2005 e320 cdi:

Glow plug preheat light didn't go off as usual today and I inadvertently started up anyway but turned the engine off right away. Turned the key on preheating coil again didn't go off. Turned the key off then tried again and everything was normal; car runs fine except the check engine light is on.
Scanned with my MBi980. No codes but scanner said to check the wires between # 6 glow plug and preheater. I have searched Google can't locate cylinder numbering????? Is # 6 at the front or the back?

Anyone with an opinion on what happened and the fix?

The other question is how to shut off the "check engine light"?