TDI Mystery - Flashing Glow Plug Light & Check Engine Light

Hello!
I recently purchased a 2010 Golf VW TDI from Auto quest in a Calgary.
A week after I picked it up, the check engine light came on. I had my mechanic scan it and we replaced a glow plug and everything was running smoothly afterwards.
A couple months later my glow plug light came on and was flashing, and my car had less power. It’s not super noticeable, but if I was driving up a long hill I would lose speed, and it wasn’t as peppy when accelerating.
I’ve taking it into the mechanic a few times, this problem is intermittent and when the lights aren’t on the car runs great and everything checks out perfectly.
The mechanic has run numerous tests and can’t seem to figure it out so I thought I would check on here to see if anyone has experienced anything like this.
The lights are on almost constantly now.
The lights are: a flashing glow plug light and the check engine light.

When the lights are on:
My car accelerates slowed and has less power, but it’s deffinitely still driveable. It loses speed uphill, especially if it’s a long or steep hill.

Turbo is checking out fine, with all the tests the mechanic has ran. He thinks it’s a communication issue?

Any ideas?!

Scan it with the correct tool? (IE *NOT* a generic OBD reader . . .). Anything that sets those warning lights *WILL* set codes!

Yes, there will be codes.. the flashing glow plug light is just a "fix me ASAP" Check Engine light.

The car knows what's wrong and is trying to tell your mechanic... if he/she can't figure it out you need a second opinion.

We have checked the codes and tested appropriately for each one! Everything checks out clear. I’ll ask him to send me the codes and tests he has done! That will help narrow it down.

Here's an important thing to clarify: is your mechanic using a VW-specific scanner or a generic one?

I ask because generic code readers can only see a small fraction of the actual universe of Volkswagen codes.

Search for threads by MOGolf; he started one that lists all the causes of a flashing GP light, but it might be outdated.