automationgame.com

Do yellow arrows affect the service costs of the car (less room for mechanics to work) or are they merely to let the player know there isn't much room left?

It's so show that the engine you're working on will fit in the car's engine bay. Once an arrow turns red, it's too big.

Yeah, but that is not his question. He wonders when the arrow changes colour, it increases maintenance costs on an engine, or its only a warning that you should be keeping an eye on increasing as it might turn our bad for maintenance costs.

Well, it does not increase the maintenance costs on an engine directly, but it will affect the car's maintenance costs once the rest of stats are introduced.

Excellent. That's what I was thinking of. I shall endeavor for green arrows where possible then.