P0670: Ford 7.3L glow-plug control-line fault

P0670 identifies the control line for the glow-plug control module in the documented California 7.3L diesel system. Ford checks the output driver's internal status for an open circuit or short. This is an electrical command-path decision, so the code alone does not identify a failed glow plug or measure the heating performance of every cylinder.

Why the operating context matters

The control-line monitor runs continuously in a 25–50 millisecond background loop, with a typical detection duration under one second and no listed entry conditions. That makes its evidence different from a heating test that needs a prolonged cold-start command. A fault can therefore be recorded without proving that the glow plugs were energized for their full operating interval.

Ford describes glow-plug on-time as dependent on oil temperature, barometric pressure and battery voltage. A short command in warm conditions is not by itself evidence of this circuit fault.

Choosing the next diagnostic branch

Keep the control circuit, the module's communication circuit and its individual glow-plug outputs separate when reviewing accompanying codes. P0683 concerns the diagnostic message path, whereas P0670 points to the driver status of the control line. Establish the correct California-system wiring and verify that command path through the applicable service procedure before condemning all glow plugs. The summary supplies a detection category, not a component replacement verdict.

Source and applicability

Ford 2002 model-year 7.3L diesel OBD system operation summary, revision February 14, 2003. Ford source, physical PDF page 5. The documented monitor explains the fault decision; vehicle-specific service information is still needed for connector identification and a complete repair procedure.