Will this heuristic always deliver the message from node a to node b? if not, explain why

Heuristic is a mental shortvut that allows people to solve problems and make judgements in a quick and efficient way.

The fact of being a process usually applied automatically and unconscious may lead to several biases and errors.

A simple example is that we tend to judge a person by the clothe they wear.

One other example is that in the medical field the heuristic thought is actually discouraged because, before making a decision, the doctor needs to thoroughly investigate the patient's medical history (two indivuduals with similar characteristics, sometimes have different backgrounds which affect the treatment that will be set for each one of them).

Regarding your question, heuristic will always deliver the message from a to b, but not always properly because of its cognitive biases.