Comprehensive Diagnostic Guide for P0670

Glow plug/Heater Circuit - Control Module

What This Code Means

• Code meaning (as per open-source OBD2 definitions): P0670 is associated with the Glow plug/heater control module, i.e., the glow plug/heater circuit or its control hardware (module/driver). This points to a fault in the glow plug heating element circuit or the controller that powers/controls it. P0670 would be categorized under Powertrain Codes in the OBD-II framework. Source: OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes.

Notes on applicability

• Glow plug/heater circuits are most commonly discussed in the context of diesel engines, where glow plugs are used to heat the combustion chamber for starting in cold conditions. However, the P0670 code definition in the open-source listing explicitly references the glow plug/heater control module concept, and some engines (including gasoline variants with glow plug/auxiliary heater elements) may use related circuits. Treat P0670 as a fault in the glow plug circuit or its control hardware for your specific engine family. Sources: OBD-II articles; Open Source code definition.

Probable symptoms you may observe

• MIL (Check Engine Light) illuminated or blinking in some cases.
• Hard starting or extended cranking in cold ambient conditions.
• Rough or misfiring idle during startup as the engine fails to heat a cylinder properly.
• In diesel applications, sluggish cold-start performance or longer cranking cycles; in some gasoline engines with glow/heater elements, similar symptoms may appear during cold starts if the circuit is involved in startup sequencing.
• Possible inconsistency in readings related to crank/engine temperature while starting, or intermittent start/stop behavior.
• Note: Symptoms can be accompanied by other DTCs if related wiring, relay, or control hardware is compromised.

Typical causes (field experience-based probabilities)
Because do not include NHTSA complaint data for P0670, probabilities are based on general ASE-field experience and typical diagnostic patterns for glow plug/heater circuit faults:

• Faulty glow plug/heater element (open or high resistance): ~50%
• Faulty glow plug relay or the glow plug heater control driver/module: ~20%
• Wiring harness/connectors (frayed, chafed, corroded, bad grounds): ~15%
• PCM/ECU control fault or intermittent driver: ~10%
• Other issues (sleeve harness damage, improper service, short to battery/ground, etc.): ~5%
  These percentages are intended as a practical guide and may vary by vehicle make/model and the exact nature of the P0670 implementation on that engine.

Diagnostic Approach

1) Gather context and confirm the code

• Confirm the DTC with a good OBD-II scan tool, and note any related codes (e.g., P0671, P0672, etc. for other glow plug circuits across cylinders, or misfire-related codes that might be secondary).
• Review freeze-frame data for engine temperature, cranking duration, and any sensor readings at the time of fault.

2) Pre-checks and safety

• Ensure battery is healthy and cranking speed is adequate. A weak battery can cause false or intermittent glow-plug-related codes.
• If working on high-current circuits, observe proper PPE and disconnect battery as needed before inspecting/repairing wiring, relays, or control modules.
• If your vehicle uses diesel, ensure you handle glow plug circuits with particular caution due to elevated heating element temperatures when energized.

3) Visual inspection

• Inspect the glow plug harness/connector for damaged insulation, corrosion, loose connections, or burnt-looking wiring near the glow plugs and the control module.
• Inspect grounds and engine block/strap connections related to the glow plug circuit.
• Check the glow plug relay (if applicable) and the control module/driver connector for signs of overheating or moisture intrusion.

4) Identify the scope of the issue

• Determine if the fault is likely a single glow plug, multiple plugs, or the control module:
  • If the code is for a specific cylinder (e.g., Cylinder 1), suspect the glow plug for that cylinder or its wiring.
  • If the code is a general P0670 without cylinder specificity, consider the overall glow plug control circuit or module.
• Check for related codes (fueling, compression, crank/cam sensor, etc.) that could influence starting behavior or engine temperature readings.

5) Electrical tests (glow plug circuit)

• Glow plug resistance/continuity: With the engine/ECU power off, test the resistance of each glow plug or the heater element within spec for your engine (reference service data). An open or high-resistance glow plug indicates a faulty element.
• Power and ground checks: While the ignition/engine is cranking or the glow plug circuit is commanded (as per service data), measure voltage at the glow plug harness connector(s). Confirm presence of battery voltage on the feed side and a proper ground return on the sensor/driver side.
• Relay/driver test: If a glow plug relay or driver module is present, test its coil resistance and verify proper switching when commanded by the ECU. Check for stuck or weak relays, or a driver/transistor failure in the control module.
• Harness continuity and shorts: Inspect wires for shorts to battery, shorts to ground, or cross-conduction between adjacent wires. Use a scan-tool live data snapshot and a digital multimeter to verify expected continuity and resistance values.

6) Control module considerations

• If all glow plugs and wiring test well, the issue may be with the glow plug control module or the ECU's driver for the circuit.
• Inspect for water ingress, corrosion, or loose pins in the control module connectors. In some cases, software updates or BCM/ECU reprogramming may address intermittent behavior.

7) Post-repair verification

• Clear the DTCs and perform a test cycle according to the vehicle's service procedure.
• Start the engine in cold conditions (and multiple starts if possible) to verify the glow plug circuit operates as expected.
• Re-scan for DTCs to confirm resolution and ensure no new codes appear.

8) Special considerations

• If multiple glow plugs fail or the harness shows widespread damage, consider replacing the glow plug harness/connector assemblies or the glow plug module/driver, and re-test.
• If the issue persists after all physical tests (and wiring appears sound), consider ECU/PCM-related fault possibilities and consult vehicle-specific service information or manufacturer bulletins.

Repair Options

• Replace faulty glow plug(s) and/or glow plug heater element(s) as needed.
• Replace faulty glow plug relay or glow plug driver module, if present.
• Repair or replace wiring harnesses/connectors with damaged insulation, corrosion, or poor grounds.
• Replace or reprogram the ECU/PCM if diagnosed as a control fault (after ruling out wiring and plug faults).
• Address any related service bulletins or updates for glow plug circuits from the vehicle manufacturer.

Testing and validation after repair

• Clear codes and perform a controlled cold-start test sequence; verify engine starts reliably when cold.
• Re-scan to confirm no residual or new codes appear; monitor live data to ensure glow plug circuit engages when commanded and that voltages are within expected ranges.
• If the problem recurs intermittently, monitor for pattern (engine temperature, ambient temperature, humidity) and reevaluate wiring and control modules.

Useful notes and references

• OBD-II code context and DTC framework (general understanding of how codes are used in powertrain monitoring): OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes.

• Code specificity to glow plug/heater control module (definition of P0670): Módulo de controle do aquecedor/vela aquecedora. This supports the interpretation that P0670 relates to the glow plug/heater circuit or its control module.

• General diagnostic methodology and the role of DTCs in emissions and powertrain monitoring: OBD-II - Emissions Testing, OBD-II - Diagnostic Trouble Codes.

• If you have vehicle-specific service information, always align the diagnostic steps with the exact engine family's glow plug circuit design, including the number of glow plugs, the driver/relay configuration, and any manufacturer-specific fault codes or test procedures.

• In cases where multiple codes are present or advanced diagnostics are required, use a scope or data-logger to observe the glow plug drive signals, timing of glow events during cranking, and ECU command behavior.

Summary

• P0670 denotes a fault in the glow plug/heater circuit or its control module. Use a systematic electrical inspection of the glow plug elements, their harnesses, grounds, and the control/driver module, followed by relay/driver testing and controller integrity checks. Replace or repair components based on test results, verify with cold-start testing, and re-scan to confirm resolution. The approach aligns with general OBD-II diagnostic practices described in Wikipedia and the open-source code definition that identifies P0670 with the glow plug/heater control module. Citations: Wikipedia; Open Source (glow plug/heater control module).

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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