P0674: Cylinder 4 Glow Plug Circuit Malfunction

If your vehicle has triggered the P0674 diagnostic trouble code, it means the engine control module (ECM) has detected a malfunction in the glow plug circuit specifically for cylinder 4. Glow plugs are heating elements in diesel engines that warm the combustion chamber before starting, especially in cold weather. When this circuit fails, your diesel engine may have difficulty starting, particularly in cold conditions, and you’ll likely see the check engine light illuminated on your dashboard.

What Does P0674 Mean?

P0674 is a powertrain diagnostic trouble code that specifically targets the glow plug heating circuit for cylinder 4. The “P” indicates it’s a powertrain code, “0” means it’s a generic OBD-II code (standardized across manufacturers), and “674” specifically identifies the glow plug circuit issue in cylinder 4.

Diesel engines rely on glow plugs to preheat the combustion chamber before ignition. Unlike gasoline engines that use spark plugs for ignition, diesel engines use compression ignition—fuel is injected into hot, compressed air. Glow plugs help achieve the necessary temperature for reliable cold starts. When the ECM detects abnormal voltage or resistance in the cylinder 4 glow plug circuit, it sets this code.

This code can appear in various diesel vehicles, including pickup trucks (Cummins, Duramax, Powerstroke), SUVs, and commercial vehicles. The severity can range from minor to moderate, depending on whether it’s a single glow plug issue or a broader circuit problem affecting engine starting capability.

Common Symptoms

• Check Engine Light: The most obvious indicator—the CEL will illuminate on your dashboard
• Difficult Cold Starts: Engine struggles to start in cold weather or early mornings
• Extended Cranking Time: Starter motor runs longer than normal before engine fires
• Rough Idle After Cold Start: Engine may run rough for several seconds after starting in cold conditions
• White Smoke on Startup: Unburned fuel creates visible white smoke, especially in cold weather
• Reduced Fuel Economy: Engine may run less efficiently due to incomplete combustion
• Hesitation During Acceleration: Particularly noticeable in cold weather when glow plugs aren’t functioning
• No Symptoms in Warm Weather: Vehicle may start and run normally in warmer temperatures, masking the issue

Possible Causes (Ranked by Frequency)

1. Failed Glow Plug (Most Common): The glow plug element in cylinder 4 has burned out or lost continuity. This is the most common cause and typically requires replacement of the individual glow plug.
2. Faulty Glow Plug Control Module: The module that regulates power to the glow plugs may be malfunctioning, affecting the cylinder 4 circuit specifically or all glow plugs.
3. Wiring or Connector Issues: Corroded, damaged, or loose wiring in the glow plug circuit for cylinder 4 can cause intermittent or complete loss of signal. Check connectors at the glow plug and control module.
4. Open or Short Circuit: A break in the circuit wire or an unintended connection to ground can trigger this code. This may be caused by rodent damage, abrasion, or manufacturing defect.
5. Low Battery Voltage: Insufficient battery power can prevent proper glow plug operation. A weak battery or poor ground connection may cause this code, especially in cold weather.
6. Engine Control Module (ECM) Malfunction: Rarely, a faulty ECM may incorrectly report a glow plug circuit problem when the circuit is actually functional. This is typically a last-resort diagnosis.
7. Fuel Quality Issues: Contaminated or poor-quality diesel fuel can affect combustion and trigger glow plug-related codes indirectly.

Diagnostic Steps

Step 1: Verify the Code and Get a Complete Scan

Use an OBD-II scanner to confirm P0674 is present and check for any additional codes. Multiple glow plug codes (P0670-P0677) suggest a control module issue rather than a single glow plug failure. Document all codes for reference.

Step 2: Visual Inspection

Inspect the glow plug wiring harness and connectors for cylinder 4. Look for:

• Corroded or discolored connectors
• Loose or disconnected wires
• Damaged insulation or burnt spots
• Water intrusion or moisture in connectors
• Signs of rodent damage

Clean any corroded connectors with electrical contact cleaner and reconnect firmly.

Step 3: Check Battery Voltage

Use a multimeter to verify battery voltage with the engine off (should be 12.6V or higher) and while cranking (should not drop below 10V). Low voltage can cause glow plug circuit issues. If battery voltage is low, charge or replace the battery and retest.

Step 4: Test the Glow Plug

Remove the glow plug from cylinder 4 (requires removing the valve cover and possibly other components). Using a multimeter set to resistance (ohms), test the glow plug element:

• A functioning glow plug typically reads 0.5-2 ohms
• An open circuit (infinite resistance) indicates a failed glow plug
• Extremely low resistance may indicate an internal short

If the glow plug tests bad, it needs replacement.

Step 5: Test Circuit Voltage

With the glow plug removed and the ignition key in the “On” position (not cranking), measure voltage at the glow plug connector:

• You should see 12V (or close to it) for several seconds during the glow plug preheat cycle
• No voltage indicates a wiring or control module problem
• Intermittent voltage suggests a loose connection

Step 6: Check Wiring Continuity

With the key off, disconnect the glow plug connector and the control module connector. Use a multimeter to test continuity between the glow plug wire and the control module:

• You should read 0 ohms (continuity)
• Infinite resistance indicates a break in the wire

Check for breaks, cuts, or corrosion along the entire wire path.

Step 7: Test the Glow Plug Control Module

If the glow plug tests good and wiring is intact, the control module may be faulty. This typically requires professional diagnostic equipment or replacement and retest.

Step 8: Clear the Code and Retest

After repairs, clear the code using your scanner and perform a cold start test (ideally in cold weather or after the engine has cooled overnight) to confirm the issue is resolved.

Repair Cost Estimates

Glow Plug Replacement: $150–$400

• Single glow plug: $50–$150 (part cost)
• Labor: $100–$250 (varies by vehicle accessibility)
• Some vehicles require valve cover removal, increasing labor time

Glow Plug Control Module Replacement: $300–$800

• Module cost: $150–$500
• Labor: $150–$300
• May require programming/coding at a dealership

Wiring or Connector Repair: $100–$300

• Simple connector cleaning: $50–$100
• Wire replacement: $150–$300

Battery Replacement (if needed): $100–$300

Professional Diagnostic: $80–$150 (if you need expert help to pinpoint the issue)

Dealership Service (all-inclusive): $400–$1,200 depending on the root cause and vehicle make/model

Can I Still Drive?

Severity: Moderate

You can typically continue driving with P0674 active, but with important caveats:

• Cold Weather Driving: Your vehicle will be difficult to start in cold conditions. If you’re in a cold climate, this could leave you stranded.
• Warm Weather Driving: In mild or warm conditions, your diesel engine may start and run normally, though fuel economy may suffer slightly.
• Safety Consideration: While not an immediate safety hazard like a brake failure, unreliable starting is a significant inconvenience and potential safety issue if you’re stranded in cold weather.
• Engine Damage Risk: Continued operation with a failed glow plug won’t damage the engine, but incomplete combustion may slightly increase emissions and reduce efficiency.
• Recommendation: Repair this issue at your earliest convenience, especially if you live in or frequently travel to cold climates. Don’t ignore it if you experience multiple failed starts.

Frequently Asked Questions

Q: Can I replace just the one glow plug, or do I need to replace all of them?

A: You only need to replace the failed glow plug in cylinder 4. However, if your vehicle has high mileage (over 100,000 miles), consider replacing all glow plugs at once since they wear together. This prevents future codes and ensures consistent cold-start performance. The additional cost is often worth the convenience.

Q: What’s the difference between a glow plug code and a misfire code?

A: Glow plug codes (like P0674) indicate a problem with the heating circuit before combustion. Misfire codes (like P0304 for cylinder 4) indicate incomplete or failed combustion during operation. You could have both codes if a failed glow plug causes poor cold starts and subsequent misfires, but they’re separate issues.

Q: Will a failed glow plug cause my diesel engine not to start at all?

A: Not necessarily. Modern diesel engines can start without functioning glow plugs in warm weather because the compression and friction heat the combustion chamber sufficiently. However, in cold weather (below 40°F), a failed glow plug makes starting difficult or impossible. This is why the code often appears in winter months.

Q: Can I drive with the glow plug warning light on?

A: Yes, you can drive, but you’ll experience difficult cold starts. In warm weather, you may not notice any symptoms. However, don’t ignore the code—have it diagnosed and repaired to ensure reliable starting, especially as temperatures drop. Continuing to drive with a failed glow plug won’t cause engine damage but will be frustrating.

Q: Is P0674 the same as other glow plug codes like P0670 or P0675?

A: No. P0674 specifically targets cylinder 4’s glow plug circuit. Other codes identify different cylinders: P0670 (general glow plug circuit), P0671 (cylinder 1), P0672 (cylinder 2), P0673 (cylinder 3), P0675 (cylinder 5), etc. If you see multiple codes, it suggests a control module or power supply issue rather than individual glow plug failures.