I was getting error "ab 04", with rpms stuck at 1500. I continued to use it not sure of the solution. Eventually it started losing power too making it unusable. After reading through these forums and learning about the tier4 issues I decided to just let it idle outside by itself. After an hour or more the error was gone and I had power and control of the throttle back. I considered the egr valve, checked the grounds, everything I could find but thankfully it ended up being the simplest solution. It just needed some alone time to think about it's emissions. Been there brother!