Fault codes P13D4 & p13D6

Hi

hoping for some answers to my problem from some knowledgeable people.

i have been getting fault codes p13d4 and p13d6 for some time now, I was told by the local garage that it was glow plug 3 that was causing this so had them all replaced.

the faults came back pretty quickly and didn’t seem to solve the issue, I thought it might have been a faulty glow plug 3 (the one with the sensor) so replaced it with a beru one around a week ago but still no luck.

the engine management light comes on and gos off randomly but always comes back on, even after clearing the codes.

the car seems to run fine.

car is :

skoda Octavia 2015 1.6tdi

WG15BEO

thanks for any advise as to what it could be in advance,