P0674 — Cylinder 4 Glow Plug Circuit/Open #

Note: The label for diagnostic trouble code P0674 can vary depending on the vehicle manufacturer.

• Cylinder 4 Glow Plug Circuit/Open (Generic/OEMs)
• Glow Plug/Heater Circuit (Cylinder 4) Malfunction (Ford, Mercedes-Benz)
• Cylinder 4 Glow Plug Control Circuit/Open (GM, Chevrolet, GMC)
• Glow Plug Circuit Open, Cylinder 4 (Volkswagen, Audi)

Summary of Code P0674 #

Diagnostic trouble code P0674 signals a detected problem in the electrical circuit controlling the glow plug for cylinder 4. A glow plug is a heating device used in diesel engines to assist in cold starting by warming the air in the combustion chamber, making ignition easier when the engine is cold. When the Powertrain Control Module (PCM) or Engine Control Module (ECM) identifies an open circuit—meaning a break or excessive resistance—in the control wiring or the glow plug element itself for cylinder 4, it triggers this code. Depending on your vehicle’s design, this fault may cause starting problems, especially in cold weather, and can increase emissions during the warm-up phase. While the engine may still run, prolonged neglect of this code can lead to harder starts and potential damage to other emissions components.

Common Symptoms for Code P0674 #

A P0674 code usually manifests through starting difficulties, but the severity of symptoms can vary depending on outside temperature and the overall health of the glow plug system. This code may also illuminate the Check Engine Light and, in some cases, trigger a specific glow plug warning indicator.

• Hard starting, especially in cold weather
• Rough idle immediately after cold start
• Excessive white smoke from exhaust on startup
• Check Engine Light illuminated
• Glow plug warning light (on some models)

Probable Causes for Code P0674 #

The most common reasons for a P0674 code relate to issues in the electrical circuit for cylinder 4’s glow plug. These problems can stem from component failure or wiring issues. Understanding the probability order can help prioritize the diagnostic process.

1. Failed glow plug (cylinder 4) — Internal heater element open circuit
2. Faulty glow plug relay or control module — Not supplying voltage to the plug
3. Open or damaged wiring or connectors between the relay/module and glow plug
4. Corroded or loose electrical connectors at the glow plug or relay
5. PCM/ECM failure (rare) — Internal circuit fault

Step-by-Step Diagnosis for P0674 #

Diagnosing a P0674 code involves methodically checking each part of the cylinder 4 glow plug circuit. This ensures you don’t overlook a simple fix before replacing more expensive components.

• Read all related codes: Scan for additional glow plug or powertrain codes that could narrow down the issue.
• Visual inspection: Check the glow plug and its wiring for obvious damage, corrosion, or loose connections.
• Test the glow plug (cylinder 4): Remove and measure its resistance with a digital multimeter. Typical values are between 0.5–2 ohms; infinite resistance means the plug is open (failed).
• Check voltage: With the ignition on, verify that voltage is reaching the glow plug terminal when the relay is activated.
• Inspect glow plug relay or control module: Test for correct operation per OEM procedures.
• Check wiring continuity: Use a multimeter to verify continuity from the relay/control module to the glow plug.
• Test PCM/ECM output: Only if previous steps check out, confirm the control module is commanding the relay/module as required.

Possible Repairs for P0674 #

Repairs are determined by pinpointing the source of the open circuit. Always address the cause rather than just clearing the code to prevent recurrence.

• Replace the faulty glow plug (cylinder 4) if it tests open or out of specification.
• Repair or replace damaged wiring or corroded connectors between the glow plug, relay/module, and PCM/ECM.
• Replace the glow plug relay or glow plug control module if it fails to supply voltage as required.
• Clean or tighten electrical connections at the glow plug and relay.
• Replace PCM/ECM (rare): Only after confirming all other components are functional and wiring is intact.

Can You Drive With Code P0674? #

Limited: You can usually drive with code P0674, especially in warm weather, as the engine may still start and run. However, starting will likely be more difficult in cold conditions, and you may experience rough idle or excess white smoke after startup. Continued driving with an inoperative glow plug increases emissions and can, over time, stress the starter or battery. It’s best to address this code as soon as possible, especially if you expect cold weather.

Vehicles Most Commonly Affected by Code P0674 #

While any modern diesel vehicle using OBD-II/EOBD diagnostics can log code P0674, it’s most frequently reported on popular models with four or more cylinders and individual glow plug monitoring. Below are some vehicles where this code is commonly seen:

• Ford Super Duty Diesel trucks (F-250/F-350 with Power Stroke engines)
• Chevrolet Silverado/GMC Sierra Duramax diesels
• Volkswagen TDI models (Jetta, Passat, Golf)
• Mercedes-Benz Sprinter vans
• Dodge Ram with Cummins diesel (though less common due to different glow plug logic)
• BMW diesel models (X5, 3-Series, etc.)

Common Mistakes & Best Practices with P0674 #

Accurately addressing code P0674 requires careful diagnosis. Skipping steps or assuming the most expensive part is to blame can lead to unnecessary repairs. Here’s what to avoid and best practices to follow:

• Replacing all glow plugs at once: Only replace those that fail testing, unless the set is due by mileage or age.
• Ignoring wiring and connectors: Damaged or corroded wiring is a frequent culprit—always inspect thoroughly.
• Clearing the code without repair: The code will return if the underlying problem isn’t fixed.
• Testing glow plugs while hot: Let the engine cool. Testing hot plugs can yield inaccurate resistance readings.
• Not verifying power supply: If no voltage reaches the plug, check the relay/module before assuming plug failure.

Quick FAQ for Code P0674 #

Here are answers to frequent questions from drivers and technicians about P0674:

Q: Will one bad glow plug keep my diesel from starting?
A: Usually, a single faulty glow plug (such as on cylinder 4) won’t prevent starting in mild weather, but in cold conditions, it can cause hard starting, rough idle, and excess smoke.

Q: Can I use a generic glow plug, or should I use the OEM part?
A: It’s strongly recommended to use OEM (Original Equipment Manufacturer) glow plugs, as they’re designed to the exact electrical and thermal specifications required by your engine. Generic parts may lead to premature failure or incorrect operation.

Q: Should I replace all glow plugs if one fails?
A: Not necessarily. If other glow plugs pass resistance and function tests, you only need to replace the faulty one. However, if several are close to the wear limit or the set is old, replacing all may prevent future issues.

If you have further questions, consult your vehicle’s service manual or a professional technician to ensure a proper diagnosis and repair.