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P0670: Glow Plug Module Control Circuit

Moderate

Quick answer

P0670 means the computer can’t properly talk to or control the glow plug control module: the smart box that switches and monitors your diesel’s glow plugs. The plugs themselves may be perfectly healthy. Check the module’s fuses, connector, and wiring before replacing it; in winter, treat this with the same urgency as any glow fault.

What it means

Modern diesels replaced the simple glow relay with a glow plug control module (GPCM): an electronic box that switches the heavy current to each plug, times the pre-glow and after-glow phases, monitors each plug’s current draw, and reports back to the engine computer. P0670 sets when the computer detects a fault in the control circuit between itself and that module: the module doesn’t respond, the control line reads open or shorted, or the module’s feedback is implausible.

Read the ladder correctly and this code saves you money. P0671–P0676 point at one cylinder’s plug; P0380/P0382 point at a feed circuit; P0670 points at the brain and its wiring. The plugs are usually innocent here, which matters, because plugs buried under intake plumbing are the labor-heavy part of the system, and the module typically bolts somewhere reachable with one connector.

Like every glow code, the stakes are seasonal. The module failing means no coordinated preheat: hard cold starts, white smoke, rough warm-up idle, or no start at all when temperatures really drop. The EcoDiesel, 3.0 Power Stroke, and 3.0 Duramax light trucks we cover in our vehicle hubs all use module-controlled glow systems, so this code reads the same way across all three: check the cheap electrical suspects, then condemn the module by elimination.

P0670 symptoms: what you'll notice

  • Hard cold starts or extended cranking, worse as temperatures fall
  • White-gray startup smoke and a rough first minute of idle
  • Check engine light, in mild climates sometimes the only symptom
  • Possible no-start in hard freezes if the module has quit entirely
  • Often accompanied by individual plug codes (P0671–P0676) when the module can no longer run its plug monitoring

Common causes

Ordered from most to least likely.

  1. 1.

    Module connector corroded, loose, or oil-soaked

    The module lives in the engine bay’s weather. A bad crimp or green pin on its connector mimics a dead module perfectly, and costs nothing to fix.

  2. 2.

    Blown module supply fuse or damaged feed wiring

    The module needs both a heavy power feed for the plugs and a small supply for its logic. Either one missing sets control-circuit faults.

  3. 3.

    Open or shorted control wiring between computer and module

    The command-and-feedback lines run through the harness; chafe points and previous repair sites are where they fail.

  4. 4.

    Failed glow plug control module

    Heat, vibration, and years of switching high current eventually kill the box itself. A common ending, but prove the wiring first.

  5. 5.

    Weak battery or poor grounds during cold starts

    Glow systems work at the hardest electrical moment a vehicle has. Low system voltage during cranking can confuse module communication and set spurious codes.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Inspect the module’s connector and fuses

    Find the GPCM (often on or near the engine, sometimes inner fender; your service info locates it), unplug its connector, and look for corrosion, bent pins, and oil. Check both its logic fuse and the high-current feed. This free step is the most common happy ending for P0670.

  2. 2 Verify battery health and grounds

    Load-test the battery and clean the engine and chassis grounds. Diesels punish weak batteries, and a module that browns out during the glow cycle sets communication faults that look like hardware failure. Cheap insurance before any parts.

  3. 3 Confirm the module’s power supplies key-on

    Multimeter at the module connector: verify the logic supply and the main feed both show battery voltage when they should. Power missing means you chase the fuse or wiring upstream, and the module just survived being replaced for nothing.

  4. 4 Test the control lines to the computer

    With everything de-powered, check continuity on the control/feedback wires between module and engine computer, then wiggle-test the harness at chafe points. A scan tool that shows glow module data or status makes intermittent drops visible.

  5. 5 Replace the module by elimination

    Good power, good grounds, good wiring, and a module that still won’t respond or run its glow cycle: that’s a condemned module. Fit the replacement, clear codes, and confirm the glow cycle commands and plug feedback look right on a cold start.

Parts & tools you may need

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Related codes

Frequently asked questions

What does code P0670 mean?
P0670 means the computer can’t properly talk to or control the glow plug control module: the smart box that switches and monitors your diesel’s glow plugs. It’s moderately serious: you can usually keep driving gently, but diagnose it soon.
Can I drive with P0670?
Once running, yes, the module only matters for preheating cold cylinders. The danger is the next cold start, and it scales with the thermometer: a mild-climate truck can limp along for a while; a truck facing freezing mornings is one cold snap from not starting. Fix it before the season makes the decision for you.
Is it the module or the glow plugs?
P0670 specifically accuses the module’s control circuit, not the plugs: individual plug failures set P0671 through P0676 instead. That said, a module that can’t communicate also can’t report plug problems, so confirm the repair with a cold-start test afterward. The good news: the module is usually the easy part to reach.
What does a glow plug module cost?
Typically $100–350 for the part, and it usually mounts accessibly with a bracket and a connector or two: modest labor, unlike the plugs themselves. Add a diagnostic hour if a shop does the elimination testing. A connector cleaning or fuse, the other common endings for this code, cost essentially nothing.
Why did individual plug codes appear with it?
The module is the system’s witness: it measures each plug’s current and reports per-cylinder faults. When the module or its circuit fails, that reporting can go haywire, throwing plug codes for cylinders that are fine. Fix the P0670 first, clear everything, and re-test on a cold start before believing any surviving plug codes.
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