Comprehensive diagnostic guide for OBD-II code P0673

Cylinder 3 Glow Plug Circuit

What This Code Means

• P0673 is a powertrain/engine-related DTC that targets the glow plug circuit for cylinder 3. The Open Source entry (glow plug/heater for cylinder 3) identifies this cylinder-specific glow plug circuit. This aligns with the standard OBD-II practice of isolating cylinder-specific glow plug faults within P067x codes. Source notes: Open Source code listing; general OBD-II code concept is described in the Wikipedia OBD-II sections. Citations: Wikipedia and the Open Source entry for cylinder 3 glow plug.

• For broader context, OBD-II codes are diagnostic trouble codes used by the vehicle's powertrain control modules to monitor engine and emissions-related systems. The Powertrain Codes section of the Wikipedia OBD-II article confirms these are part of the system the PCM/ECU uses to monitor and report issues. Citations: Wikipedia - OBD-II: Diagnostic Trouble Codes; Wikipedia - OBD-II: Powertrain Codes.

Symptoms

• Cold-start difficulties: hard starting or extended cranking on cold starts.
• Rough idle or misfire symptoms that seem to originate around cylinder 3 on non-warmups.
• Check Engine Light illumination with P0673 stored in the ECM/PCM.
  Note: These symptom patterns are consistent with a glow plug circuit issue in a diesel (or a vehicle using glow plugs for cold-start aid). The specific symptom set should be considered alongside other codes (e.g., misfire codes like P0303) to differentiate glow plug issues from mechanical or injector problems.

What you should do first (verification steps)

• Confirm the code: Use an OBD-II scan tool to verify P0673 and note any accompanying codes (e.g., P0303 cylinder 3 misfire, or P067x other glow plug circuit codes). Check freeze-frame data for engine temperature, run status, and related sensor readings at the time the code was set.
• Check related system data: If the vehicle is diesel or uses glow plugs, look for multiple cylinder codes or only cylinder 3. Also note if the fault occurs primarily during cold starts. This helps prioritize whether you're chasing a single glow plug circuit issue or a broader electrical/control issue.

Step-by-Step Diagnosis

1) Visual inspection

• Inspect the cylinder 3 glow plug/heater and its wiring harness, connectors, and the immediate surroundings for damage, arcing, grounding issues, corrosion, or melted insulation.
• Inspect the glow plug relay (if the vehicle uses a relay to drive glow plugs) and the control circuit wiring for damage or poor connector engagement.

2) Glow plug circuit test (cylinder 3)

• With ignition off, perform a resistance test of the cylinder 3 glow plug. Compare results to the vehicle's service specs. An open circuit or excessively high/low resistance usually indicates a faulty glow plug.

3) Relay and power supply verification

• Verify that voltage is being supplied to the glow plug when the ignition is on (the glow cycle is active). Check the ground path as well. If there is no voltage or poor voltage due to wiring or relay fault, fix the circuit rather than the glow plug first.

4) Control circuit verification (ECM/PCM output)

• Confirm that the ECM/PCM is actually commanding the glow plug circuit (i.e., the ignition-control output to the glow plug relay or direct drive is active when cold-start is required). If the ECM isn't driving the circuit correctly, the fault could be the ECM or a software/interrupt issue.

5) Glow plug replacement decision

• If cylinder 3 glow plug resistance is out of spec or the plug is open/shorted, replace only the faulty glow plug. Also inspect/replace any related gaskets or seals if you're removing the housing assembly.

6) Wiring and connector integrity

• If the glow plug itself tests fine, test the wiring harness for continuity from the ECU/relay to the glow plug terminal, including any inline fuses, fuses boxes, or junctions. Repair or replace damaged wiring or connectors as needed.

7) Related causes and cross-checks

• If multiple cylinder glow plugs or circuits show faults, consider a common supply or control issue (e.g., power supply problems to the glow plug system, a shared relay, or ECM grounding/diagnostic issues).

8) Recheck and verification after part replacement

• After replacing a glow plug or repairing the circuit, clear the codes and perform a cold-start test to ensure P0673 does not return. If the fault reappears, re-check the circuit for intermittent faults or ECM-related issues.

9) Consider related codes

• If misfire codes (e.g., P0303) appear in conjunction with P0673, re-test to determine whether misfire symptoms persist after the glow plug repair. The root cause could still be glow plug-related if the cylinder fails to glow properly, leading to starting and running issues that mimic misfire symptoms.

Common Causes

• Faulty cylinder 3 glow plug (open/short or high resistance): Most frequent single cause in practice for P0673. Probability range (rough): 40-60%.
• Glow plug relay or control circuit fault (relay coil, driver transistor, wiring to relay): Common second cause. Probability range: 20-30%.
• Wiring harness/connectors to glow plug or ECU (pin/connector corrosion, damaged insulation): Moderate likelihood. Probability range: 15-25%.
• ECM/PCM fault or software/diagnostic issue (less common, but possible): Lower likelihood. Probability range: 5-10%.

Repair and Verification

• If cylinder 3 glow plug is faulty: replace the glow plug, inspect wiring and connector, then re-test. If the fault persists, investigate the relay/control circuit and ECM.
• If wiring/connector suspected: repair or replace wiring harness or connectors, re-test circuit continuity and resistance.
• If relay or control circuit suspected: replace glow plug relay or fix control wiring; re-test with ignition on to confirm voltage to the glow plug.
• After any repair: clear DTCs, perform a cold-start test, and confirm that P0673 does not reappear under normal operating conditions. Consider road testing to verify function in real conditions.

Documentation and next steps

• Record the diagnosed cause, parts replaced, wiring repairs, and software/ECM considerations.
• If symptoms persist or recur after repair, escalate to a deeper ECM/vehicle-level diagnostics and consider professional service for potential PCM faults or software updates.

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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