Mirai key fob battery issue

Experiencing issues such as keyless entry failure, error messages, and difficulty locking/unlocking could indicate a low battery in your key fob. However, have you considered radio interference or a malfunction in the vehicle's receiver? It may be wise to investigate these possibilities before assuming it is solely a battery issue.

Radio interference? Seriously? I never even thought of that. How would I check for that? Is there an easy way to rule it out before I start taking the key fob apart?

While radio interference is less common, it can occur. If you are experiencing these issues consistently in one particular location, that could be an indication. If the problem persists regardless of location, it is more likely the fob battery. Given the symptoms and the vehicle's mileage (80272 km), replacing the battery is a reasonable first step, or you could visit a trusted workshop for a formal diagnosis.

I put a new battery in the fob today, it cost 37€. Problem solved! It seems it really was just the battery being flat.