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F55

F55 Fault — CO sensor issue

Vaillant · ecoTEC plus Boilers

What does F55 mean?

A fault has been detected with the CO (Carbon Monoxide) sensor. This could be due to a cable harness issue, a defective all-gas sensor, or a fault with the main electronics.

Common Causes

  • Cable harness issue
  • All-gas sensor defective
  • Electronics defective

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Check the cable harness connected to the CO sensor for any damage or loose connections.

  2. 2

    If the all-gas sensor is defective, it needs replacement (contact a technician).

  3. 3

    If checks fail, the PCB (electronics) may be defective (contact a technician).

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
Open Manual (Page 5)Search more codes in this manual