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P0677Cylinder 7 Glow Plug Circuit

An electrical fault has been detected in the glow plug circuit for cylinder 7 on a diesel engine.

Definition

On diesel engines with individually monitored glow plugs, the glow plug control module measures voltage and current on each plug circuit during the pre-glow and post-glow cycles. P0677 sets when the cylinder 7 circuit shows an open, a short, or a current draw outside the expected range. Symptoms include hard starting and white smoke in cold weather, rough running for the first minute after a cold start, and an illuminated MIL, while warm-weather starting is often unaffected. Prolonged cold-start misfire raises emissions and can dilute the engine oil with unburned fuel.

Common causes

  • Failed cylinder 7 glow plug (open heating element)
  • Corroded, loose, or burned glow plug connector terminal
  • Wiring harness damage (open or short) between the glow plug control module and cylinder 7
  • Failed output driver in the glow plug control module
  • Poor engine ground causing excessive circuit resistance
  • Incorrect replacement glow plug with the wrong resistance rating

Common misdiagnoses

  • Replacing the full set of glow plugs when only the cylinder 7 circuit is at fault
  • Replacing the glow plug relay/control module when the plug itself is open
  • Battery or charging system repairs for what is a single-circuit fault
  • PCM replacement

Troubleshooting steps

  1. 1.Read all stored codes

    Multiple glow plug circuit codes point to the control module or its power feed, while a single cylinder 7 code points to that plug or its wiring.

  2. 2.Measure glow plug resistance

    Disconnect the cylinder 7 plug terminal and measure to ground; most plugs read roughly 0.5-6 ohms, and an open (OL) reading confirms a failed plug.

  3. 3.Inspect the connector and harness

    Look for burned or spread terminals and chafed wiring where the harness runs near the valve cover or exhaust heat.

  4. 4.Verify supply during the glow cycle

    Command the glow plugs on with a scan tool and confirm battery voltage (or the PWM signal on ceramic-plug systems) at the cylinder 7 terminal.

  5. 5.Test the module output

    If the plug and wiring test good but only cylinder 7 has no output, the glow plug control module's driver has failed.

  6. 6.Clear the code and retest after a cold soak

    The monitor typically runs only during pre-glow on a cold engine, so verify the repair on a cold start.

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