P0671 Code? Here’s How to Fix It – Quickly and Easily

Among OBD-II OBD2 problem codes, P0671 is a generic SAE definition indicating a Cylinder 1 Glow Plug Circuit fault. It means the control module detected an electrical issue in the circuit for the first cylinder.

Because this code points to a specific circuit condition, exact application verification is required. The code indicates an electrical fault, and testing is necessary to isolate the issue to the glow plug, wiring, or control module.

Key Takeaways

• Condition detected: The PCM identified an electrical fault in the Cylinder 1 glow plug control circuit.
• Electrical checks required: Diagnosis involves measuring the glow plug resistance, inspecting the wiring harness, and verifying module output.
• System impacts: A faulty glow plug circuit can cause hard cold starts, rough idle, and increased emissions in diesel engines.
• Component testing: The wiring, connectors, and Cylinder 1 glow plug must be evaluated to isolate the root cause.

P0671 Code Description and Implications

P0671 is a generic SAE definition for Cylinder 1 Glow Plug Circuit. Diesel engines use glow plugs to heat the combustion chamber, ensuring reliable ignition during cold starts and reducing cold-start emissions.

When the powertrain control module detects that the electrical circuit for the Cylinder 1 glow plug is open, shorted to ground, or shorted to voltage, it stores P0671. This indicates an electrical failure in the circuit rather than a mechanical engine fault.

Common Causes of the P0671 Code

• Failed glow plug: The internal heating element of the Cylinder 1 glow plug may be burned out or open.
• Damaged or chafed wiring: The harness routing to the glow plug may be damaged by engine heat or vibration.
• Corroded or loose connectors: Moisture or carbon buildup at the glow plug connector can cause high resistance or an open circuit.
• Failed glow plug control module: The module or relay responsible for supplying power to the glow plugs may have an internal fault on the Cylinder 1 driver circuit.

Symptoms Associated With P0671

Possible symptoms include:

• Illuminated Check Engine Light (MIL) or glow plug warning light.
• Extended cranking time or hard starting, especially in cold weather.
• Rough idle or misfire immediately after a cold start until the engine warms up.
• Increased white exhaust smoke during cold starts due to unburned fuel.

Diagnosing the P0671 Trouble Code

Diagnosing a glow plug circuit code requires a digital multimeter, a scan tool, and the vehicle’s specific wiring diagrams. The first step is to verify the exact factory definition.

1. Visual inspection: Inspect the wiring harness and connector at the Cylinder 1 glow plug for heat damage, chafing, or corrosion.
2. Measure glow plug resistance: Disconnect the harness and measure the resistance across the Cylinder 1 glow plug terminal and ground. Compare the reading to the manufacturer’s specification.
3. Check circuit continuity: Test the wiring from the control module to the glow plug connector for opens or shorts.
4. Verify module output: If the wiring and glow plug test correctly, use a scan tool or test light to verify the control module is outputting the correct voltage during the pre-heat cycle.

Repairing the P0671 Issue

Repairs must follow confirmed diagnosis. Never replace the powertrain control module based solely on the presence of a P0671 code.

• Glow plug replacement: If the glow plug fails resistance testing, replace the Cylinder 1 glow plug. It is often recommended to replace all glow plugs if they are of similar age.
• Wiring repair: If the harness is chafed or broken, repair the wires using high-temperature solder and heat shrink tubing.
• Connector replacement: If the plug is corroded, replace the pigtail connector.
• Control module replacement: If the module driver circuit fails testing, replace the glow plug control module.

Cost Considerations for P0671 Repairs

Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.

Replacing a single glow plug is generally straightforward, though some engines require intake manifold removal for access to the cylinder head.

Prevention Strategies for Future Occurrences

• Protect wiring harnesses: Ensure the glow plug wiring is securely routed away from hot exhaust components and sharp edges.
• Use correct replacement parts: When replacing glow plugs, use components that meet the exact OEM specifications to prevent premature failure or module damage.

Troubleshooting Tips and Best Practices

• Do not assume the glow plug is dead: A P0671 code frequently points to a corroded connector or a broken wire. Always verify circuit continuity before replacing the glow plug.
• Check for related codes: Look for other glow plug circuit codes or control module codes that might indicate a broader harness issue or a failed control module.
• Check for TSBs: Some manufacturers issue Technical Service Bulletins for updated glow plug designs or revised control module software.

More OBD-II Codes

Frequently Asked Questions

Can a bad ground cause a P0671 code?

Yes. The glow plug relies on the engine block for its ground path. If the engine ground strap is corroded or loose, it can create high resistance, triggering a circuit fault code.

Is it safe to drive with a P0671 code?

You can typically drive the vehicle, but cold starting will be difficult. In warm weather, the engine may start normally. However, extended cranking can drain the battery or damage the starter.

Will replacing the glow plug fix a P0671 code?

It will fix the issue if the internal heating element is burned out. However, if the root cause is a broken wire or corroded connector, replacing the glow plug will not resolve the circuit fault.

Does P0671 mean my PCM needs to be replaced?

No. P0671 indicates an electrical fault in a specific glow plug circuit. The root cause is usually a failed glow plug, damaged wiring, or a bad control module. Proper circuit testing is required first.

Should I replace all glow plugs if Cylinder 1 fails?

While not strictly required to clear the code, replacing all glow plugs simultaneously is often recommended if they have similar mileage, as the remaining plugs are likely nearing the end of their service life.

What Should You Do About the P0671 Code?

Because the P0671 code indicates an electrical fault in the Cylinder 1 glow plug circuit, it requires verifying the wiring, connectors, and glow plug resistance before replacing components. Confirm the exact factory definition for your vehicle.

Inspect the wiring harness for damage, check the connector, and test the relevant system and components. Follow the manufacturer’s diagnostic procedure to isolate the faulty component, repair the confirmed root cause, and verify the repair.