MSPA spa will not heat; display shows E1 shortly after power‑up or when trying to raise temperature.

Safety first: Unplug the MSPA spa from the mains and ensure no one is in the water before working on it. Water and electricity are dangerous together.

Step-by-step checks:

• Power reset: Disconnect power for 5–10 minutes, then reconnect and see if E1 clears. Temporary glitches can trigger sensor errors.
• Inspect sensor cable: Locate the heater housing on the MSPA external control unit or integrated motor unit. Follow the thin cable from the heater tube to the control box. Check for cuts, pinches, or rodent damage. If damaged, the NTC sensor assembly must be replaced.
• Check connectors: Open the control box cover (if user‑serviceable on your MSPA model). Find the temperature sensor plug on the PCB (often labeled NTC or TEMP). Reseat the connector firmly and ensure no corrosion or moisture is present. Dry gently if needed.
• Measure resistance (for advanced users): With power disconnected, unplug the sensor from the PCB and measure resistance across the two sensor leads with a multimeter. At room temperature (~20–25°C), most MSPA NTC sensors read roughly 8–15 kΩ. A reading of open circuit (OL) or near 0 Ω indicates a failed sensor.
• Replace sensor: If resistance is out of range or the cable is damaged, replace the MSPA heater NTC temperature sensor assembly. Route the new cable along the original path and secure it away from sharp edges and hot surfaces.

If E1 persists after sensor replacement, the fault may be on the main control PCB and a professional MSPA service technician should be consulted.