P0671 — Cylinder 1 Glow Plug Circuit/Open

Overview of the code

The diagnostic trouble code P0671 indicates an issue specifically related to the glow plug circuit for cylinder 1. Essentially, this code signals that the vehicle’s engine control unit (ECU) has detected a problem in the electrical circuit that warms up the glow plug in cylinder 1, which is essential during cold starts, especially in diesel engines.

Please note that labels, numbering, and descriptions related to this code can vary between different vehicle manufacturers. Always refer to the specific OEM (Original Equipment Manufacturer) documentation for precise diagnostics.

Fast Facts:

• System affected: Glow plug circuit (diesel engines)
• Severity: Moderate — may affect cold start performance
• Main symptoms: Rough starting, engine hesitation during cold start, possible check engine light (MIL) illumination
• Driveability: Limited — vehicle may start but perform poorly in cold conditions

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Manufacturer variations for trouble code P0671

Different vehicle manufacturers often assign different generic or specific labels to the same fault, depending on their diagnostic conventions. The code P0671 may be referenced using various labels in manufacturer-specific fault codes.

For example, some known variations include:

• Volkswagen / Audi: P0671 – Cylinder 1 Glow Plug Circuit/Open
• Ford: P0671 – Glow Plug Relay Circuit
• Mercedes-Benz: P0671 – Cylinder 1 Glow Plug Circuit Problem
• Peugeot / Citroën: P0671 – Pre-heating Indicator Circuit / Glow Plug Circuit Open for Cylinder 1

Always verify with the specific vehicle repair manual or diagnostic tool to confirm the exact label.

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What does trouble code P0671 mean?

This code points to a fault in the electrical circuit that supplies power to the glow plug in cylinder 1 of a diesel engine. The ECU constantly monitors the glow plug circuit for electrical continuity and proper voltage.

When it detects an open circuit (break in wiring or connection) or short to ground or voltage, it triggers this DTC. Typically, this fault occurs either during engine cold start, when glow plugs are actively used, or when there’s an electrical malfunction.

Common triggers include:
– A blown glow plug
– A malfunction in the relay controlling glow plugs
– Damaged wiring or poor connector contact
– Faulty ECU output or sensor signals

The fault often appears during cold weather or after multiple start attempts, as the ECU detects inconsistent or missing glow plug activation.

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Severity and risks of trouble code P0671

The presence of P0671 generally indicates a moderate issue that primarily affects engine starting and cold operation. If ignored, the problem could lead to:
– Difficulties during cold starts; engine may hesitate or struggle to ignite
– Increased emissions due to incomplete combustion at startup
– Possible damage to glow plug components if left unresolved
– Reduced fuel economy, especially in colder conditions

However, because the fault relates to a specific glow plug circuit, it usually does not cause immediate danger or prevent driving altogether.

**Can the vehicle be driven?**
Limited — it’s often possible to drive the vehicle, but starting might be more difficult, and cold weather conditions may exacerbate performance issues.

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Symptoms of trouble code P0671

When this fault is present, you may notice several symptoms related to engine starting and operation:
– **Hard or slow starting in cold weather**
– **Rough or vibrating engine during startup**
– **Longer cranking times**
– **Engine hesitation or stumble upon ignition**
– **Illumination of the check engine light (MIL) on the dashboard**
– Possible warning messages on the vehicle’s display related to engine pre-heating

The severity of symptoms often depends on whether multiple glow plugs are affected or just the one in cylinder 1.

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Most likely causes of trouble code P0671

This fault is typically caused by disruptions in the electrical circuit supplying the glow plug of cylinder 1. Several causes are commonly encountered:

1. Faulty glow plug: The glow plug itself may be burned out or damaged, leading to an open circuit.
2. Relay or control module issue: The relay responsible for switching current to the glow plugs, or the ECU’s output circuit, may malfunction.
3. Damaged wiring or connectors: Corrosion, frayed wires, or poor connections can interrupt the circuit.
4. Blown fuse or fuse link: The fuse protecting the glow plug circuit might be blown, cutting power.
5. ECU malfunction or sensor error: While less common, a fault within the ECU or an incorrect sensor reading can cause false triggers.

Determining the exact cause requires systematic testing, but the first causes are usually related to hardware components.

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How to diagnose trouble code P0671

Diagnostic process should always be aligned with manufacturer recommendations but generally includes:

1. Visual inspection: Check wiring harnesses, connectors, and fuse condition around the glow plug circuit.
2. Live data reading: Use a diagnostic scanner capable of reading live ECU data to verify the glow plug signal during cold engine start.
3. Resistance testing: Disconnect the glow plug and measure its resistance; compare it against specifications. An open circuit indicates a faulty glow plug.
4. Check relay operation: Test or replace the glow plug relay to confirm it’s functioning correctly.
5. Inspect circuit continuity: Use a multimeter to verify wiring continuity from ECU output to the glow plug connector.
6. Check fuse and power supply: Ensure fuses are intact and voltage supply is present at the glow plug circuit.

Always clear trouble codes after repairs and verify if the fault reappears.

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Possible repairs for trouble code P0671

Repairs should target the most probable causes to restore proper glow plug circuit function:
– **Replace the defective glow plug**: Basic repair, usually straightforward but requires access to cylinder 1 glow plug.
– **Repair or replace damaged wiring or connectors**: Can involve soldering, crimping, or connector replacement; a professional task.
– **Replace blown fuse or fuse link**: Simple task; verify the reason for the fuse blow before replacing.
– **Test and replace relay**: If the relay is faulty, swapping it out is a quick fix.
– **ECU diagnosis and replacement**: This is a less common, more advanced repair and should be performed by a professional.

It’s essential to accurately diagnose the exact component at fault to avoid unnecessary replacements.

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Vehicles commonly associated with trouble code P0671 in Europe

This fault code is mostly encountered in diesel vehicles equipped with glow plug systems. Various brands are affected, including:
– Volkswagen Group (Volkswagen, Audi, Seat, Skoda): Engines from the TDI family often show this fault.
– Ford: Certain diesel models with robust glow plug systems.
– Mercedes-Benz: Common in diesel models with cylinder-specific glow plug monitoring.
– Peugeot / Citroën: Diesel engines with electrical glow plug controls.

While this list is not exhaustive, understanding common vehicle groups helps streamline diagnostics.

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Frequent mistakes with trouble code P0671

Avoid common pitfalls that may hinder effective diagnosis:
– Replacing glow plugs without confirming the cause: Not all glow plug circuit faults are due to the plug itself.
– Ignoring wiring and connector conditions: Damage or corrosion can mimic glow plug failure.
– Overlooking the relay and fuse condition: These are inexpensive and straightforward to test and replace.
– Failing to clear codes after repairs: Residual codes may lead to repeated faults or misdiagnosis.
– Assuming the ECU is faulty: ECU-related faults are less common and should be confirmed after hardware checks.

Following systematic diagnostics reduces unnecessary repairs and increases reliability.

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FAQ — trouble code P0671

Can this code disappear on its own?
Typically, no. The fault persists until the underlying cause, such as a blown glow plug or faulty relay, is repaired. Some temporary issues might clear if wiring connection issues are re-established, but persistent faults require specific repairs.

Can I keep driving?
In general, yes, but with limitations. The vehicle might start less easily in cold weather, and starting may be sluggish. Long-term driving without fixing the fault can lead to increased emissions and engine stress, especially during cold starts. It is advisable to address the fault promptly.

Why does the code return after clearing?
This usually indicates an unresolved fault, such as a damaged glow plug, wiring issue, or relay problem. The ECU continuously monitors the glow plug circuit, so if the root cause isn’t repaired, the fault will reappear.