I was doing some work recently which involved having the manifold off and noticed I had an error on one glow plug so I thought I'd go ahead and replace all 6 (all 6 glow plugs and modules were replaced about a year ago previously). I didn't have any bad symptoms like a rough/lumpy cold start in regards to the one glow plug error.
Once I changed the glow plugs (I went for these
https://www.ebay.co.uk/itm/372618154461) straight away on cold starts, especially in the colder mornings the car would run lumpy for 5-10 secs. I checked for errors and now it brings up an error on all 6 glow plugs (DDE: Glow plug, cylinder 'X' activation). So either all 6 plugs are defective or the glow plug module is, most probably the latter.
So my question is, should you always replace the glow plug module together with the plugs? If you don't do this, can this cause the glow plug module to go bad? Seems a bit of a coincidence that as soon as the new 6 plugs were installed it runs worse and I go from one plug error to six.