A board

It made sense, and I understood everything! Just a couple small changes to make it sound a little more natural! Great job!

A week ago we replaced a damaged board with a refurbished(1) one and the system started working normally. We've noticed unusually low values of the  parameter A in comparison with the parameter B for 2 days. We would appreciate it if you could give us some advice on this matter.

(1) in English, when you have a brand new piece of equipment you would say "a new one". But as you know, the replacement part is usually not brand new from a factory. For example, if you have a water pump that has a broken seal, the pump will be removed from the engine and sent back to the factory, and the seal will be replaced and the pump will be put back into a box so that it can be sold as a "refurbished" part. If a mechanic or an engineer orders a "refurbished" part he knows that it has been used, and that something broke on it, but that it was repaired. Sometimes we say "refurbished part" and sometimes we say "rebuilt part". When a person uses the term "rebuilt part" they are usually referring to a mechanical thing, like an engine, a motor, or a pump. The word "refurbished" is more common when you are talking about electronic parts.

https://en.wikipedia.org/wiki/Refurbishment_(electronics)