There were no prototypes built and given that this locomotive was on the cutting edge of technology in its day, that turned out to be a mistake. After a short time they started having problems and failures in service and it took SNCB over five years to sort them all out. The first problem area was the ACEC Type G transmission. They all had to be rebuilt with stronger gears and that fix solved the problem. Oddly enough, the Type G transmission was the only part of the Class 20 that had been fully tested before entering service. The reason it caused problems in service was that it was tested on locomotive 124.001/2401/2383 (all the same one) which had four much less powerful traction motors compared to the six much larger motors of the Class 20. The other major problem was the thyristor control system for the traction motors. Some major modifications were made over time and reliability greatly improved. Even after these changes they had to be driven gently for the rest of their service lives. They could still run at their top speed of 160 KmH (99 MPH) but maximum acceleration was forbidden.