P0670 on the 6.7L Power Stroke means the glow plug control module is reporting a circuit fault. Cold-start hard, hard-start gets worse as temperatures drop. Here's the diagnostic and what the module replacement runs.
What this issue is
P0670 on the 6.7L Power Stroke — full code description “Glow Plug Module Control Circuit” — means the ECM is reporting a fault in the circuit that controls the glow plug control module. The glow plug control module (GPCM) manages the power delivery to the glow plugs during cold-start operation, and when the module or its control circuit develops a fault, cold-start performance suffers.
The symptom progression is temperature-dependent — most owners don’t notice the issue in summer, then can’t start the truck on the first cold morning. By the time you’re calling because the truck won’t start in 25-degree weather, the issue has been brewing for a while.
This is a moderate-scope repair when caught at the module level. If individual glow plugs have also failed, the scope grows but is still manageable.
Symptoms you’ll notice
- Check Engine Light with P0670 — the trigger code
- Sometimes paired with P067X codes — P0671 through P0678 indicate individual glow plug failures (one per cylinder)
- Hard cold-start, especially at low ambient temperatures
- Extended cranking required to fire on cold mornings — what should take 2-3 seconds takes 8-15 seconds or more
- White smoke on cold start, clearing as the engine warms — uncombusted fuel making it through cylinders that aren’t quite hot enough to ignite cleanly
- Rough idle when cold — uneven combustion across cylinders that haven’t all reached glow-aided temperature
- No-start in cold weather in severe cases — the truck simply won’t fire when it’s cold enough
- Warm starts work fine — the temperature dependence is the most distinctive symptom
If you have P0670 in summer, the truck may run fine. The problem appears the first time it gets cold.
Why it happens
The glow plug control module is an electrical component that manages high-current loads (the glow plugs draw significant amperage during their preheat cycles). Several failure modes:
1. Internal module failure. The module’s internal electronics fail with age, heat cycling, and current load. Sometimes a specific channel within the module fails (which can present as both P0670 plus a specific P067X code for the affected cylinder).
2. Connector or wiring issues. The high-current connections between the module and the glow plugs see significant electrical and thermal stress. Corrosion, loose connections, or damaged wiring can produce P0670 even when the module itself is healthy.
3. Ground connection issues. The module’s ground reference matters for proper operation. Bad grounds cause unpredictable behavior.
4. Failed individual glow plugs creating module fault conditions. When individual glow plugs short or open in specific failure modes, the module can interpret the abnormal current as a circuit fault and trigger P0670 alongside the more specific P067X codes.
5. Module relay or fuse issues. Less common, but the power supply to the module itself can be the problem.
How we diagnose it
Sequence:
-
Pull all stored codes. P0670 alone indicates module-side issue; P0670 with P067X codes indicates both module and individual glow plug issues; P067X codes alone (no P0670) indicates only specific glow plugs without a module fault.
-
Visual inspection of the module and harness. Check connector condition at the module, look for damaged wiring or corrosion at the high-current connections.
-
Module ground and power check. Verify the module is getting clean power and has a good ground reference. Voltage drop tests across the connections identify supply-side issues.
-
Glow plug resistance test. With glow plugs accessible, measure resistance across each plug. Out-of-spec resistance (open circuit, short, or significantly different from the others) indicates failed individual plugs.
-
Module functional test — using the factory scan tool, command the glow plug system on and monitor what the module reports. Patterns in the data tell us whether the module is functional or failed.
By the end of this sequence, we know whether you need module-only replacement, individual glow plug replacement, or both.
What the repair looks like
GPCM replacement only — module is unbolted, harness is disconnected at the connector, new module installed, harness reconnected. Modest scope. We use OEM Motorcraft GPCM for reliability — aftermarket modules have a poor track record on this platform.
Individual glow plug replacement — accessible from the top of the engine. Each glow plug threads into the cylinder head; access requires removal of some related components but isn’t a major teardown. Done in sets when multiple are bad (or proactively when the originals are aged).
Wiring or connector repair — when diagnostic identifies the issue is harness-side rather than module-side. Specific to what’s damaged.
Combination repair — when both module and individual glow plugs need replacement, doing both at once leverages the labor overlap.
Pricing
Final pricing depends on labor, parts, and diagnostic findings — we walk through every line item with you before work begins. Call the shop or request an estimate with your year, mileage, and symptoms for a quote on your specific situation.