I had the same problem, the A4 from the Easterner train set. Worked fine on the transit controller. As soon as I inserted the chip and tried to run it under default settings, nothing worked but on checking I got the "overcurrent message". Sent it back to Hornby (Loco and Next18 chip). It was returned as repaired and working. Put it on the track and again nothing. Overcurrent message returned. Put blanking plate back in Loco runs fine on transit controller again. I had already sent the Loco and original decoder back as it would not accept any downloads. It was returned as fixed as above. So Loco is back in box and chip is sitting on the shelf. Don't think I will be continuing with my planned layout build now in TT120.