P0674: Cylinder 4 Glow Plug/Heater Circuit - Comprehensive Diagnostic Guide

P0674: Cylinder 4 Glow Plug/Heater Circuit - Comprehensive Diagnostic Guide

Overview

• What the code means (typical interpretation): P0674 is a powertrain diagnostic trouble code indicating a fault in the glow plug/heater circuit for cylinder 4. In many OBD-II implementations this code points to the cylinder 4 glow plug (heater) circuit having an electrical fault (open, short, high resistance, or control issue). This falls under the broader category of powertrain codes (P0xxx) in OBD-II, which monitor engine and emissions-related systems. Source guidance on OBD-II codes and their role in powertrain monitoring can be found in on Diagnostic Trouble Codes and Powertrain Codes.
• Language note: The provided open-source entry uses Portuguese wording which translates to glow heater/plug for cylinder 4, reinforcing that this code relates to the glow plug/heater in cylinder 4.

Symptoms

• Morning cold starts or engine starting in cold ambient conditions may be sluggish or extended cranking.
• MIL (Malfunction Indicator Lamp) may be ON with P0674 stored.
• Misfire-like symptoms on cylinder 4 or rough idle could appear if the glow plug circuit affects combustion timing/heat in early start.
• In some cases, symptoms are intermittent or only present during specific temperature ranges or after certain driving cycles.
  Note: These symptom patterns align with the general role of glow plug/heater circuits within engine starting and cold-weather operation of OBD-II powertrain codes.

Probable Causes

• Faulty glow plug (heater) in cylinder 4 (open, shorted, or degraded): ~40-50%
• Wiring harness/connector between PCM and cylinder 4 glow plug (damaged insulation, corrosion, loose or oxidized terminals): ~20-35%
• Glow plug relay or the supply/ground path for the cylinder 4 glow plug (faulty relay, blown fuse, poor terminal contact): ~15-25%
• Powertrain control module (PCM) control issue or misinterpretation by PCM (rare but possible): ~5-10%
• Battery voltage or charging issues causing insufficient glow-time or weak heat (low voltage conditions): ~5-15%
• Other related wiring or miscellaneous faults (e.g., ground faults, adjacent cylinder interference): small remainder

Diagnostic Approach

1) Confirm and document

• Verify the stored code(s) in the vehicle's scan tool, and pull freeze-frame data to see engine conditions at the time of the fault (engine load, coolant temp, voltage, RPM, etc.).
• Check for any related cylinder glow plug or heater codes (e.g., codes for other cylinders) that might indicate a common supply/ground issue or PCM problem.
• Reference the general role of DTCs and powertrain codes; P0674 is understood as a glow-plug/heater-circuit issue in cylinder 4.

2) Visual inspection and basic mechanical checks

• Inspect cylinder 4 glow plug connector and wiring for visible damage, chafing, corrosion, bent pins, or disconnected harness.
• Check battery health and charging voltage; if voltage is low at start, glow plugs may not heat properly.
• Look for signs of water intrusion or wiring harness damage near the cylinder head or glow plug area.
  Note: Visual inspections are a standard early step when diagnosing glow-plug circuits and are consistent with general DTC diagnostic practice.

3) Electrical tests focusing on the cylinder 4 glow plug circuit

• Measure the resistance of the cylinder 4 glow plug element (disconnect the harness, isolate the plug, and compare to typical spec for your engine). If the resistance is out of spec or if the plug is open/shorted, replace the glow plug.
• Check the circuit continuity from the PCM/relay feed to the glow plug and from the glow plug ground back to chassis. Look for high resistance, breaks, or poor connections.
• Inspect the glow plug relay and its control signal from the PCM (if equipped). With the ignition on, verify that the relay coil energizes and that the contact side provides voltage to the glow plug circuit when commanded. Check for proper voltage present at the glow plug connector when commanded.
• Check fuses and the power supply path to the glow plug circuit; ensure there is no blown fuse or fuseable link interrupting the circuit.
  Note: These steps reflect standard electrical testing practices for glow plug circuits and are aligned with the general scope of OBD-II powertrain fault testing.

4) Control-side assessment (PCM/ECU)

• If resistance and wiring checks are good, verify whether the PCM is commanding the glow plug to heat via the live data/relay control status in the scan tool.
• Check for PCM-related fault codes or patterns that might indicate a control-module issue. PCM faults are less common but possible for P0674.
  Note: The possibility of PCM-related issues is acknowledged in general discussions of powertrain codes; PCM faults are less frequent than component-level faults but must be considered if circuit and component tests pass.

5) System-level checks and driving condition confirmation

• After repairs or replacements, clear the code and perform a drive cycle under both cold and warm conditions to verify that the fault does not return.
• If the code reappears, consider re-checking all connections and re-testing the glow plug, as intermittent issues can be caused by loose connections or marginal voltage.
  Note: OBD-II diagnostic practices emphasize rechecking after repairs and drive-cycle verification.

Recommended tests and practical procedures (condensed checklist)

• Visual: Inspect cylinder 4 glow plug harness, connector, and nearby wiring for damage or corrosion.
• Power/ground integrity: Verify battery voltage is adequate; check that the glow plug circuit has proper voltage when commanded and a solid ground path.
• Glow plug resistance: Measure cylinder 4 glow plug resistance; replace if out of spec or open.
• Circuit integrity: Test continuity from the PCM/relay to the glow plug and from the plug ground to chassis; repair as necessary.
• Relay/PCM: Test glow plug relay operation and PCM output to ensure proper control.
• Post-repair testing: Clear codes and perform an appropriate drive cycle to confirm the fault does not return.

Safety Considerations

• Disconnect the battery before removing or disconnecting electrical connectors near the glow plug to avoid short circuits or electric shock.
• Allow the engine and exhaust components to cool before handling glow plugs or related components, as glow plugs become extremely hot during operation.
• When testing high-amp circuits (glow plug circuits can draw significant current), use proper PPE and insulated tools, and follow standard electrical safety practices.

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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