The most common problem with a yellow LED is due to a (too) late descaling.

Our standard advice is to check the cell nucleus for limescale once every 2 to 3 weeks and descale if necessary with cleaning vinegar or citric acid. It is even better to check every week, at a fixed time, so that you find out whether there is a lot of lime in the water.
In areas with hard to very hard water, such as Portugal and Spain, even more often.
It may even be necessary to check for limescale every 3 to 4 days!

By descaling the cell core at regular intervals, adjusted to the lime content in the water, you avoid damage to the electrolysis unit. Descaling only takes about 30 minutes and ensures an optimal lifespan of the cell nucleus.

Note: If you fail to descale the electrolysis unit enough in time, the lime will accumulate under the cell nucleus. The accumulated hard lime then crushes the clamps that hold the cell nucleus, a new cell nucleus is then the solution.
Or worse: the tip at the top of the electrolysis unit breaks off, resulting in the need to replace the most expensive part, the electrolysis unit! This is visible in the bottom photo. Despite the insertion of a new cell nucleus, the yellow LED remains lit.
The risk is thus minimized by regular checks. This prevents costly replacement of the entire electrolysis unit with cell core. Prevention is better than replacement!

Electrolysis unit with top (without cell nucleus). GOOD!

Electrolysis unit without top (without cell nucleus). WRONG!

It needs to be renewed.....