There are a few different issues that can crop up with an fan coil coolers, and they often manifest themselves in similar ways. For example, you might notice that your system isn’t cooling as effectively as it used to, or that it’s taking longer to cool a space than normal. These are both signs that something is wrong with the coil itself, and there are a few possibilities for what’s actually happening:

Dirty coil: Perhaps the most common problem, a dirty coil can cause all sorts of issues. As the name implies, dirt and debris will collect on the coil over time; this provides insulation that prevents heat from being properly absorbed. In some cases, the coil might be so covered that air can’t even flow through it efficiently.
Refrigerant leaks: These are somewhat common in fan coil coolers, and they can cause a host of issues. A refrigerant leak will cause the coil to freeze over, which then prevents it from absorbing heat as intended. This can quickly lead to an AC unit that doesn’t work at all, so it’s important to have any leaks repaired as soon as possible.
Corrosion: Corrosion is another problem that’s relatively common infan coil coolers. This often happens when the coil itself is old, or when it wasn’t installed properly to begin with. In some cases, though, corrosion can be caused by a refrigerant leak; this is because the chemicals within refrigerant can eat away at metal if they’re not contained properly.