Understanding OBD-II Diagnostic Trouble Code P0674 can be crucial for diagnosing issues in diesel vehicles. As an expert at autelfrance.com, I’m here to provide a comprehensive guide to this code, focusing on its meaning, symptoms, causes, and how to diagnose and potentially resolve it. This article aims to be a superior resource compared to basic explanations, offering in-depth information and SEO optimization for English-speaking automotive enthusiasts and professionals.
What Does OBDII Code P0674 Really Mean?
The P0674 diagnostic trouble code is a generic powertrain code, as it applies across various vehicle makes and models manufactured from 1996 onwards. However, specific repair procedures can vary slightly depending on the vehicle’s manufacturer and model. This code signals an issue within the cylinder #4 glow plug circuit.
Diesel engines utilize glow plugs to heat the cylinder head, especially during cold starts. Unlike gasoline engines, diesels rely on intense heat generated by compression to ignite fuel. When the engine is cold, the heat of compression can be rapidly lost to the cold cylinder head, making ignition difficult. Glow plugs are preheaters designed to combat this issue.
A glow plug is essentially a pencil-shaped heating element installed in the cylinder head, positioned near the combustion initiation point, whether in the main chamber or a pre-chamber. When the engine control module (ECM) detects a cold engine, based on readings from oil and transmission sensors, it activates the glow plugs to assist in starting.
Typical Diesel Engine Glow Plug:
Alt Text: Close-up of a diesel engine glow plug, highlighting its metallic construction and heating tip.
The ECM grounds the glow plug timer module, which in turn activates the glow plug relay. This relay then delivers power to the glow plugs. The duration of activation is carefully controlled to prevent overheating and damage to the glow plugs, as they generate heat through electrical resistance and become extremely hot when energized. This intense heat pre-warms the cylinder head, ensuring that the heat of compression is sufficient to ignite the injected fuel for a successful engine start.
Code P0674 specifically indicates a malfunction in the glow plug circuit for cylinder number 4. This means that the glow plug in cylinder 4 is not heating up as expected, and the entire circuit needs inspection to pinpoint the exact fault.
Important Note: If you are also encountering a P0670 trouble code alongside P0674, it is advisable to address the P0670 diagnostic process first before proceeding with the diagnosis of P0674. P0670 relates to the general glow plug control circuit, and resolving it might indirectly fix the P0674 issue.
Common Symptoms of a P0674 Code
While a single faulty glow plug might not cause dramatic symptoms, especially in warmer conditions, the presence of a P0674 code indicates a problem that should be addressed. In colder climates or after the engine has cooled down significantly, the symptoms can become more noticeable. The most common symptoms include:
- Check Engine Light Illumination: The engine management computer (PCM/ECM) will detect the fault in the glow plug circuit and illuminate the Malfunction Indicator Lamp (MIL), commonly known as the check engine light.
- Hard Starting or No Start in Cold Weather: This is a primary symptom. Without the assistance of a functioning glow plug in cylinder 4, the engine may struggle to start, or fail to start altogether, in cold ambient temperatures.
- Reduced Engine Power Until Warm-Up: The engine might run sluggishly or lack power immediately after starting, until it reaches its normal operating temperature. This is because cylinder 4 is not contributing optimally during the initial cold running phase.
- Engine Misfire: A colder than normal cylinder head temperature in cylinder 4 can lead to misfires, especially during the engine’s warm-up period. This can result in rough idling or hesitation.
- Hesitation During Acceleration: The engine might hesitate or stumble when you try to accelerate, particularly when the engine is still cold.
- Extended Preheat Period (If Applicable): In some diesel vehicles, a preheat light indicates when the glow plugs are active. If the P0674 code is present, this preheat period might be abnormally long, or the preheat light might not turn off at all.
Potential Causes of DTC P0674
Several factors can lead to the P0674 trouble code. Diagnosing the root cause is essential for effective repair. The most common culprits include:
- Faulty Cylinder #4 Glow Plug: The glow plug itself may be defective. Over time, glow plugs can burn out, similar to light bulbs, due to wear and tear or electrical stress.
- Open or Shorted Glow Plug Circuit: There might be a break (open circuit) or an unintended connection to ground (short circuit) in the wiring that supplies power to the cylinder #4 glow plug. This could be anywhere in the wiring harness between the glow plug control module/relay and the glow plug itself.
- Damaged Wiring Connector: The electrical connector at the glow plug itself, or connectors along the wiring path, could be corroded, loose, or damaged, leading to poor electrical contact or circuit interruption.
- Faulty Glow Plug Control Module (GPCM) or Relay: Although less common for a single cylinder code, a malfunctioning glow plug control module or relay could be failing to properly energize the circuit for cylinder #4’s glow plug.
Step-by-Step Diagnostic Procedures and Possible Solutions
To accurately diagnose and resolve the P0674 code, you’ll need some basic tools and a systematic approach. A digital multimeter (DVOM) is essential for electrical testing, and an OBD-II code scanner will be needed to read and clear codes. Always consult your vehicle’s repair manual for specific procedures and safety precautions.
Step 1: Initial Inspection and Code Verification
- Visually inspect the glow plug and wiring: Locate the cylinder #4 glow plug (refer to your vehicle’s service manual for cylinder numbering). Check the wiring and connector at the glow plug for any obvious signs of damage, such as frayed wires, corrosion, or loose connections.
- Record the P0674 code: Use an OBD-II scanner to confirm that the P0674 code is present and to check for any other related DTCs. Note down all codes present.
- Clear the DTCs: After recording the codes, clear them using the scanner and then attempt to start the engine. If the P0674 code returns immediately, proceed with further diagnostics.
Step 2: Glow Plug Resistance Test
- Disconnect the glow plug connector: Carefully disconnect the electrical connector from the cylinder #4 glow plug.
- Set up your DVOM: Set your DVOM to measure resistance (Ohms – Ω).
- Measure glow plug resistance: Place the red lead of your DVOM on the glow plug terminal (the metal tip where the connector attaches) and the black lead on a good ground point on the engine block (unpainted metal surface).
- Compare to specification: The resistance should typically be very low, usually between 0.5 to 2.0 Ohms. However, always verify the exact resistance specification for your specific vehicle model in the factory service manual. An excessively high resistance (or infinite resistance – open circuit) indicates a faulty glow plug that needs replacement. A reading outside the specified range suggests a faulty glow plug.
Step 3: Circuit Continuity and Voltage Test
-
Check for power at the glow plug connector (with ignition ON):
- Caution: Be careful when working with electrical circuits.
- With the glow plug connector still disconnected, set your DVOM to measure DC voltage.
- Turn the ignition key to the “ON” position (engine OFF).
- Place the red lead of your DVOM into the power supply side of the glow plug connector (you might need a wiring diagram to identify the correct pin) and the black lead to a good ground.
- You should see battery voltage (approximately 12V) for a brief period when the glow plug system is activated (usually for a few seconds after turning the key ON, especially if the engine is cold). If there’s no voltage, it indicates a problem upstream in the circuit, such as the glow plug relay, control module, or wiring.
-
Check continuity of the ground path:
- Turn the ignition OFF.
- Set your DVOM to measure continuity (or resistance).
- Place one lead on the ground side of the glow plug connector (again, refer to wiring diagram) and the other lead on a known good ground.
- You should have continuity (very low resistance). If there’s no continuity (infinite resistance), there’s a break in the ground path.
Step 4: Wiring Inspection
-
Inspect wiring for damage: Carefully examine the entire length of the wiring for the cylinder #4 glow plug circuit, from the glow plug connector back to the glow plug relay/control module. Look for:
- Chafing or rubbing: Wires rubbing against metal parts can wear through the insulation, causing shorts.
- Cuts or breaks: Physical damage can sever wires.
- Corrosion: Moisture and road salt can corrode wires and connectors, especially in exposed areas.
- Loose or damaged connectors: Ensure all connectors are properly seated and in good condition.
-
Test wire continuity: If you suspect a wire break, use your DVOM to perform a continuity test on individual wire segments.
Possible Solutions Based on Diagnostic Findings:
- Faulty Glow Plug: If the resistance test in Step 2 indicates a faulty glow plug, replace the cylinder #4 glow plug. Ensure you use the correct type of glow plug for your vehicle.
- Wiring Issue: If you find damaged wiring, repair or replace the affected section of the wiring harness. Pay close attention to proper wire splicing and insulation techniques to ensure a reliable repair.
- Connector Problem: Clean corroded connectors or replace damaged connectors. Ensure connectors are securely plugged in.
- Glow Plug Relay or Control Module: If voltage tests in Step 3 indicate a lack of power supply and wiring and glow plugs are good, suspect a faulty glow plug relay or control module. Testing these components may require more advanced diagnostic procedures and potentially a scan tool that can command the glow plug system. Consult a repair manual or seek professional help for these components.
After Repairs:
- Clear DTCs: After completing any repairs, use your OBD-II scanner to clear the P0674 code and any other related codes.
- Test Drive: Perform a test drive to verify that the symptoms are resolved and that the check engine light does not reappear.
- Re-scan for Codes: After the test drive, re-scan the vehicle’s computer system to ensure no codes have returned.
By following these diagnostic steps, you should be able to effectively troubleshoot and resolve the P0674 DTC, ensuring the proper functioning of your diesel engine’s glow plug system and reliable cold starts. Remember to always prioritize safety and consult professional resources when needed.
