So after the timing belt, water pump, plugs, etc major service (B14), the next code I got some 12k km later was a B156. The 5 means replace the engine coolant that was done anyways when the water pump was replaced. Yet, the maintenance minder says change coolant again. Is this legit or is it a bug?

 

No, it's not a bug. It has no way of knowing you replaced the coolant before since that wasn't part of the B14 service. The 4 in that code suggests inspection of the water pump and not necessarily replacement.

There's no need to replace it again this soon but then you run the risk of it reaching the end of its life before you get the 5 code again.

 

You will be alright till the next coolant change code. I also plan on replacing the water pump when the timing belt gets changed. The real rule with these new long life coolants is 5 years or 100K miles. Don't waste your money.