In March 2025, I had a glow plug fail in the third cylinder from the timing belt. Everything was fine after the replacement, until yesterday when my car passed the MOT. However, I was reminded that I have a code, P0161D, and the mechanic noted on paper that a new glow plug is needed for the fourth cylinder.

I did some research for that code, but I could not find any information specifically about faulty glow plugs. It says to check the relay under the fuse box in the engine bay or the wiring harness connecting the glow plugs number 4.

What does the term "A-ground circuit" mean?

My car is 2.0TDI PD 140BHP 2009

The glow plugs weren't Chinese cheap, I bought OEM NKG https://www.ebay.co.uk/itm/156118910533

Check the wiring to number 4. I think the ground circuit is the path taken back to the neutral side of battery from glow plug. It could be a faulty plug,wiring isssue or the glow plug control timer unit itself. If they have just been replaced they should come out fairly easily for testing. You could always swap 4 and say three to see if the fault code changes cylinder to rule out glow plug or confirm fault in wiring.

Alasdair

I think your better removing them whe the engine has been run and is hot. Thats what my old mechanic friend always said. He reckons its less likely to snap them. As yours have been replaced it shouldnt be a problem.

Alasdair