Codes 15918 & 15920 glow plugs

[Dusty605]

Hi, I have the above codes coming up on Carista. There is definitely an issue as when she's cold on a cold day, she is a cow to start. Is it most likely to be the glow plugs themselves or a wiring or module issue? Is there any way of testing without stripping it all down? It's still driving fine once started and restarts easily enough. Thanks.