P0670 – Glow Plug Module Control Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit

Definition source: SAE J2012/J2012DA (industry standard)

P0670 indicates a fault detected in the glow plug module control circuit. This is an electrical-circuit DTC, meaning the control module has identified an abnormal condition in the command, feedback, power, ground, or communication path used to operate the glow plug module. While the definition is consistent, the exact monitor strategy, enabling criteria, and what the module considers “faulted” can vary by vehicle, so always verify the diagnostic routine, connector views, and pin functions in the appropriate service information. Treat P0670 as a circuit diagnosis first: confirm the concern, capture data, and test the circuit methodically before replacing components.

What Does P0670 Mean?

P0670 means the vehicle’s control module has detected a problem in the Glow Plug Module Control Circuit. Per SAE J2012 DTC structure conventions, the code identifies the affected system and the monitored fault entry; however, the official definition is the single source of truth for what this DTC represents. In practical diagnostic terms, the module is reporting that the electrical path used to control the glow plug module is not behaving as expected. The issue may be in wiring, connectors, power or ground supply to the circuit, the control module’s driver/logic, or the glow plug module itself, depending on vehicle design.

Quick Reference

• Subsystem: Glow plug module control circuit (control/command/feedback path for glow plug operation).
• Common triggers: Open/shorted wiring, poor connections, incorrect power/ground to the circuit, or control/feedback signal not matching commanded operation.
• Likely root-cause buckets: Wiring/connector issues; power/ground distribution faults; glow plug module internal fault; control module driver/logic fault; corrosion or water intrusion at connectors (varies by vehicle).
• Severity: Typically moderate; may cause hard starting when cold and increased smoke, and can lead to no-start in low temperatures.
• First checks: Verify battery condition; scan for related codes; inspect connectors/pins and harness routing; confirm module power and ground integrity; check for loose terminals.
• Common mistakes: Replacing glow plugs first, skipping power/ground checks, or condemning a module without confirming circuit integrity and connector pin fit.

Theory of Operation

The glow plug system preheats the combustion chamber during cold starts. A control module (varies by vehicle) requests glow operation based on inputs such as coolant temperature and operating state. The glow plug module then switches current to the glow plugs, typically using high-current drivers, while the control module monitors the control circuit for expected electrical behavior.

P0670 sets when the monitored control circuit indicates a fault, such as an abnormal command/feedback relationship, a missing or unstable control signal, or a circuit integrity problem affecting control of the glow plug module. Because architectures vary, the “control circuit” may involve a dedicated control line, multiple control/feedback lines, or a communication path between modules; the diagnostic focus remains verifying circuit power, ground, continuity, and signal integrity under real operating conditions.

Symptoms

• Hard start: Extended cranking or delayed start, especially in colder conditions.
• No-start: Increased likelihood of a no-start when ambient temperatures are low.
• Rough idle: Uneven running immediately after start that improves as the engine warms.
• Smoke: Increased exhaust smoke during cold start/warm-up due to incomplete combustion.
• Reduced readiness: Emissions readiness or self-tests may not complete if the fault persists.
• Warning light: Malfunction indicator lamp illuminated and stored fault memory for the glow plug module control circuit.

Common Causes

• Open circuit, high resistance, or intermittent connection in the glow plug module control circuit wiring harness
• Connector issues at the glow plug module or control module (loose fit, corrosion, backed-out terminals, pin damage)
• Poor power feed to the glow plug module (blown fuse, faulty relay, or excessive voltage drop in the supply path)
• Poor ground path for the glow plug module (loose ground point, corrosion, damaged ground strap, voltage drop under load)
• Short to ground or short to power in the control circuit between the control module and glow plug module
• Glow plug module internal fault affecting its ability to communicate/operate on the control circuit
• Control module fault (driver/output stage or internal monitoring issue) or software/calibration issue (varies by vehicle)
• Aftermarket wiring modifications or previous repairs causing incorrect pinout, splices, or inadequate conductor sizing

Diagnosis Steps

Tools that help: a scan tool capable of reading freeze-frame data, clearing codes, and viewing relevant live data; a digital multimeter; a test light as appropriate; and basic back-probing leads. A wiring diagram and connector pinout from service information are essential because circuit routing and module locations vary by vehicle.

1. Confirm the DTC and capture context. Record stored and pending codes, freeze-frame data, and readiness status. Note whether P0670 resets immediately or only after a cold start/after key-on. Address any battery-voltage or control-module communication codes first, since they can skew circuit monitoring.
2. Verify the complaint and operating conditions. Cycle the key and observe whether the glow plug system status (if available in live data) changes as expected. If the vehicle requires specific enabling conditions for the monitor, follow service information to reproduce them.
3. Perform a focused visual inspection. Inspect the glow plug module, harness routing, and connectors for chafing, melted insulation, signs of water intrusion, loose retention, and previous repair splices. Gently tug on the harness near connectors to check for broken conductors under the insulation.
4. Check fuses and power distribution for the module. Using the wiring diagram, identify all feeds to the glow plug module and any related relays. Confirm the correct fuses are intact and that the module receives power when it should. If a fuse is blown, do not replace it repeatedly without checking for a short or damaged wiring.
5. Check grounds with a voltage-drop test under load. Do not rely only on continuity. With the circuit loaded (command the system on if possible, or use the vehicle’s built-in self-test conditions), measure voltage drop across the glow plug module ground path(s). Excessive drop indicates resistance at the ground point, fasteners, terminal crimp, or ground strap (exact limits vary by vehicle).
6. Check the control circuit for opens and shorts. With power off as directed by service info, isolate the control circuit between the control module and glow plug module. Test for continuity (open/high resistance) end-to-end, and test for shorts to ground and shorts to power. If readings change when you move the harness, suspect an intermittent break or rubbed-through section.
7. Connector pin and terminal testing. Inspect terminal tension and pin fit at both ends of the control circuit. Look for spread terminals, corrosion, or backed-out pins. Correct any pin-fit issues and ensure connector seals and locks are fully seated before further testing.
8. Command and observe with live data (if supported). Use bi-directional controls to command glow plug operation or module activation where available. Log live data for commanded state versus reported/feedback status (naming varies by vehicle). A mismatch can support a circuit or module-control issue, but confirm with wiring tests before condemning a module.
9. Perform a wiggle test while monitoring the circuit and scan data. With the engine idling (or during key-on tests if the engine will not start), gently flex the harness and connectors while observing live data and/or multimeter readings. If the fault appears/disappears, pinpoint the section that triggers the change and recheck for damaged conductors or terminals.
10. Differentiate wiring versus module faults. If power and ground are verified good under load and the control circuit passes open/short checks, follow service information to determine whether the glow plug module or the control module is the next suspect. If allowed, substitute known-good inputs/loads only in a controlled manner; avoid bypassing circuits in ways that could damage drivers.
11. Clear codes and verify the fix. After repairs, clear DTCs and run the monitor under the required conditions (often including a cold start). Confirm P0670 does not return and that related readiness/monitor status completes normally.

Professional tip: Treat P0670 as a circuit diagnosis first. Many repeat comebacks happen when power and ground are checked only with an ohmmeter and not with a voltage-drop test under actual load. If the fault is intermittent, a short live-data log plus a gentle harness wiggle test often locates the exact connector or harness segment faster than repeated parts replacement.

Possible Fixes & Repair Costs

Repair cost for P0670 varies widely because it depends on what the diagnostic tests confirm (wiring vs module vs power/ground), how accessible the components are, and whether additional harness or connector repair is required.

• Repair or replace damaged wiring in the glow plug module control circuit (chafed insulation, broken conductors, melted sections)
• Clean, secure, and repair connector issues (corrosion removal, terminal tension repair, pin fit correction, connector replacement as needed)
• Restore proper power and ground to the glow plug module (repair blown feed issues, poor grounds, or high-resistance connections)
• Replace the glow plug module only after verifying correct power/ground and confirmed control-circuit fault behavior
• Repair related circuit protection and distribution components (as applicable by vehicle design) after confirming the root cause
• Perform required control-module relearn/configuration steps if service information specifies them after replacement

Can I Still Drive With P0670?

You can often drive with P0670, but cold starting and warm-up quality may be affected, and repeated failed starts can leave you stranded. If the engine will not start, stalls, enters reduced-power operation, or you have any safety-related warnings, do not drive—diagnose and repair first. Because vehicle behavior varies by design, verify symptoms and monitor data with service information and a scan tool.

What Happens If You Ignore P0670?

Ignoring P0670 can lead to persistent hard-start/no-start conditions in colder temperatures, rough operation during warm-up, increased starter and battery stress from extended cranking, and recurring warning indicators. Ongoing circuit faults can also worsen connector/terminal damage over time, making future repairs more involved.

Related Module Glow Codes

Compare nearby module glow trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P0682 – Cylinder 12 Glow Plug Circuit
• P0681 – Cylinder 11 Glow Plug Circuit
• P0680 – Cylinder 10 Glow Plug Circuit
• P0679 – Cylinder 9 Glow Plug Circuit
• P0678 – Cylinder 8 Glow Plug Circuit
• P0677 – Cylinder 7 Glow Plug Circuit

Key Takeaways

• P0670 indicates a fault in the glow plug module control circuit, not a confirmed glow plug failure by itself.
• Start with circuit basics: power, ground, connector condition, and harness integrity before replacing modules.
• Intermittent faults are common; use wiggle testing and drive-cycle logging to catch them.
• Cold-start complaints are typical, but symptoms and severity vary by vehicle.
• Fixes should be based on test results, not parts swapping.

Vehicles Commonly Affected by P0670

• Diesel-powered vehicles equipped with a glow plug system and an electronically controlled glow plug module
• Light-duty trucks and utility vehicles used in frequent cold-start or short-trip operation
• Commercial or fleet vehicles with high idle time and repeated start cycles
• Vehicles operated in wet, salty, or high-corrosion environments affecting connectors and grounds
• High-mileage vehicles where harness routing wear and terminal tension loss are more likely
• Vehicles with prior engine-bay repairs where connectors may be disturbed or pin fit compromised
• Vehicles with battery, starter, or power distribution issues that can expose marginal circuit connections

FAQ

Does P0670 mean the glow plugs are bad?

No. P0670 is specifically about the glow plug module control circuit. Individual glow plug faults may set different codes or be found through testing, but this code alone does not confirm failed glow plugs.

Can a weak battery cause P0670?

It can contribute indirectly. Low system voltage or poor battery connections can make control modules and high-current circuits behave unpredictably. However, you should confirm the actual fault is in the glow plug module control circuit through proper circuit testing rather than assuming the battery is the root cause.

What should be checked first for a circuit-related glow plug module code?

Check the simplest, highest-probability items first: connector seating, corrosion, damaged pins, harness chafing, and verified power/ground integrity to the glow plug module. Then confirm the control circuit’s behavior with scan-tool data and targeted electrical tests as outlined in service information.

Will clearing the code fix P0670?

Clearing the code only resets the stored fault information. If the underlying circuit problem remains, the monitor will typically detect the fault again and the code will return, sometimes after a key cycle or when the glow plug system is commanded on.

When is glow plug module replacement appropriate for P0670?

Replace the glow plug module only after confirming the module has correct power and ground, the control circuit wiring and connectors test good, and the fault can be reproduced or verified as internal to the module per service-information procedures.

For an accurate repair, treat P0670 as an electrical circuit fault and confirm the failed section with structured testing before replacing any components.