Flow NTC is normally blue, return NTC red on Vaillant ecoFIT Pure 435. Do not swap.

NTCs are non-polar — but keep leads clear of ignition HT leads.

Do I need to notify the manufacturer?

Only if the Vaillant ecoFIT Pure 435 is in warranty — Vaillant require log of any part swap for labour claims.

How to Replace the Ntc Sensor on a Vaillant ecoFIT Pure 435 (UK)

Replace an NTC thermistor on Vaillant ecoFIT Pure 435 for wildly inaccurate flow temps or an F22/F75/L1 low-water fault.

Each NTC on Vaillant ecoFIT Pure 435 clips onto the copper flow pipe with a spring clip. Fit fresh heat-sink compound before refitting.

Cross-check resistance with a multimeter — 10 kΩ at 25 °C for standard blue-body NTCs.

Refill the Vaillant ecoFIT Pure 435 to 1.2 bar cold. Bleed high-point radiators and the internal AAV.

Run a full flue-gas analysis after 15 minutes. Log the reading against the OEM commissioning envelope in the Benchmark book.

Which colour?: Flow NTC is normally blue, return NTC red on Vaillant ecoFIT Pure 435. Do not swap.
Wire polarity?: NTCs are non-polar — but keep leads clear of ignition HT leads.
Do I need to notify the manufacturer?: Only if the Vaillant ecoFIT Pure 435 is in warranty — Vaillant require log of any part swap for labour claims.

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