Q: P0684 glow plug control circuit light on?

The codes do not necessarily mean the glow plugs themselves are faulty. And, in any event, glow plugs are easily tested and should be tested prior to replacement so you don’t inadvertently replace good parts. Other causes of the codes you have cited include loose or corroded circuit connections, a short in the wiring, such as from the PCM to the GPCM, the bus bar, or from the glow plug to the bus bar, and insufficient voltage due to a battery issue. The GPCM can also fail. The only way to know for sure is to perform a diagnostic such as that outlined in the factory service manual. All components have to be tested to pinpoint the exact cause of the problem. If you request a check engine light diagnostic, a certified mechanic will resolve this on a mobile basis, right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.