A few things can cause this. The velocity fuse in the manifold at bottom of mast but I don't think this is your problem because you didn't mention the cab lowering slowly. The truck has to see a steady pulse at predetermined intervals, if not then it will give the code. The other will be the encoders. The truck takes a snapshot of the total currents of the encoders when the truck is first setup. Everytime the truck is turned on from this point it compares it to the initial current and it has to be within (i believe) 80%. If not it shuts down the travel and gives this code. The other is the signal wires are damaged and truck thinks it is somewhere it is not. Which kinda is the same as the first but due to wiring not hydraulics. Most likely one of your encoders are bad. You can go into the diagnostics and programming in the dash and check the encoders without having a meter. When you find the problem encoder and get it replaced, you need to do a defaults in the dash to have the truck read the new encoder currents. Should take care of your problem. If you are able to lift the truck above 30 inches then turn the truck off then back on (assuming you already checked what Garland m said about prox switch.) Lower the truck all the way down. Will reset truck if your encoders are ok. Can get a little more in detail if needed... Just let me know.