Had a similar problem with an "ERR" displayed.  (error message without a code attached i.e. ERR2 or ERR5... just plain "ERR")
Turns out after several phone calls and trips to where I bought it, (I'll never suggest these guys for business ever! But I digress) The main internal circuit board's capacitor (part of the board) was not up to snuff and the whole circuit board had to be replaced with one that had a larger service capacitor (its a big cylindrical black-like tube structure wired to the board).  The technician that installed it told me that when it comes to treadmills, it's all in the electronics...  pay for a better unit that has better circuit boards with capacitors that can handle the stress loads.
                
 Bob McCulloch  Ottawa, Ont.
                
 April 2008