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Hi Sir or Madam, I am John Mc, a Ford dealer technician.I hope I can help solve your problem quickly and accurately.
P0671 and p0678 are glow plug codes for the number 1 and 8 glow plugs. You could have a problem with those glow plugs, wiring or valve cover gaskets. You probably are just in need of new glow plugs. I would suggest replacing the entire set of plugs (good maintenance on a 10 year old engine) and retest. This should solve your problem. If not you will need to check those wires and if all the wiring, valve cover gaskets, and under valve cover wiring are ok you will need to replace your glow plug module.
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If you have received all 8 codes then I would suggest replacing the glow plug control module. Also make sure to check all the main fuses under the hood.