I own a 2008 Chevrolet Silverado 2500 with a persistent P0678 diagnostic trouble code. I've already replaced the glow plugs three times (using parts from O'Reilly) and the glow plug control module (from a dealership), but the P0678 code continues to return. Any insight into what might be causing this? Thank you for your help.
If the resistance of the glow plug (GP) exceeds 1 ohm, it should be replaced. As noted, is there a full 12 volts applied to the glow plug when the Powertrain Control Module (PCM) commands it on? Compare this reading with the adjacent cylinder. Confirm that the wiring from the control module to the glow plug or at the connector itself is intact—this could be the source of the issue.
Verify the wire gauge and diameter (wiring dia) of the glow plug harness, especially between the control module and the glow plug. A mismatched or degraded wire may not handle current properly and could cause intermittent faults despite replacement efforts.
Check the voltage supplied to glow plug #8. Test one or two other cylinders as well and share both readings. A low or inconsistent voltage could indicate poor wiring. If all glow plugs show similar voltage levels, then the wiring is likely fine—next step would be measuring current draw (in amps) for glow plug #8 and another cylinder. An ammeter clamp would be ideal. Each glow plug may require 8–12 amps to operate properly.