Small wirelessly monitoring

Hello all. Here comes a question from a beginner. I have a challenge with a program for my boat where I want to. some type of function (433mhz, Bluetooth etc) where I want to monitor that the device is available, let's own every 20 seconds. it should be as small as possible. I will use 1 mini pro on each device. Imagine any kind of Ping or similar. If you remove a part, the engine does not work. It must not be cordless but must be wireless. The break function in the engine is complete so I need help with communication.

Please Gussen

Your description is not at all clear. For example I reckon cordless and wireless are much the same thing.

What range is this thing supposed to work over?

What are the circumstances where you want to know if something is UNavailable - what would cause the unavailability?

Having a wireless transmitter transmit a short message every few seconds and having a receiver raise an alarm if the messages stop coming may be one option.

Simple nRF24L01+ Tutorial