1. DTC P0380, P0670 & P0671-P0674 Definitions & System Architecture
Modern common rail diesel engines rely on fast-acting glow systems not merely to achieve compression ignition during freezing sub-zero mornings, but to stabilize idle combustion, minimize hydrocarbon emissions, and assist Diesel Particulate Filter (DPF) thermal regeneration. When an electrical discontinuity, high resistance, or communication disruption occurs in this circuit, the Engine Control Module (ECM) logs one or more specific OBD-II diagnostic trouble codes:
- P0380: Glow Plug / Heater Circuit "A" Malfunction. Represents a system-level supply or primary relay fault where the main current feed to the glow busbar or control unit is broken, the master 50A-80A Maxi-fuse is blown, or relay coil continuity is lost.
- P0670: Glow Plug Control Module (GPCM / GCU) Control Circuit / Open. Indicates a communication or driver line failure between the engine ECM and the intelligent solid-state Glow Control Unit, typically caused by broken LIN-bus wiring, loss of 12V switched ignition feed, or internal driver MOSFET failure.
- P0671 through P0674: Cylinder 1 to 4 Glow Plug Circuit / Open. Pinpoints a localized circuit disruption isolated to an individual cylinder, caused by an open-circuit heating coil inside that specific glow plug, a loose terminal boot, or severed branch harness wiring.
Operates before engine cranking. Solid-state driver delivers full PWM saturation to reach incandescent tip temperatures, heating compressed air to exceed the 450°C auto-ignition threshold of diesel fuel.
Maintains modulated glow tip temperatures while the engine idles cold. Drastically reduces unburned raw fuel emissions, cures white tailpipe smoke, and smooths cylinder-to-cylinder torque balance.
During active DPF soot oxidation, the ECM pulses the glow plugs under cruising conditions to introduce parasitic electrical load on the alternator, forcing exhaust temperatures up toward 600°C.
2. Glow Plug Physics: Metallic vs Ceramic & Cold Resistance Specs
Accurate diagnosis requires knowing whether the engine utilizes traditional 11V-12V metallic sheathed glow plugs, low-voltage metallic plugs (4.4V to 5V), or high-performance ceramic glow plugs (7V to 4.4V). Testing a modern ceramic or low-voltage plug with antiquated 12V test methods will permanently destroy the component within milliseconds.
| Glow Plug Technology | Nominal Voltage | Cold Resistance (at 20°C) | Maximum Current Surge | Failure Threshold |
|---|---|---|---|---|
| Standard Metallic Sheath | 11.0V - 12.0V | 0.6 - 1.5 Ω | 15 - 25 Amps / plug | > 2.5 Ω or Infinite (OL) |
| Low-Voltage Rapid Metallic | 4.4V - 5.0V | 0.3 - 0.6 Ω | 25 - 35 Amps / plug | > 1.2 Ω or Infinite (OL) |
| High-Tech Ceramic (Self-Regulating) | 7.0V (PWM) / 4.4V | 0.2 - 0.5 Ω | 30 - 45 Amps / plug | > 1.0 Ω or Infinite (OL) |
Ceramic glow plugs feature an ultra-dense silicon nitride (Si3N4) sheath surrounding a tungsten heating element designed exclusively for high-frequency Pulse-Width Modulated (PWM) regulation at 4.4V to 7V. Connecting jumper wires directly from a 12V vehicle battery to a ceramic plug produces an instant thermal shock exceeding 1,400°C within 1.5 seconds. The ceramic tip will shatter, releasing razor-sharp ceramic shards into the cylinder bore.
3. Step-by-Step Electrical Troubleshooting Tree
When diagnosing P0380, P0670, or P0671-P0674, follow this logical sequence using a calibrated digital multimeter (DMM) and a high-current DC clamp meter before replacing any parts:
Isolate the Glow Plug and Measure Internal Cold Resistance
Switch off the ignition. Disconnect the electrical connector cap from each glow plug. Set your digital multimeter to the lowest resistance scale (200 ohms) and touch the test leads together to record lead resistance (typically 0.1 to 0.3 ohms). Place the black lead firmly onto the engine block casting (clean bare metal ground) and the red lead onto the glow plug center threaded terminal. Subtract the lead resistance from the displayed value.
- Reading 0.6 to 1.5 ohms (or 0.2 to 0.5 ohms on low-voltage plugs): The internal heating coil is mechanically intact. Proceed to Step 2.
- Reading "OL" (Infinity) or > 5.0 ohms: The internal filament has fractured or burned open. The glow plug is defective and must be replaced.
Inspect the High-Current Supply and Maxi-Fuse (DTC P0380)
If DTC P0380 is active, locate the engine bay pre-fuse distribution block. Measure battery voltage at the heavy-gauge input terminal (B+) of the Glow Control Unit (GPCM). Voltage must match open-circuit battery voltage (12.4V to 12.8V).
- 0.0V measured at GPCM input: Inspect the dedicated 50A to 80A bolt-down Maxi-fuse. A blown master fuse indicates a high-current short-to-ground in the main harness or a dead-shorted glow plug.
- Voltage drop under load > 0.5V: Severe corrosion or loose ring terminals on the main starter/alternator feed harness. Clean and torque connections.
Verify GPCM Control Circuit and Communication Bus (DTC P0670)
Disconnect the multi-pin control harness plug at the GPCM. Turn the ignition Key-ON, Engine-OFF (KOEO).
- Terminal 15 Switched Ignition: Must read battery voltage (12.0V+). If zero, check the ignition relay and cabin fuse box.
- GPCM Chassis Ground: Measure voltage drop between the GPCM ground pin and battery negative terminal. Must read under 0.05V DC. Clean ground points if higher.
- LIN-Bus / PWM Communication Line: Using a digital multimeter in DC volts, a functional LIN-bus line will read between 9.0V and 11.5V DC with active bus toggling. An oscilloscope will reveal a crisp 12V-to-ground square wave packet. If the line is stuck at 0V (short to ground) or 12V (open pull-up), the GPCM cannot report status, setting P0670.
Perform Dynamic In-Rush Current Testing with a Clamp Meter
Connect a digital DC clamp meter around the common glow plug feeder harness (or around each individual cylinder output lead). Have an assistant cycle the ignition Key-ON.
On a functional 4-cylinder engine with healthy 11V plugs, the total initial in-rush current must spike to approximately 60 to 80 Amps before tapering smoothly to 35 to 45 Amps after 4 seconds as the elements heat up. If current spikes to only 40 Amps, exactly two glow plugs are drawing current, confirming two dead cylinders even if harness voltages appear normal.
4. Seized Glow Plug Extraction Protocol & Shear-Torque Limits
Glow plugs spend years embedded inside the aluminum cylinder head, directly exposed to combustion blow-by, carbon fouling, and galvanic corrosion between the steel plug shell and the aluminum head threads. Attempting to force a stubborn plug with a standard ratchet will twist the body and shear the hexagonal head, creating an engineering disaster.
| Thread Size | Hex Drive Size | Recommended Tightening Torque | Maximum Shear Breaking Limit (Breakaway Point) |
|---|---|---|---|
| M8 × 1.0 | 8 mm | 8 - 12 N·m | 20 - 25 N·m |
| M9 × 1.0 | 9 mm | 10 - 15 N·m | 25 - 30 N·m |
| M10 × 1.0 / 1.25 | 10 mm | 15 - 20 N·m | 35 - 40 N·m |
| M12 × 1.25 | 12 mm | 20 - 25 N·m | 45 - 55 N·m |
Professional 5-Step Safe Extraction Procedure
- Thermal Expansion: Never attempt to remove glow plugs on a stone-cold engine block. Run the engine until the coolant temperature reaches 80°C to 85°C. Aluminum expands nearly twice as fast as steel, significantly loosening thread grip.
- Chemical De-Carbonization: Blow all dirt and grit out of the plug wells using compressed air. Spray a dedicated synthetic carbon dissolver and penetrating fluid around the threads. Allow it to soak for at least 2 to 4 hours (or overnight for high-mileage engines).
- Torque Limiter Setup: Set a reversible torque wrench to 5 N·m below the shear breaking limit (e.g., set to 15 N·m for an M8 plug, or 25 N·m for an M10 plug). If the torque wrench clicks without the plug rotating, STOP immediately.
- Pneumatic Micro-Vibration: Use a specialized pneumatic glow plug impact extractor running at low air pressure (2 to 3 bar). The high-frequency micro-impacts (over 2,000 taps per minute) shatter carbon crystalline bonds inside the thread galling without subjecting the plug shaft to continuous shearing torque.
- Thread Reaming & Ceramic Grease: Before threading in the new plug, run a clean glow plug aperture reamer coated in grease down the well to sweep out all carbon scale. Coat the threads of the new glow plug with high-temperature metal-free ceramic assembly paste (rated to 1,400°C) and torque strictly to manufacturer specification. Never use copper slip on aluminum heads.
5. Driver Action Guide: Symptoms, Costs & Mechanic Counter-Questions
Severity: Moderate to High
Can I Continue Driving?
Yes for short journeys, but with significant caveats. If only 1 out of 4 plugs has failed (P0671), the vehicle will start in warm weather with minor idle roughness for 10 seconds. However, if all plugs are dead (P0380/P0670) or operating in winter sub-zero temperatures, extended cranking will deplete the battery and flood cylinders with raw fuel. Crucially, on modern Euro 5/6 platforms, an active glow plug DTC automatically disables passive and active DPF regeneration, which will clog the particulate filter within 500 to 1,000 km.
Fair Workshop Repair Estimates
| Repair Level | Component Replaced | OEM Parts Cost | Labor & Diagnostic Hours | Fair Total Workshop Invoice |
|---|---|---|---|---|
| Level 1 | Single OEM Glow Plug (e.g., Bosch, Beru, NGK) | $15 - $35 | 0.5 - 1.0 hr ($50 - $100) | $65 - $135 |
| Level 2 | Full Set (4 Glow Plugs) + Well Cleaning | $60 - $140 | 1.0 - 2.0 hrs ($100 - $200) | $160 - $340 |
| Level 3 | Glow Plug Control Module (GPCM / GCU) | $80 - $220 | 0.5 - 1.0 hr ($50 - $100) | $130 - $320 |
| Level 4 | Master 60A Pre-Fuse + Terminal Harness Repair | $10 - $30 | 1.0 - 1.5 hrs ($100 - $150) | $110 - $180 |
Dishonest repair shops frequently exploit glow plug codes to claim that rough cold starting and white tailpipe smoke require a $3,000 fuel injector replacement or an engine compression overhaul. Arm yourself with these three precise diagnostic questions:
- "Can you write down the exact digital multimeter cold resistance readings (in ohms) measured across each cylinder's glow plug pin to ground?" (A healthy plug reads 0.6-1.5 Ω; if they cannot provide exact numbers, they have not tested them).
- "Have you verified battery voltage and ground drop at the Glow Control Module (GPCM) main power lug before condemning the glow plugs themselves?" (Prevents replacing good plugs when only a $15 master fuse is blown).
- "If a plug is seized, will you use an 80°C warm engine block and a pneumatic vibration extractor within the manufacturer's 25 N·m shear limit rather than forcing it with a breaker bar?" (Protects you from cylinder head removal bills).
6. Related Diagnostics, Tools & Cross-References
Diesel glow system malfunctions operate in close technical synergy with common rail fuel delivery and cylinder compression balance. Use these FuelPartsDB resources to cross-reference related components:
- Master fault code diagnostic protocol: DTC P0380 Diagnostic Guide, DTC P0670 GPCM Protocol, and DTC P0671 Cylinder 1 Glow Protocol.
- Cold weather hard starting: Diesel Cold Start Problems: Extended Cranking & Rail Pressure Guide.
- Acrid unburned white smoke: Cold Start White Smoke & Rough Idle Root Causes.
- Cylinder-by-cylinder contribution: Cylinder Balance & Injector Deviation Workshop SOP.
Frequently Asked Diagnostic Questions
What is the normal electrical resistance of a diesel glow plug?
For standard 11V-12V metallic glow plugs, cold resistance between the center terminal and the threaded body typically measures 0.6 to 1.5 ohms at 20°C (often reading ~0.8 to 1.2 ohms on a digital multimeter after zeroing test lead resistance). Modern low-voltage rapid-heat metallic or ceramic glow plugs (4.4V to 7V operated via high-frequency PWM) measure much lower, typically 0.2 to 0.5 ohms. A reading of infinity (open circuit / OL) or anything above 5.0 ohms confirms an internal heating element fracture.
Can I test a modern glow plug by applying 12V directly from a car battery?
Only on traditional 11V/12V stamped metal plugs. You must NEVER apply direct 12V battery voltage to modern low-voltage ceramic or rapid-heating metallic glow plugs rated at 4.4V or 7V (widely used in Euro 5/6 and modern Tier 4 diesel engines). Direct 12V will cause thermal runaway, vaporizing or exploding the ceramic heating tip within 1 to 2 seconds, which can drop shattered fragments into the combustion chamber and destroy the piston and cylinder liner.
Why does the check engine light stay on after installing brand-new glow plugs?
If all glow plugs are new and verified good, the fault usually originates in the Glow Control Module (GCU/GPCM), the high-current Maxi-fuse, or the wiring harness. A corroded connector terminal, burned relay contacts, or lost LIN/PWM bidirectional communication between the GCU and the engine ECM will keep DTC P0380 or P0670 active. Additionally, many modern platforms require clearing the stored DTC and resetting the glow adaptation counter using diagnostic software before the MIL extinguishes.
What should I do if a glow plug is seized and will not turn?
Stop immediately before exceeding the plug's shear torque limit (typically 20 Nm to 35 Nm depending on thread diameter, e.g., M8 or M10). Applying excessive leverage will snap the hex head off the threaded body, requiring costly cylinder head removal or EDM machining. Warm the engine to 80°C operating temperature, apply dedicated carbon-penetrating synthetic fluid around the well, allow it to soak overnight, and use a calibrated torque-limiting pneumatic vibro-extractor operating at low air pressure.
Featured Assemblies
Verified technical specifications, pinout schematics, and hold-down clamp torque figures for assemblies cited in this protocol: