P0670 Code: Glow Plug Control Module Control Circuit/Open
P0670 Code: Test Module Power and Command Before Glow Plugs
P0670 is a system-control fault. Save the cold-start event, identify the exact preheat architecture, and prove supply, ground, command, module response, and cylinder loads in that order.
P0670 means the ECM detected a fault in the glow plug control module circuit. The exact OEM description may say control circuit, circuit open/short, electrical malfunction, or glow time module. It does not identify one failed glow plug. Preserve code status and cold-start conditions, scan for cylinder-specific and voltage companions, then verify the battery, fuse links, loaded power and ground, connector, low-current command or communication wiring, module response, and individual glow loads. UR1000 can organize supported scan evidence, while a meter, current clamp, or scope provides direct electrical proof.
What Does P0670 Mean on a Diesel?
Glow plugs warm the combustion chambers so diesel fuel can ignite reliably when the engine and surrounding air are cold. A control module or relay decides when to preheat, how long to continue afterglow, and how much current each branch receives. The ECM may command that module through a dedicated line or network and monitor status in return.
P0670 concerns that shared control relationship. On one vehicle it may monitor a command circuit; on another it may describe the glow-time module, its supply, feedback, or an electrical malfunction recognized inside the controller. This is why a generic description is a starting point rather than a universal pin-level test.
P0670 is not the same as P0671 through P067x. Those codes point toward a named cylinder circuit on many platforms. P0670 can appear alone or beside them, but the complete code set determines whether diagnosis starts with a shared supply/module or an individual output.
P0670 Symptoms and Cold-Start Clues
The symptom pattern depends on temperature, engine design, number of affected cylinders, battery condition, and whether the module fails completely or intermittently. The same vehicle may start normally in summer and struggle after an overnight winter soak.
The MIL may remain on, and some vehicles also display a coil-shaped glow or diesel-system warning.
The engine may need extra cranking after an overnight soak because cylinder temperature rises too slowly.
Unburned fuel can create white exhaust haze until combustion stabilizes; persistent smoke needs broader diagnosis.
One or more cold cylinders may misfire or shake briefly, then smooth out as engine heat builds.
A warm diesel can restart normally even when the preheat circuit cannot perform its cold-start job.
P0671 through P067x, voltage, communication, or module codes can reveal whether the fault is shared or cylinder-specific.
Common P0670 Causes in Test Order
Start with voltage and shared paths because preheat demand and starter demand occur close together. Do not condemn the controller because it reports no output while its battery feed, ignition feed, ground, wake-up command, or enable conditions are missing.
Weak Battery, Supply, Fuse, or Fusible Link
Glow systems draw high current. Low cranking voltage or a weak shared feed can disable the module or distort its self-test.
How to prove it: Load-test the battery and every specified module supply and ground during the commanded preheat state.
Check firstCorroded or Heat-Damaged Connector
Moisture, engine heat, poor terminal tension, or an unlocked connector can interrupt the ECM-to-module command or high-current path.
How to prove it: Inspect seals and pin fit, then prove the fault with a loaded voltage-drop test rather than appearance alone.
Check firstOpen or Shorted Control/Communication Wire
The low-current command, wake-up, feedback, or network circuit can fail even while the heavy glow-plug cables look intact.
How to prove it: Use the VIN-specific diagram to isolate each conductor and protect controller drivers during testing.
Test directlyFailed Glow Plug Control Module or Relay
Internal power transistors, relays, thermal protection, or electronics may stop the module from responding or delivering current.
How to prove it: Verify its power, ground, command, communication, and output under OEM conditions before replacing it.
Test directlyFailed Glow Plug or Incorrect Load
A shorted, open, swollen, or incorrect plug can set a cylinder code and may stress a monitored module channel.
How to prove it: Compare supported cylinder data or current and test plugs by the manufacturer procedure; P0670 alone does not identify a cylinder.
Advanced branchCalibration or Rare ECM Fault
Some platforms use learned glow-plug offsets or module software; a controller driver fault remains possible after the external circuit passes.
How to prove it: Check service information and calibration only after documenting good supply, ground, wiring, module load, and communication.
Advanced branch
How Glow Plug Control Systems Differ
Older diesels may use a heavy relay and common bus, while newer systems often use a solid-state module that monitors individual current. Some communicate digitally; others use dedicated command and feedback wires. The module may be engine-mounted, fender-mounted, or integrated into another power-distribution assembly.
Relay and Common Bus
One high-current path feeds several plugs. A shared supply, relay contact, or bus fault can affect every cylinder together.
Solid-State GPCM
Electronic channels can monitor current, temperature, duty cycle, and individual plug behavior while reporting status to the ECM.
Networked Module
Command and diagnostic data may travel on a communication bus, so P0670 can coexist with P0683/P0684 or U-codes.
Learned or Coded System
Some replacements require calibration, offset learning, configuration, or software. Confirm the service procedure before installation.
Volkswagen documentation, for example, uses manufacturer-specific monitoring language and thresholds, while General Motors bulletins show that glow-plug module software and cylinder monitoring strategy can matter on other platforms. Treat those documents as examples of why the VIN-specific procedure overrides a generic resistance chart.
Using UR1000 for P0670 Diagnosis
The iCARZONE UR1000 provides full-system scanning, supported freeze frame, live data, ECU information, reports, and active tests. Useful available parameters may include glow request, preheat or afterglow status, coolant and intake temperature, system voltage, module communication, cylinder current, duty cycle, and learned values.
Scan every relevant module, not only generic OBD-II engine data. A GPCM, body/power-distribution controller, or charging-system code can explain why P0670 appeared. If an output test is available, observe command, module status, battery voltage, and cylinder response together rather than treating a click or icon as proof.
Coverage varies by VIN and ECU. Do not promise a specific glow-module active test, PID, coding function, or relearn until compatibility is confirmed. UR1000 cannot physically measure resistance, current, continuity, insulation breakdown, or loaded voltage drop through the diagnostic connector.
iCARZONE UR1000 Bidirectional Scan Tool with ECU Coding
Use UR1000 to preserve the P0670 cold-start event, scan related modules, graph supported preheat data, and document verification. Confirm glow-system coverage for the exact VIN.
How to Diagnose P0670 in 6 Steps
Work from the exact wiring diagram and safety procedure. Glow circuits can carry substantial current, and some plugs use low operating voltage or pulse-width control. Never jumper battery voltage to an unknown module output or ECM driver.
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1
Save P0670 and the Cold-Start Evidence
- Read current, pending, permanent, and history codes before clearing anything; save freeze frame and a full-system report.
- Record coolant temperature, intake-air temperature, battery voltage, engine speed, and how long the vehicle had been cold-soaked.
- List cylinder-specific P0671–P067x, P0380, P0683/P0684, supply-voltage, communication, and charging-system companions.
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2
Identify the Exact Preheat Architecture
- Confirm VIN, engine code, glow-plug type, module name, connector views, fuse paths, grounds, and controller relationship.
- Determine whether the design uses a separate relay, solid-state GPCM, integrated power stage, current sensing, or network communication.
- Do not import resistance, amperage, duty-cycle, wire-color, or pinout values from another diesel platform.
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3
Prove Battery Condition and Module Power
- Test battery state and cranking stability first; a diesel can crank yet pull voltage below the module operating threshold.
- Check every specified fuse and fusible link, then load-test power and ground at the module while preheat is requested.
- If a fuse has opened, locate the excessive load or short before installing another fuse of the same rating.
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4
Inspect Connectors, Harnesses, and Glow Leads
- Look for heat discoloration, green corrosion, backed-out pins, coolant or oil contamination, abrasion, rodent damage, and prior splices.
- Inspect the high-current leads and the smaller ECM/module circuits separately; they do not fail or test in the same way.
- Use a controlled wiggle test only while monitoring a safe signal; secure wiring away from exhaust and moving parts afterward.
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5
Use Supported UR1000 Data and Active Tests
- Graph available preheat request, module status, cylinder current, duty cycle, temperature, voltage, and communication parameters.
- Run a glow or module output test only when the exact vehicle and ECU provide it and all enable conditions are satisfied.
- A scan command organizes the test, but direct current, resistance, continuity, or voltage-drop proof still needs separate equipment.
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6
Test the Module and Loads, Repair, Then Verify
- Follow the OEM sequence for isolated module commands, output current, individual plugs, control wires, and controller pins.
- Repair only the proven supply, ground, connector, harness, plug, module, software, or ECM branch; perform required coding or learning.
- Clear eligible codes after saving results, repeat a genuine cold soak, and confirm clean pending-code and start-quality results.
How to Fix and Clear P0670 Safely
Power or Ground
Correct the weak battery, terminal, cable, fuse link, relay contact, splice, or ground path that fails under preheat and cranking load. If protection opens again, locate the excessive current instead of fitting a higher rating.
Connector or Harness
Replace heat-damaged terminals and seals with specified parts, restore conductor size and routing, and use an approved sealed repair. Command and network wires may require different repair rules than heavy output cables.
Glow Plug
Replace only plugs that fail the OEM test or have reliable cylinder evidence. Match the exact plug type; mixing voltage or control characteristics can create repeat faults. Follow removal precautions for an aluminum head.
Module or Relay
Replace it only after proving supply, ground, command, communication, and loads. Complete required setup, coding, initialization, or learned-value procedures with stable support voltage.
ECM or Software
Apply service information only after the external circuit passes. Controller replacement is the least likely branch and may require secured programming equipment.
Cold-Start Verification
- Save the before-and-after scan reports and electrical readings.
- Clear eligible codes only after the repair and required setup.
- Let the engine and coolant reach the required cold-soak condition.
- Observe preheat request, voltage stability, module response, cranking time, and smoke.
- Rescan for pending and module-specific faults before declaring success.
P0670 Diagnostic and Repair Cost Factors
These are broad planning ranges, not quotes. Engine access, seized plugs, broken-plug extraction, module coding, harness damage, part availability, and local labor can change the total substantially.
| Repair branch | Broad range | What drives cost |
|---|---|---|
| Scan and electrical diagnosis | $120–$350 | Cold-soak reproduction, reports, current and voltage-drop testing |
| Fuse, relay, terminal, or ground repair | $80–$500+ | Access, heat damage, cable size, and cause of repeat failure |
| Harness or connector repair | $150–$900+ | Engine routing, sealed terminals, shared circuits, and damage length |
| One or more glow plugs | $180–$1,200+ | Cylinder count, intake removal, seized plugs, and breakage risk |
| Glow plug control module/relay | $250–$1,200+ | Part design, location, coding, setup, and software |
| Controller/programming branch | $400–$2,000+ | Proof, programming access, support voltage, and module replacement |
Where P0670 Diagnosis Differs by Vehicle
| Architecture | Verify first | Common wrong assumption |
|---|---|---|
| VW/Audi TDI | Engine code, module message, supply, command, and manufacturer threshold | A generic plug-resistance value applies to every TDI |
| GM Duramax | GPCM codes, cylinder companions, battery condition, calibration and bulletins | Any glow code means the controller hardware failed |
| Ford Power Stroke | Exact relay/GPCM generation, connectors, feeds, grounds, and individual outputs | All model years use the same relay layout |
| Mercedes/Sprinter or other European diesel | Correct diagnostic connector, module communication, plug type, and adaptation | A warm successful start proves the system is healthy |
DIY Limits and When to Use a Professional
- Save codes, freeze frame, and a full report with UR1000.
- Test the battery and inspect accessible fuses and grounds.
- Identify the exact module and OEM wiring diagram.
- Inspect accessible connectors for heat, corrosion, and damage.
- High-current clamp or scope diagnosis under command.
- Removing seized or swollen glow plugs from the head.
- ECM-driver, network, or intermittent harness diagnosis.
- Programming, coding, initialization, or broken-plug extraction.
Use professional help when wiring overheats, a fuse opens repeatedly, the engine will not start in unsafe weather, a plug may break during removal, or repair requires high-current waveform analysis or controller programming.
Frequently Asked Questions About P0670
What does P0670 mean?
Does P0670 mean all glow plugs are bad?
Can I drive with P0670?
Can a weak battery cause P0670?
Can UR1000 test the glow plug module?
How should P0670 be cleared?
Sources and Technical Review Notes
- Volkswagen 2013 Beetle TDI diagnostic reference — manufacturer P0670 wording and monitored glow-module behavior.
- General Motors glow plug control module bulletin — illustrates cylinder-specific codes and the need to check module calibration on applicable vehicles.