Before I bought the A4 (2006, 2.0 TDI, 140BHP) about a month ago, the EML was on so I read the codes which were:
P0670 Glow plug circuit fault.
P0671 Glow plug #1.
P0673 Glow plug #3.

The seller replaced plugs 1 & 3 and all was good.

2 days later the EML came on again showing the same codes..

So I decided to swap plugs 1 & 2 with each other to see what happens. Checked the part numbers all correct and you could see that 1 & 3 had recently been replaced.

EML on again, this time only showing P0670. So I put the plugs back in their original position and waited for EML to reappear the next day. Read the codes and again only showing P0670, which proves the plugs are all good.

So yesterday I replaced the Glow plug/Injector wiring harness, and again today the EML appeared showing code P0670.

I’m thinking now that maybe its the glow plug relay.
Anybody have any idea where the relay is located or have any other idea what the problem might be?