CAT light and glow plugs

[Macsprocket1947]

My 1.6 TDI had no warning lights showing when I first bought it a few months ago. 2 of the glow plugs were open circuit so I replaced all 4 which was tricky as the plastic ends on the leads were disintegrating and had to be fished out of the plug recesses. At some stage the cat warning light came on, steady not flashing and I was UNRELIABLY advised from diagnostics that it was because the glow plug leads were disconnected. Since the leads were lying on top of the engine, it was an easy answer. I used Redex and a few days later the warning light stopped coming on. With the ignition check there is both a DPF light and a cat light plus others. When testing the electrics I could not get even a momentary voltage on the glow plug leads. Since the car is in Greece in reality the glow plugs are not needed. Any ideas on why the cat light came on appreciated plus a guide on how to replace the glow plug leads.
Thanks