I have for some time been having trouble with my Cat track loader refusing to register the safety arm bar status. Basically if the safety bar light stays on you go nowhere and cannot proceed with the start up sequence. I found a high resistance connection on the bonding strap between the tilt cab and the main frame of the machine. The clamping bolt was very rusty. The current path is via the underside of the bolt head and through the bolt threads to the internal threads of the weld on boss on the machine main frame. All the threads were cleaned and loaded with anti seize, all assembled and worked correctly first time. I think that galvanic corrosion had caused the high resistance joint. We all know about this type of problem but sometimes we forget. BEWARE THE RUSTY BOLT !!!!