P0674 – Cylinder 4 Glow Plug Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Cylinder 4

Definition source: SAE J2012/J2012DA (industry standard)

P0674 is a powertrain diagnostic trouble code indicating a fault in the cylinder 4 glow plug circuit. In diesel applications, the glow plug system preheats combustion chambers to support reliable cold starts and stable combustion during warm-up. When the control module detects an electrical problem affecting the cylinder 4 glow plug circuit, it can store P0674 and may illuminate the malfunction indicator lamp. The exact enabling conditions, monitoring strategy, and how quickly the code sets can vary by vehicle, so always confirm the circuit layout, connector locations, and test specifications using the correct service information before making repairs.

What Does P0674 Mean?

P0674 means the control module has detected a fault in the Cylinder 4 Glow Plug Circuit. The definition is circuit-focused: it indicates an electrical integrity problem in the wiring path and components used to command and power the glow plug for cylinder 4, rather than confirming a mechanical engine issue. While SAE J2012 defines how DTCs are structured and categorized, the actionable takeaway for P0674 is to diagnose the cylinder 4 glow plug electrical circuit end-to-end (power feed, control/driver path, connectors, and the glow plug element) using vehicle-specific service information and test procedures.

Quick Reference

• Subsystem: Cylinder 4 glow plug circuit (glow plug, connector, harness, and control/driver path).
• Common triggers: Open or high resistance in the cylinder 4 glow plug circuit, poor terminal contact, damaged wiring, failed glow plug element, or an issue in the control output/driver.
• Likely root-cause buckets: Wiring/connector faults, glow plug fault, power/ground distribution issue (varies by vehicle), or control module/driver issue.
• Severity: Usually moderate; may cause hard starting and rough running when cold, typically improves when warm.
• First checks: Confirm cylinder numbering, inspect the cylinder 4 glow plug connector and harness routing, check for corrosion/loose pins, and verify the glow plug circuit is not open.
• Common mistakes: Replacing the glow plug without checking the connector/harness, misidentifying cylinder 4, or ignoring a shared power feed/ground problem that affects multiple plugs.

Theory of Operation

The glow plug system uses an electronic control strategy to heat the combustion chamber area before and during initial engine operation, especially in cold conditions. Depending on vehicle design, a control module may command a glow plug control unit/relay or an integrated driver stage to supply current to each glow plug, sometimes with individual control per cylinder. The glow plug itself is a heating element; when energized, it draws significant current and quickly heats.

The module typically monitors the circuit’s electrical behavior to determine whether the commanded glow plug operation matches expectations. If the cylinder 4 circuit appears electrically abnormal (such as an open circuit, excessive resistance, or an output that does not respond as expected), it may flag a fault and store P0674. Specific monitoring methods and logic vary by vehicle, so test steps should follow service information.

Symptoms

• Hard start: Extended cranking or delayed start, most noticeable in cold ambient temperatures.
• Rough idle: Shaking or uneven idle during the first moments after starting.
• Cold misfire: Intermittent stumble or uneven combustion until initial warm-up improves conditions.
• White smoke: Increased light-colored exhaust during cold start due to incomplete combustion (varies by vehicle and conditions).
• MIL on: Malfunction indicator lamp illuminated, sometimes with additional glow plug-related codes.
• Reduced smoothness: Noticeable hesitation or poor throttle response during warm-up that clears as temperature rises.

Common Causes

• Open circuit, damaged wiring, or broken conductor in the cylinder 4 glow plug control circuit
• Loose, corroded, contaminated, or poorly seated connector/pin fit at the glow plug, harness junction, or controller/module connection
• High resistance in the cylinder 4 glow plug circuit due to corrosion, partial breaks, or overheated terminals
• Faulty cylinder 4 glow plug (internal open or resistance out of specification)
• Power feed or ground issue affecting glow plug operation (shared supply/ground points vary by vehicle)
• Glow plug control module/driver fault (channel for cylinder 4 not switching as commanded)
• Harness damage from heat, abrasion, vibration, or improper routing near hot or moving components
• Aftermarket wiring repairs, splices, or incorrect connector replacements introducing resistance or intermittents

Diagnosis Steps

Tools you’ll typically need include a scan tool capable of viewing data and running output controls (if supported), a digital multimeter, and basic back-probing tools. A wiring diagram and connector end views from service information are important because the glow plug circuit design varies by vehicle. For intermittent faults, plan to log scan data and perform wiggle testing safely with the engine off and key on as required.

1. Confirm the DTC and capture context: Verify P0674 is present. Record freeze-frame data and note any related glow plug or powertrain codes. Clear codes and see if P0674 resets immediately (key-on) or only after a cold start attempt, since the monitor may run under specific conditions that vary by vehicle.
2. Identify cylinder 4 and the correct circuit path: Use service information to confirm cylinder numbering and locate the cylinder 4 glow plug, its connector, and the control module/driver (integrated or separate, varies by vehicle). Confirm which pins/wires correspond specifically to cylinder 4.
3. Perform a focused visual inspection: Inspect the cylinder 4 glow plug connector and nearby harness routing. Look for heat damage, chafing, oil contamination, broken locks, pushed-out terminals, corrosion, or evidence of prior repairs/splices. Correct obvious wiring/connector issues before deeper testing.
4. Check for an output test (if available): With the scan tool, look for a glow plug output control or functional test. Command the glow plug system on (as supported) and observe whether cylinder 4 shows different behavior than other cylinders (status, commanded state, or feedback, depending on what data is available). If no output test is supported, proceed with circuit checks per service information.
5. Verify power feed and ground integrity to the control system: Using wiring diagrams, confirm the module/controller has proper power and ground under load. Perform voltage-drop testing on the main power and ground paths while the system is commanded on (or during the time the system would normally energize). Excessive drop indicates a supply/ground problem that can cause a circuit fault to be detected.
6. Test the cylinder 4 glow plug element: Disconnect the glow plug connector (as applicable) and measure glow plug resistance; compare to service information and to other cylinders as a sanity check. An open or out-of-spec glow plug can trigger a circuit fault. If resistance appears normal, continue—wiring or driver faults can still exist.
7. Check for opens and high resistance in the cylinder 4 control circuit: With ignition off and appropriate modules powered down as directed by service info, perform continuity checks between the controller/driver output and the cylinder 4 glow plug connector. Then perform a loaded check (preferred) or a voltage-drop check of the same path when commanded on. High resistance or an open indicates harness/terminal issues.
8. Check for shorts to power/ground where applicable: With connectors unplugged and following service information precautions, test the cylinder 4 circuit for unintended continuity to ground or to power. Pay special attention to areas where the harness contacts brackets, engine components, or heat shields. Repair any insulation damage or pinched wiring.
9. Compare cylinder-to-cylinder behavior: If the system provides per-cylinder feedback or current sense (varies by vehicle), compare cylinder 4 readings to the other cylinders under the same conditions. A clear outlier supports focusing on the cylinder 4 plug, connector, or driver channel rather than global power/ground.
10. Wiggle test for intermittents: While monitoring relevant scan data and/or meter readings, gently manipulate the cylinder 4 connector, harness segments, and module connector. Look for dropouts, sudden changes, or fault re-occurrence. If the code is intermittent, this step often reveals poor pin fit, broken strands inside insulation, or marginal splices.
11. Evaluate the driver/module only after circuit integrity is proven: If the glow plug and circuit test good (no opens/high resistance, no shorts, solid power/ground), and the controller is not driving cylinder 4 as commanded while other cylinders operate normally, suspect an internal driver fault or control issue. Follow service information for any module pinpoint tests and required re-learn/programming steps (varies by vehicle).

Professional tip: When diagnosing a circuit fault, prioritize loaded testing over unloaded continuity checks. A circuit can show “good continuity” yet fail under current due to corrosion, poor terminal tension, or a partially broken conductor. Use voltage-drop testing during a commanded glow event (or the closest available equivalent) and compare cylinder 4 to a known-good cylinder to quickly isolate resistance and connection issues.

Possible Fixes & Repair Costs

Repair costs for P0674 vary widely because the correct fix depends on what testing proves: the cylinder 4 glow plug itself, the wiring/connector integrity, the power/ground feed, or the control side. Labor time also varies by vehicle access and corrosion.

• Repair or replace damaged wiring to the cylinder 4 glow plug circuit (chafing, heat damage, broken conductors)
• Clean, tighten, or replace corroded/loose connectors or terminals (including poor pin fit) at the glow plug, harness, or control unit
• Replace the cylinder 4 glow plug only after confirming it fails the specified electrical checks
• Restore missing power feed or ground for the glow plug control circuit (verify with proper load testing)
• Replace a faulty glow plug control module/relay only after confirming command and output behavior are incorrect and wiring is verified
• Correct routing and add protective loom/retainers where the harness repeatedly rubs or overheats

Can I Still Drive With P0674?

You can often drive with P0674, but expect harder starting and rougher operation when the engine is cold because the cylinder 4 glow plug circuit may not be heating as intended. If you experience a no-start condition, severe misfiring, heavy smoke, or any warning that affects braking/steering, do not drive; diagnose the electrical fault first. Even when it runs normally warm, address the code promptly to avoid repeated extended cranking and related wear.

What Happens If You Ignore P0674?

Ignoring P0674 can lead to progressively worse cold-start performance, longer crank times, rough idle immediately after startup, and increased smoke during warm-up. Repeated hard starts can stress the battery and starter, and ongoing electrical resistance or poor connections can worsen over time, making the circuit fault harder to resolve later.

Last updated: March 7, 2026

Vehicles Commonly Affected by P0674

• Vehicles equipped with diesel engines that use individual cylinder glow plugs
• Applications with a separate glow plug control module or relay controlling multiple glow plugs
• Engines where the glow plug harness runs near high-heat components, increasing insulation and terminal degradation risk
• Vehicles operated in cold climates with frequent cold starts that heavily use the glow plug system
• Higher-mileage vehicles where connector tension, pin fit, and ground integrity may degrade
• Vehicles exposed to corrosion (road salt, moisture) affecting terminals and harness splices
• Vehicles that have had recent engine work where glow plug connectors may be disturbed or mis-seated
• Applications with tight packaging where cylinder 4 access is limited and harness routing is more prone to abrasion

For an accurate repair plan, confirm whether P0674 is current or history, then use service information to identify the exact cylinder 4 glow plug circuit routing and test points before replacing any components.