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P codepowertrainMedium severity

P06BCCylinder 4 Glow Plug Circuit Range/Performance

The glow plug circuit for cylinder 4 is drawing current outside the expected range, usually because that glow plug is degraded or its circuit has excess resistance.

Definition

The glow plug control module on diesel engines monitors the current through each glow plug individually during pre-glow and after-glow operation. P06BC sets when the cylinder 4 circuit's current or calculated resistance is outside the expected window, indicating a worn or open plug, a high-resistance connection, or an incorrect plug. Drivers notice hard cold starts, cold misfire or rough running until warm, and white smoke at startup, along with a glow plug lamp or MIL; the stored code can also fail an emissions inspection even when warm-weather symptoms are absent.

Common causes

  • Failed or worn cylinder 4 glow plug (most common)
  • Corroded or loose terminal at the glow plug or its connector
  • Damaged wiring between the glow plug control module and the plug
  • Wrong glow plug part number with incorrect resistance rating
  • Glow plug control module output channel fault

Common misdiagnoses

  • Glow plug control module replaced when one plug is out of spec
  • Battery replaced for the slow cold-start complaint
  • Fuel system serviced for cold smoke caused by the inoperative glow plug

Troubleshooting steps

  1. 1.Measure cylinder 4 glow plug resistance

    Disconnect the plug and measure resistance to ground — expect roughly 0.5–2 ohms on most modern plugs, and compare against the other cylinders to spot the outlier.

  2. 2.Check circuit current during an active glow cycle

    Use an amp clamp on the cylinder 4 feed and compare current to a known-good cylinder; low or absent current confirms the fault in this circuit.

  3. 3.Inspect the connector and harness

    Check for corrosion, heat damage, and chafing between the module and the plug — added resistance shifts the reading out of the module's window.

  4. 4.Confirm the correct plug is installed

    Verify the part number matches the specified rating, especially after prior glow plug service.

  5. 5.Replace the plug and verify

    Install the correct plug carefully to its torque spec (glow plugs snap easily — typically only around 8–15 Nm), then rerun a glow cycle and confirm current matches the other cylinders.

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