Follow the glow plug wire back to the relay and check that.
But be warned,
more than half of these I see have shorts in the wiring harness.
Which can throw up all kinds of faults.
That fault description sounds a little OBD generic,
Better quality diagnostic tools will give better results,
like short to earth,short to +,open.............
Get it rescanned.
The plugs on these are only supposed to come on at VERY low temperature,
1/4drive, May 2014