OBD-II Diagnostic Trouble Code
P0670 — Glow Plug Control Module Circuit
P0670 means the ECM detected a fault in the circuit between it and the glow plug control module or glow plug relay. That's the circuit used to command the glow plugs on, or to read glow plug system status. It applies to diesels with glow plugs. Many heavy-duty diesels use an intake air heater instead and will set heater-specific codes.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Glow Plug Control Module Circuit
Code
P0670
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
P0670 (Glow Plug Control Module Circuit) — see existing entry.
Before you replace parts — try this first
P0670 on the Cold Start system is most often failed glow plug control module or relay — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Hard starting or no-start in cold weather, white smoke and rough running right after a cold start, and a glow plug indicator that may not light or stays on. Warm-engine starts are usually normal.
Common causes
- CommonFailed glow plug control module or relay ($150–$600)
- OccasionalOpen or shorted control wire between the ECM and the glow plug module ($100–$400)
- OccasionalBlown fuse or fusible link, or corroded power feed to the glow plug module ($30–$250)
- OccasionalPoor ground at the glow plug module ($60–$200)
- RareFailed ECM driver ($1200–$3500)
How to fix
Check the fuses and the power and ground at the glow plug module
When to use: First step
Command the glow plugs with a scan tool and check for the control signal and output voltage at the plugs
When to use: To isolate the module versus the wiring
Replace the glow plug module or relay
When to use: When the command arrives but there's no output
Repair the control wiring
When to use: When the command doesn't reach the module
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
- → P0675 — Cylinder 5 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0670?
P0670 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0670 on a commercial truck?
Total repair cost ranges $30–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Failed glow plug control module or relay — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0670 fail a DOT inspection?
P0670 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0670 and related codes?
P0670 is closely related to: P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit), P0674 (Cylinder 4 Glow Plug Circuit). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0670 is set without the companions, the issue is more isolated.
How do I prevent P0670 from coming back after a repair?
For P0670 (Glow Plug Control Module Circuit), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.