Comprehensive diagnostic guide for OBD-II code P0671

What This Code Means

• P0671 is a glow plug system fault tied to cylinder 1. In many open-source mappings, the DTC description is listed as a glow plug/heater for cylinder 1 . This supports the interpretation that P0671 relates to a fault in the heater element or its circuit for cylinder 1.
  • Supporting source: Open Source listing (glow plug for cylinder 1)
  • Supporting context on DTCs: Wikipedia's OBD-II references explain that Diagnostic Trouble Codes are generated by control systems and that Powertrain Codes are a major category of DTCs (OBD-II context and code types).

What This Code Means

• The engine control module (ECM/PCM) detected a fault in the glow plug circuit for cylinder 1. This can involve:
  • A faulty glow plug (heater element) in cylinder 1
  • A broken or high-resistance glow plug wiring harness or connector for cylinder 1
  • A faulty glow plug relay or the ECM/PCM driver that energizes the glow plug
  • Wiring faults (open circuit, short to ground, corrosion)
• This code is most common on engines that use glow plugs (diesel) or pre-heating systems; on engines that don't utilize glow plugs for starting, this exact DTC is less typical. The diagnostic path still focuses on the cylinder 1 glow plug circuit.

Typical symptoms reported with glow-plug-related cylinder 1 faults (inform symptom-driven diagnostics)

• Hard starting or extended cranking on cold starts
• Rough idle or misfire indications around cylinder 1 during cold operation
• MIL (Check Engine Light) illumination
• Occasional white/blue exhaust smoke on startup (diesel engines in particular)

Note: Symptoms can vary by engine type (diesel vs. gasoline) and by how aggressively the OEM uses the glow/plug preheat system. emphasize the DTC and glow-plug concept rather than engine-specific symptom catalogs.

Diagnostic Approach

1) Confirm and scope the issue

• Use a scan tool to confirm P0671 and review freeze-frame data and any related codes (P0670-P0676 family codes are commonly seen with glow-plug circuits on cylinder-specific codes).
• Note whether other cylinder-related or misfire codes appear (e.g., P0301 for cylinder 1 misfire) which can complicate interpretation.

2) Gather vehicle context

• Determine engine type: diesel with glow plugs or any gasoline engine with a pre-heating system. This guides expected circuit behavior and component layout.
• Check for known manufacturer specifics: some brands have different wiring schemes or rely more heavily on PCM control.

3) Visual inspection (simple, non-invasive)

• Inspect cylinder 1 glow plug connector and wiring harness for damage, corrosion, or loose pins.
• Inspect the glow plug relay/relay wiring and any PCM-controlled lines for signs of overheating, chafing, or short circuits.
• Look for obvious signs of water ingress, oil contamination, or battery/wiring issues near the glow plug circuit.

4) Electrical and circuit testing

• Accessory/Key-on voltage check:
  • With ignition on (engine off) verify that the glow plug circuit has the expected supply voltage when the PCM commands preheat.
• Wiring continuity:
  • Check for open circuits or shorts between the glow plug connector and the power/ground reference.
• Glow plug resistance:
  • Measure the resistance of the glow plug for cylinder 1 with the plug removed and cooled. An open circuit or abnormal resistance typically indicates a faulty glow plug.
• Relay and driver testing:
  • Verify the glow plug relay operation and coil resistance.
  • If possible, swap with a known-good relay to confirm whether the relay is the fault.
  • If the relay tests and wiring check out, consider PCM/ECU driver sanity (some diagnostics involve observing whether the PCM actually energizes the circuit during a fault condition).

5) Component-specific checks

• Glow plug in cylinder 1:
  • If the plug shows open circuit or excessive resistance, replace the glow plug and re-test the circuit.
• Wiring harness/connector:
  • Repair or replace damaged wiring or connectors. Ensure proper pin alignment and secure connections; treat any corrosion as a failure point.
• Glow plug relay/ECU driver:
  • If the circuit is energizing intermittently or not at all, test or replace the relay. If the relay tests good but the ECM driver is suspected (e.g., other cylinders' plugs also have issues or the ECU is not commanding the circuit correctly), escalate to PCM diagnostics or replacement per OEM procedures.

6) Confirm behavior after repairs

• Clear DTCs and perform a controlled test drive under varying conditions (cold start, warm start, idling, and under load as appropriate).
• Re-scan to confirm P0671 is cleared and that there are no new codes or pending codes related to glow-plug circuits.
• Monitor for any repeat occurrences of P0671 or related glow-plug circuit faults.

Probable Causes

• Open circuit or high resistance in cylinder 1 glow plug circuit or its wiring/connectors: 40-60%
• Faulty cylinder 1 glow plug (heater element): 15-25%
• Faulty glow plug control relay or PCM driver: 10-20%
• Wiring harness damage or poor connector condition (corrosion, loose pins): 10-15%
• ECM/PCM fault (less common, but possible, especially if other glow-plug circuits show anomalies): 5-10%

Notes on cause probabilities

• These ranges reflect common failure patterns seen in practice and are not vehicle-specific; exact likelihood depends on vehicle age, climate, and maintenance history. No dedicated NHTSA complaint data is provided , so these probabilities are anchored in typical diagnostic experience.
• If multiple related cylinder glow-plug circuits show faults or if P0671 repeats after a repair, consider more comprehensive PCM evaluation or reprogramming as recommended by the OEM.

Additional diagnostic considerations

• If the vehicle has multiple glow-plug circuits (P0670, P0672, etc.) and only P0671 is present, focus on cylinder 1: plug, wiring, and its specific control path.
• If cold-start performance is your primary symptom and other sensors check out, you should still verify that the glow-plug system is commanded properly by the PCM during the preheat cycle.
• If compression or mechanical integrity tests show issues in cylinder 1, those problems could manifest as or contribute to misfire codes; ensure a holistic approach (compression test, injector/valve condition) if misfire codes appear.

Safety Considerations

• Glow plugs reach high temperatures; avoid contact when the engine is hot. Allow cooling before handling plugs or wiring.
• Disconnect the battery or follow proper lockout/tagout procedures when performing electrical tests to avoid accidental energization.
• Follow OEM procedures for testing sequences and use the correct wiring diagrams for your vehicle.

Documentation

• DTC: P0671 (Cylinder 1 Glow Plug Circuit Fault)
• Reproduction conditions: cold start vs. warm start; any observed misfire or idle symptoms
• Freeze-frame data and any related codes
• Components tested and results: glow plug, wiring, relay/driver, PCM
• Actions performed: replacement(s) or repair(s) with part numbers if applicable
• Verification results: post-repair re-scan results, drive test outcomes
• Follow-up recommendations if code recurs (e.g., PCM health check, software update, further cylinder testing)

Source notes and references

• Diagnostic Trouble Codes overview and the role of DTCs in OBD-II systems (Powertrain Codes context) as described in Wikipedia:
  • OBD-II: Diagnostic Trouble Codes
  • OBD-II: Powertrain Codes
  • OBD-II: Emissions Testing
  • These sources describe how DTCs are used by control modules to identify issues in the powertrain and emission-related systems, forming the basis for interpreting a code like P0671.
  • Citation examples: "Diagnostic Trouble Codes are generated by vehicle control modules when issues are detected; Powertrain Codes are a major category within OBD-II."
• Open-source code definition mapping for P0671 (glow plug cylinder 1)
  • Aquecedor/vela aquecedora do cilindro 1 (glow plug for cylinder 1) - supports the interpretation that P0671 corresponds to the cylinder 1 glow plug circuit fault.
  • Citation example: as the code description in an MIT/open-source repository
• Safety and general diagnostic approach aligns with standard automotive diagnostic practice (test, verify, and isolate the glow plug circuit with a combination of visual inspection, electrical testing, and component replacement as needed).

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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