I have recently had problems with the error light coming on when I try and apply a train of many pulses at higher voltages. Is there something wrong with my D185?

The D185 has certain safety limits that prevent excessive energy from being delivered to the patient. If you set the D185 to Normal mode and try to deliver too many pulses at too high a voltage, the unit will give an ERROR. You should consult the section titled Operating Modes in the D185 Operator’s Manual where you will find a graph that illustrated what these limits are.