P0670: Glow Plug Control Module (GPCM / GCU) Control Circuit / Open
Diagnostic Trouble Code (DTC) P0670 indicates that the Engine Control Module (ECM/PCM) has detected an electrical communication or control circuit continuity failure with the electronic Glow Plug Control Unit (GCU / GPCM). Unlike older vehicles that utilized simple mechanical relays, modern common rail diesel engines employ an intelligent solid-state Glow Control Unit featuring integrated microcontrollers and smart power MOSFETs. The GCU communicates with the ECM via a dedicated bidirectional Pulse-Width Modulated (PWM) diagnostic line or a single-wire LIN (Local Interconnect Network) serial bus. DTC P0670 is set when the ECM fails to establish communication with the GCU, detects an open circuit in the GCU enable line, or senses that the module has lost its main ignition or battery power supply.
🚗 Operational Safety, Typical Repair Costs & Diagnostic Verification
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🚦 Driving Safety & Operational Risk
Immediate mechanical assessment and tow requirementsGlow Plug Control Module (GPCM) Circuit Open - Cold Start Fault
The Engine Control Unit cannot communicate with or command the Glow Plug Control Module (GPCM / GCU). Once the engine has started, glow plugs are not needed to keep it running. You can drive completely normally once warm. However, cold morning starts will be accompanied by extended cranking and rough running.
MECHANICAL HAZARDS & ENGINE RISK IF DRIVEN:Severe starter motor wear and battery discharge from prolonged winter cranking, and DPF regeneration inhibition (many ECUs cancel DPF regen when glow plug faults exist). - 02
💰 Typical Workshop Repair Budget
Expected parts replacement and garage labor cost tiersMinor DIY / Filter Purge$20 - $60: High-current pre-fuse inspection and power supply wire contact cleaning.Sensor cleaning, fuel filter renewal, fuse replacement, or harness contact cleaning.
Sensor / Valve Replacement$120 - $280: Brand new OEM Glow Plug Control Module (Beru / Hitachi / Bosch).Single component fix (e.g. pressure regulator valve, inlet metering unit, or reconditioned injector).
Major Overhaul / Pump & Injectors$350 - $600: Complete control module, power feed cable, and glow plug set replacement.High-pressure injection pump overhaul, new OEM injector set, or DPF assembly replacement.
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🧰 Initial Visual & Physical Verifications
5-minute zero-tool garage triage steps before calling a workshop1Check the high-current mega fuse: Look in the engine bay fuse box for a 50A to 80A bolt-down fuse supplying the glow plug relay.2Inspect the module connector: Unclip the GPCM plug and check for melted plastic around the large 12V power supply pin.3Check cold morning behavior: If the car cranks 5-10 seconds before coughing to life with white smoke, the module isn't powering the plugs. - 04
🛡️ Diagnostic Verification & Mechanic Shield
Exact questions to ask your technician to prevent unnecessary parts replacement"Say: 'Please check the thick red 12V battery power feed cable to the module for voltage drop under load.'"
"Say: 'Test the LIN-bus or PWM control signal wire from the ECU to the module before assuming the module is dead.'"
""Check the heavy-gauge fusible link and engine block ground wire resistance (must be < 0.2 ohms) before replacing the glow plug control unit.""
Technical Note: Read these technical questions to your mechanic before approving expensive parts replacements:
30-Second Workshop Triage
- Check Engine Light (MIL) and flashing glow plug spiral indicator illuminated on the dashboard
- Prolonged engine cranking and hard starting when ambient temperature drops below 10°C (50°F)
- Rough, irregular idle with severe engine vibration during the first minute after cold start
- Internal electronic component failure, blown MOSFET driver, or solder joint fatigue inside the Glow Control Unit (GCU)
- Blown high-current 60A-80A maxi-fuse supplying main battery power to the Glow Control Unit
- Blown low-current ignition fuse (typically 5A-10A) supplying wake-up power to GCU Terminal 15
Connect scan tool live telemetry: compare Target Fuel Rail Pressure vs Actual Rail Pressure and log metering valve command PWM duty cycle during idle and road load.
Expensive Diagnostic Pitfall & Technical Reality
P0670 requires replacing all 4/6 glow plugs at high cost, and if plugs are stuck in the cylinder head, head removal is required ($1,500 - $2,500).
P0670 is NOT an individual glow plug fault (which are P0671 - P0678). P0670 means the ECM cannot communicate with the Glow Plug Control Module (GPCM) over the dedicated bidirectional LIN-Bus or PWM control wire. The glow plugs themselves are almost never damaged by this code. In over 85% of cases, the fault is caused by an unseated/corroded multi-pin GPCM connector, a blown small 10A ignition fuse that powers the module's micro-controller logic, or water ingress corroding the module pins under the battery tray.
Spending $1,200+ attempting to pull delicate glow plugs (risking sheared threads in head) vs replacing a $15 control fuse or an $85 plug-and-play Beru/Hitachi module located safely on the firewall, saving over $1,100.
Most Common Engines & Fuel Injection Platforms Affected
This fault code predominantly affects the following common rail diesel platforms. Cross-reference your specific engine family below for verified factory failure modes, key sub-assembly part numbers, and dedicated repair links:
Mercedes-Benz 2.1 CDI / 3.0 CDI (OM651 / OM642)
Beru Instant Start System (ISS)Typical Symptom: Glow plug coil light stays illuminated for 60 seconds AFTER start-up, P0670 stored.
Known Factory Failure Mode: Internal aluminum bond wires in the Beru module snapping due to thermal cycling fatigue.
A6429007801 / Beru GSE114BMW 2.0d / 3.0d (N47 / M57)
Beru / Bosch Glow SystemTypical Symptom: Long cranking in winter, automatic DPF regeneration ceases to trigger, P0670 stored.
Known Factory Failure Mode: Heavy-duty power cable connection corrosion and internal power MOSFET failure in module.
12217801201 / Beru GSE102Ford 6.7L / 6.4L Powerstroke V8 Diesel
Electronic GPCM Solid-State SystemTypical Symptom: Extended cold cranking accompanied by white smoke clouds, Wrench Light active with P0670.
Known Factory Failure Mode: Dual multi-pin harness connectors backing out due to heavy-duty vibration or blown 50A Maxi-Fuse under-hood.
4C2Z-12B533-AA / Motorcraft DY-1077Volkswagen / Audi 2.0 TDI (EA189 / EA288)
Bosch EDC17 J179 Glow Control SystemTypical Symptom: Flashing glow plug spiral indicator on cluster, delayed winter starting, P0670 stored in ECU.
Known Factory Failure Mode: J179 smart relay module micro-circuit failure or hairline fracture on under-hood E-Box strip fuse.
038907281D / 04L907282 / Bosch 0281003083Diesel Glow Plug Diagnostics & Circuit Troubleshooting Guide
Workshop troubleshooting guide for diesel glow plug fault codes P0380, P0670, and P0671 to P0674. Resistance specs, GPCM relay testing, and removal torque.
Control Circuit & Actuator Logic
1. Dynamic In-Demand Fuel Metering
The electronic Glow Control Unit (GCU) operates as a slave node on the vehicle's electronic architecture. It connects via two primary harnesses: a high-current connector (receiving unswitched battery positive through a 60A-80A maxi-fuse and outputting individual current lines to each cylinder's glow plug) and a low-current control connector (incorporating 12V ignition wake-up, logic ground, and the LIN-bus / PWM communication line to the ECM). The GCU modulates individual glow plug voltages (varying from 4.4V to 11V DC depending on plug type) to achieve fast heating in under 2 seconds while preventing element burnout.
2. Closed-Loop PID Deviation Monitoring
During every ignition cycle, the ECM transmits initialization commands over the LIN-bus / PWM interface to verify GCU internal self-test status. The GCU responds with module identification, operating status, and individual cylinder current feedback. If the ECM receives no response within 500 to 1,000 milliseconds, or if the control wire voltage is pulled continuously to ground or floats open at 12V, the ECM records DTC P0670. The ECM disables all pre-heating, post-heating, and DPF regeneration intermediate heating routines.
3. Diagnostic Trap: Normally Open (NO) vs Normally Closed (NC) Valves
Electronic GCU Failsafe Shutdown: When a communication loss or control circuit fault occurs (P0670), the GCU internal micro-controller immediately de-energizes all internal MOSFET output stages to prevent uncontrolled continuous heating that would melt the glow plugs. Consequently, the glow plugs remain completely inoperative until communication and power are restored.
Electrical & Hydraulic Physical Benchmarks
Step-by-Step Workshop Diagnostic Protocol
Verify Main Battery Power & Ignition Wake-Up Feeds at GCU
Locate the Glow Control Unit (typically mounted on the engine block, wheel arch, or near the battery). Disconnect the module connectors. Using a digital multimeter referenced to battery negative, test the large red battery positive wire (Terminal 30); it must read constant battery voltage (> 12.2V). Turn ignition ON and test the switched ignition wire (Terminal 15); it must read > 11.8V. If 0V, inspect fuses.
Test GCU Chassis Ground Circuit Integrity
With the connector unplugged and ignition OFF, measure resistance between the module ground terminal and the battery negative terminal using the multimeter 200 Ω scale. Resistance must be < 0.3 Ω. Perform a voltage drop test under load to verify that high-current return is clean and uncorroded.
Oscilloscope LIN-Bus / PWM Communication Signal Verification
Reconnect the GCU. Back-probe the communication line connecting the ECM to the GCU with an oscilloscope. Switch ignition ON. For LIN-bus architectures: you must observe a crisp digital square wave pulsing between dominant low (< 1.5V) and recessive high (battery voltage ~12V) at 19.2 kbaud. A flat line at 0V indicates a short to ground; a flat line at 12V indicates an open circuit or inactive ECM.
Harness Continuity & Short-to-Ground Test
Disconnect the main harness connector from the ECM and the small control connector from the GCU. Measure continuity on the communication wire between ECM and GCU pins (< 0.4 Ω). Check resistance to chassis ground (must read infinite / OL). Check for short to 12V.
Scan Tool Bi-Directional GCU Activation Test
Using an OEM diagnostic scan tool, navigate to Special Functions -> 'Glow Control Module Actuation Test'. Command the module ON while measuring voltage at the glow plug harness caps. The module should output pulsed voltage to all channels. If power and communication are good but no voltage is output, the GCU has suffered internal MOSFET failure.
Inspect Connector Pins for Water Intrusion & Corrosion
Inspect the multi-pin connector on the GCU for green copper corrosion or backed-out pins. Because the GCU is often mounted low on the engine or near splash shields, water intrusion through degraded silicone seals is a very common root cause of P0670.
Component-Level BOM: Preheating Electronic Control & LIN-Bus Communication Architecture
Preheating Electronic Control & LIN-Bus Communication Architecture
GSE102 / 0 522 120 702 / A6429005801
Solid-state electronic power stage with onboard microprocessor communicating with ECM over LIN
Littelfuse Mini 10A Red / Underhood Fuse Box
Supplies switched +12V battery power to the module's internal logic microprocessor
Tyco / AMP 12-Pin Heavy-Duty Timer Connector
Carries high-current glow feeds and low-voltage LIN-Bus communication lines
Glow Plug Control Module (GPCM) Diagnostic & Logic Voltage Reference
High-Pressure Pump Adaptation Reset & Road Test Drive Cycle
Following the replacement of the Glow Control Unit (GCU) or repair of its communication wiring for DTC P0670:
1. Clear all trouble codes from the Engine Control Module memory using an OBD-II diagnostic tool.
2. Access ECM Special Functions -> 'Glow Control Unit Identification / Adaptation'. Confirm that the ECM successfully queries and reads the hardware and software part numbers of the new GCU over the LIN-bus.
3. Switch ignition OFF for 60 seconds to allow the ECM main relay to power down and register the new module.
4. Switch ignition ON with a cold engine and verify that the spiral glow indicator illuminates and extinguishes in accordance with ambient temperature.
5. Start the engine and verify smooth, immediate idle without misfires, followed by clearing of all emissions readiness monitors.
Differential Diagnosis & Related Faults
Workshop Field Case Studies
2013 Mercedes-Benz ML350 BlueTEC (W166 OM642 3.0 V6)(192,000 km)
Frequently Asked Technical Questions
What does engine fault code P0670 mean?
Diagnostic Trouble Code (DTC) P0670 indicates that the Engine Control Module (ECM/PCM) has detected an electrical communication or control circuit continuity failure with the electronic Glow Plug Control Unit (GCU / GPCM). Unlike older vehicles that utilized simple mechanical relays, modern common rail diesel engines employ an intelligent solid-state Glow Control Unit featuring integrated microcontrollers and smart power MOSFETs. The GCU communicates with the ECM via a dedicated bidirectional Pulse-Width Modulated (PWM) diagnostic line or a single-wire LIN (Local Interconnect Network) serial bus. DTC P0670 is set when the ECM fails to establish communication with the GCU, detects an open circuit in the GCU enable line, or senses that the module has lost its main ignition or battery power supply.
What are the most common symptoms of P0670?
Check Engine Light (MIL) and flashing glow plug spiral indicator illuminated on the dashboard, Prolonged engine cranking and hard starting when ambient temperature drops below 10°C (50°F), Rough, irregular idle with severe engine vibration during the first minute after cold start, Clouds of acrid white or grey exhaust smoke emitted during cold start cranking, Automatic DPF soot regeneration completely disabled, leading to progressive particulate filter restriction, Diagnostic scan tool reports 'No Communication with Glow Plug Control Module' or 'GPCM Circuit Open'.
What causes P0670 on a diesel engine?
Internal electronic component failure, blown MOSFET driver, or solder joint fatigue inside the Glow Control Unit (GCU); Blown high-current 60A-80A maxi-fuse supplying main battery power to the Glow Control Unit; Blown low-current ignition fuse (typically 5A-10A) supplying wake-up power to GCU Terminal 15; Severed, broken, or chafed LIN-bus / PWM communication wire between the ECM and the GCU; Corroded, oxidized, or water-damaged terminal pins inside the GCU electrical harness connectors; Corroded or loose module chassis ground cable preventing the GCU internal logic from powering up.
Is it safe to drive with code P0670 active?
🟡 Caution: Limp Mode Active / Service Soon
How much does it cost to fix fault code P0670?
Typical repair costs for fault code P0670 depend on the underlying root cause and diagnostic tier: Level 1 (Minor / Maintenance): $20 - $60: High-current pre-fuse inspection and power supply wire contact cleaning. (Sensor cleaning, fuel filter renewal, fuse replacement, or harness contact cleaning.); Level 2 (Component Replacement): $120 - $280: Brand new OEM Glow Plug Control Module (Beru / Hitachi / Bosch). (Single component fix (e.g. pressure regulator valve, inlet metering unit, or reconditioned injector).); Level 3 (Major Sub-Assembly): $350 - $600: Complete control module, power feed cable, and glow plug set replacement. (High-pressure injection pump overhaul, new OEM injector set, or DPF assembly replacement.). Independent workshop benchmark rates. Data indicates over 70% of occurrences are resolved within Level 1 or Level 2 without replacing entire sub-assemblies.
Can I replace an electronic GCU with a standard 4-pin relay?
Absolutely not. Modern common rail diesel ECMs communicate with the GCU via microsecond LIN-bus data telegrams and require closed-loop current feedback for each individual cylinder. Installing an analog relay will destroy communication, keep P0670 permanently active, and burn out low-voltage glow plugs.
Why does a faulty GCU prevent DPF regeneration?
Modern common rail engines activate the glow plugs during DPF active regeneration to stabilize in-cylinder post-injection combustion and add heavy electrical load onto the alternator to elevate exhaust temperatures. If the ECM cannot control the GCU, it aborts all DPF regenerations as a safety precaution.
Where is the Glow Control Module usually located?
Depending on vehicle make, the GCU is typically bolted to the engine block underneath the intake manifold (e.g. BMW M47/M57/N47/N57), mounted on the engine bay bulkhead near the brake booster (e.g. Mercedes CDI, VAG TDI), or located inside the under-hood battery fuse distribution box.