address ALARM 1:

• Power-cycle the machine: Turn the machine off for 2–3 minutes, then back on. If ALARM 1 reappears immediately, continue with diagnostics.
• Inspect the brew temperature probe connection: Remove the top or side panel (depending on model). Locate the brew boiler and its temperature probe (usually threaded into the top or side of the brew boiler with two small wires leading to the control board). Ensure the connector is fully seated and not corroded or wet.
• Check wiring harness: Follow the probe wires back to the main control board. Look for pinched, cut, or burned insulation. Reseat the connector at the board.
• Basic resistance test (for qualified users with a multimeter): With the machine unplugged, disconnect the probe from the harness and measure resistance across the probe terminals. Compare to La Marzocco specifications for your probe type (PT100 or NTC) at room temperature. An open circuit (infinite resistance) or a dead short (near 0 Ω) indicates a failed probe.
• If the probe is faulty: Replace the brew boiler temperature probe with the correct La Marzocco part for your model. Use food-safe thread sealant if specified, and do not overtighten into the boiler.
• If wiring and probe test good: The temperature input channel on the main control board may be defective. At this point, contact a qualified La Marzocco technician or service center to test and, if needed, replace the control board.

Do not bypass or jumper the probe to clear the alarm; this can cause uncontrolled heating and serious damage.