P0671 — Cylinder 1 Glow Plug Circuit/Open #

Note: The exact wording of this code’s definition may differ depending on the vehicle’s manufacturer.

• Cylinder 1 Glow Plug Circuit/Open (Generic OBD-II)
• Cylinder 1 Glow Plug Control Circuit (Ford, GM, Volkswagen, Mercedes-Benz, etc.)
• Glow Plug/Heater Circuit “A” (Cylinder 1) (Toyota, Nissan)
• Glow Plug 1 Circuit Malfunction (Renault, PSA, Fiat)

Summary of code P0671 #

The P0671 diagnostic trouble code (DTC) indicates that the engine control module (ECM) has detected an open or fault in the glow plug circuit for cylinder 1. Glow plugs are electric heating elements used in diesel engines to preheat the air in the combustion chamber, improving cold starting and reducing emissions. When the ECM identifies a fault (such as an open circuit, excessive resistance, or poor continuity) in the glow plug or its wiring, it sets the P0671 code and usually illuminates the check engine light (CEL). While the engine may still run, starting and performance during cold weather can be affected. Prompt diagnosis is important to prevent further issues or inefficient operation.

Common symptoms for code P0671 #

Drivers experiencing the P0671 DTC may notice a range of symptoms, especially in colder conditions. The most obvious is difficulty starting the engine when temperatures are low, but other, sometimes subtle, issues can also arise. Here’s what to watch for:

• Hard starting in cold weather
• Longer engine cranking time before ignition
• Rough idle immediately after starting
• Increased white smoke from the exhaust on startup
• Check engine light (CEL) illuminated
• Engine runs roughly or misfires until warmed up

Likely causes of P0671 (by frequency) #

Several issues can trigger the P0671 code, but some are more common than others. The fault usually relates to the glow plug itself or its electrical circuit. Here are the most frequently encountered root causes, in order of likelihood:

1. Faulty glow plug (burnt out or open circuit)
2. Damaged or corroded glow plug wiring or connector
3. Failed glow plug relay or control module
4. Poor ground connection at the cylinder head or engine block
5. ECM (engine control module) failure (rare)

Step-by-step diagnosis for P0671 #

Diagnosing code P0671 requires a logical, stepwise approach to accurately identify the cause and avoid unnecessary part replacements. Here’s a proven method to follow:

• Verify the presence of code P0671 with an OBD-II scanner and clear any stored codes.
• Inspect glow plug #1 wiring and connector for corrosion, loose terminals, or damage.
• Use a digital multimeter to measure resistance across glow plug #1; compare with OEM specifications (typically 0.5–2 ohms when cold).
• If resistance is high or infinite, replace the glow plug. If within spec, proceed.
• Check for voltage at glow plug #1 connector during pre-glow cycle (key ON, engine OFF, when cold).
• If no voltage is present, test the glow plug relay or control module for function and output.
• Verify ground continuity from the connector to the engine block.
• If all checks pass, suspect a possible ECM fault (rare); consult OEM-specific wiring diagrams and service bulletins.

Possible repairs for P0671 #

Repairs should always be based on accurate diagnosis. Addressing the root cause will clear the code and restore optimal engine performance. Typical repairs for P0671 include:

• Replace faulty glow plug #1
• Repair or replace damaged glow plug wiring or connectors
• Replace failed glow plug relay or control module
• Clean and secure ground connections at the engine block or cylinder head
• In rare cases, repair or reprogram the ECM as per OEM guidance

Is it safe to drive with code P0671? #

Limited: While driving with code P0671 may not immediately damage your engine, it can lead to hard starting, rough running, and increased emissions—especially in cold weather. Prolonged operation with a non-functioning glow plug can also cause excessive unburned fuel and carbon buildup. For best reliability and efficiency, address the issue as soon as possible.

Vehicles most commonly affected by P0671 #

This code is most frequently encountered on diesel-powered vehicles from various manufacturers, especially those using individual glow plug monitoring. Here are some of the most commonly affected brands and models:

• Ford Super Duty F-Series (Powerstroke diesel engines)
• Chevrolet Silverado / GMC Sierra (Duramax diesel engines)
• Volkswagen Golf, Jetta, Passat (TDI diesel engines)
• Mercedes-Benz Sprinter and passenger vehicles (CDI diesel engines)
• Renault, Peugeot, Citroën (HDI diesel engines)
• Fiat Ducato, Doblo, Scudo (Multijet diesel engines)
• Toyota Hilux, Land Cruiser (D-4D diesel engines)

Common mistakes & best practices with P0671 #

It’s easy to jump to conclusions when confronted with a glow plug code, but some pitfalls can lead to wasted time or unnecessary parts replacement. Here’s what to avoid and what to do instead:

• Mistake: Replacing all glow plugs without testing — Always test resistance first.
• Mistake: Ignoring wiring or connector corrosion — Thoroughly inspect and clean all electrical connections.
• Mistake: Overlooking the relay or control module — Test all components in the circuit, not just the glow plug.
• Best Practice: Use a digital multimeter for accurate resistance and voltage checks.
• Best Practice: Consult OEM repair manuals and wiring diagrams for your specific vehicle.
• Best Practice: Clear codes and verify repair with a cold start after corrective action.

Express FAQ for P0671 #

Q: Can I ignore the P0671 code if my diesel starts fine in warm weather?
A: Even if starting seems normal now, the problem can get worse—especially as temperatures drop. It’s best to repair the issue to avoid future cold start failures and increased emissions.

Q: Should I replace all glow plugs if one fails?
A: Not necessarily. Only replace the faulty plug(s) unless others show signs of wear or high resistance. However, if the engine has high mileage, a full set may be cost-effective in the long run.