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F01

F01 Fault — Return sensor NTC interruption

Vaillant · ecoTEC plus Boilers

What does F01 mean?

There's an interruption with the return sensor NTC. This could be due to a disconnected plug, a loose connection on the PCB, a break in the cable, or a defective sensor.

Common Causes

  • Return sensor NTC plug not plugged in or has come loose
  • Multiple plug on the PCB not plugged in correctly
  • Interruption in cable harness
  • NTC defective

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Check that the NTC sensor is connected properly and securely.

  2. 2

    Check the resistance of the NTC sensor (requires specialist tools).

  3. 3

    Check the continuity of the wiring harness connected to the NTC sensor (requires specialist tools).

Browse all codes in this manual (100)
Verified technical data. Last updated: April 2026

Related Faults

F.0F.0 — Flow–NTC open circuitF.1F.1 — Return–NTC open circuitF.10F.10 — Flow NTC short circuit
Source: Vaillant ecoTEC plus Boilers
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