P0682 — Cylinder 12 Glow Plug Circuit/Open #

Note: The definition and wording of DTC P0682 may vary depending on the vehicle manufacturer (OEM).

• P0682: “Cylinder 12 Glow Plug Circuit/Open” (General/SAE)
• P0682: “Cylinder 12 Glow Plug Control Circuit Malfunction” (Ford, GM)
• P0682: “Glow Plug Circuit Open – Cylinder 12” (Chrysler/RAM)
• P0682: “Glow Plug #12 Circuit Fault” (Isuzu)

Summary of DTC P0682 #

Diagnostic Trouble Code (DTC) P0682 indicates that the engine control module (ECM) or powertrain control module (PCM) has detected an open circuit or malfunction within the glow plug system for cylinder #12. Glow plugs are electrical heating elements used in diesel engines to warm up the combustion chamber for easier starting, especially in cold conditions. An “open circuit” means that the electrical flow is interrupted, often by a break in the wiring, a faulty glow plug, or a control module issue. When this code sets, the engine may struggle to start, especially in cold weather, and may exhibit rough idle or excessive smoke at startup. Prompt diagnosis is recommended to prevent further engine or emissions issues.

Frequent Symptoms for Code P0682 #

When DTC P0682 is active, drivers or technicians may notice several symptoms, most commonly related to cold engine starts. These symptoms can vary in severity depending on the number of glow plugs affected and environmental conditions.

• Hard starting, especially in cold weather
• Rough idle immediately after startup
• White or gray exhaust smoke on cold start
• Longer cranking times
• Reduced engine performance at low temperatures
• Check Engine Light illuminated

Probable Causes for Code P0682 #

The most common causes of a P0682 code relate to electrical faults within the glow plug circuit. Understanding the root cause is crucial for effective repairs. Causes are listed below in order of frequency based on typical diagnostic experience:

1. Faulty glow plug in cylinder 12 (burned out or open internally)
2. Damaged or corroded glow plug wiring/connectors
3. Failed glow plug relay or controller
4. Corroded or loose electrical grounds
5. Defective engine control module (ECM/PCM) (less common)

Step-by-Step Diagnostic for Code P0682 #

Diagnosing P0682 requires systematic inspection and basic electrical testing. Follow these steps to accurately pinpoint the fault:

• Verify the code with a scan tool and check for additional related codes.
• Visually inspect the glow plug for cylinder 12 and its wiring harness for signs of damage, corrosion, or disconnection.
• With the engine cool, test the resistance of the cylinder 12 glow plug using a digital multimeter. Compare readings to OEM specifications (usually less than 2 ohms).
• If resistance is out of spec or open, replace the glow plug.
• If resistance is correct, check for voltage at the glow plug terminal when the key is turned to “ON” (with the engine cold).
• If voltage is absent, test the glow plug relay or controller and inspect associated wiring for continuity.
• Check for proper ground connections at the glow plug and relay/controller.
• If all components test good, suspect a possible ECM/PCM issue and confirm with advanced diagnostic tools or OEM service information.

Possible Repairs for Code P0682 #

The appropriate repair for P0682 depends on the identified cause. Here are common solutions linked to the root causes:

• Replace the faulty glow plug in cylinder 12.
• Repair or replace damaged/corroded wiring and connectors in the glow plug circuit.
• Replace the glow plug relay or controller if found defective.
• Clean and tighten all related ground connections.
• In rare cases, reprogram or replace the ECM/PCM after confirming all other components are functional.

Is It Safe to Drive with Code P0682? #

Driving with code P0682 is generally possible, but limited. While a single glow plug failure may not cause immediate engine damage, it can severely affect cold start performance, leading to difficult starting, rough idle, and increased emissions. In extreme cold, the engine may not start at all. Prolonged operation with multiple glow plug faults can contribute to unburned fuel and possible damage to the diesel particulate filter (DPF) or catalytic converter. Prompt repair is advised to avoid further complications.

Vehicles Most Commonly Affected by Code P0682 #

This code is most often found on larger diesel engines, particularly those with 12 cylinders. It is commonly associated with certain heavy-duty trucks and commercial vehicles. Examples include:

• Chevrolet Silverado/GMC Sierra HD (with Duramax V8 or V12 diesels)
• RAM Heavy Duty trucks (Cummins 12-cylinder variants, export models)
• Isuzu NPR/NQR (diesel models with 12-cylinder engines)
• Ford F-Series Super Duty (with certain international 12-cylinder diesels)
• Industrial/agricultural equipment using large OEM diesel engines with glow plugs

Common Mistakes & Best Practices for P0682 #

A few frequent errors can complicate the repair of code P0682. Following best practices ensures a smooth diagnosis and repair process:

• Assuming the glow plug is always the culprit—always test before replacing.
• Overlooking corroded or loose connectors, which are a frequent cause of open circuits.
• Failing to check related wiring for shorts or opens along the harness.
• Neglecting to clear the code and perform a cold start test after repairs.
• Using non-OEM glow plugs with incorrect specifications—always match the OEM part number for best results.

Quick FAQ for P0682 #

Q: Will my engine be damaged if I ignore P0682?
A: One faulty glow plug won’t usually cause direct engine damage, but persistent cold start issues and unburned fuel can harm emission systems over time. Prompt repair is the best practice.

Q: Can I replace a single glow plug, or should I replace them all?
A: It is acceptable to replace only the faulty glow plug, but if several have failed or are the same age, replacing the whole set can prevent future issues and ensure even starting performance.