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P codepowertrainMedium severity

P0671Cylinder 1 Glow Plug Circuit

A fault has been detected in the cylinder 1 glow plug circuit on a diesel engine, preventing that plug from heating properly.

Definition

Diesel engines rely on glow plugs to heat the combustion chamber for cold starting, and the glow plug control module monitors current and voltage on each plug's circuit. P0671 sets when the cylinder 1 circuit shows an open, a short, or out-of-range current draw, meaning the plug is not heating as commanded. With one plug down, cold-weather symptoms include extended cranking, a stumble or dead miss on cylinder 1 until the engine warms, and white smoke from unburned fuel; warm restarts are usually unaffected. The MIL or glow plug lamp illuminates, and engines that use glow plugs during DPF regeneration may log reduced emissions performance.

Common causes

  • Burned-out (open) cylinder 1 glow plug — the most common failure
  • Corroded or loose terminal connection at the glow plug
  • Damaged wiring between the glow plug control module and cylinder 1
  • Carbon buildup insulating the plug tip and raising circuit resistance
  • Faulty glow plug control module output channel

Common misdiagnoses

  • Replacing the glow plug control module when a $20 plug is open
  • Replacing all glow plugs when only one circuit is faulted (though matched replacement is sometimes reasonable on high-mileage sets)
  • Replacing batteries or the starter for slow cold starts
  • Replacing the cylinder 1 injector to chase a cold miss

Troubleshooting steps

  1. 1.Measure the cylinder 1 glow plug's resistance

    Disconnect the lead and measure from terminal to engine ground; expect approximately 0.5-2 ohms (up to ~6 ohms on some designs), with OL indicating an open element.

  2. 2.Compare readings across all plugs

    Measuring every plug takes minutes and identifies weak siblings; on a high-mileage engine, plugs reading high relative to the others are near end of life.

  3. 3.Inspect the terminal and lead

    Look for a loose terminal nut, oil intrusion in the connector, and chafed insulation along the feed wire.

  4. 4.Verify voltage at the plug during pre-glow

    Command a glow cycle (cold start or scan tool) and confirm the specified supply voltage reaches the cylinder 1 terminal; voltage present with a good plug means a connection problem at the terminal.

  5. 5.Replace the plug carefully

    On engines prone to seized plugs, penetrating oil and gradual back-and-forth removal on a warm engine reduce breakage risk; torque the new plug to spec (typically 8-15 Nm).

  6. 6.Retest the circuit after repair

    Clear the code and run two full pre-glow cycles, confirming the code stays clear and current draw matches the other cylinders if the module reports it.

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