It sounds like you may have a wiring issue or, as you said, bad glow plugs. I have never seen faulty glow plugs set a code. It is unnecessary to pull the valve covers to check glow plugs. Unplug the wire harness at the valve cover and ohm the outer most pins (furthest from the center); these are the glow plug terminals. If no other symptoms are occurring, it’s probably safe to drive. You will, however, experience white-blue smoke and fuel-smelling exhaust when cold.