P0680 Code Symptoms, Causes, Diagnosis & Repair Step-by-Step Guide

P0680 Code – Symptoms, Causes, and How to Fix It

Drivers first become aware of a P0680 fault when the engine refuses to start or cranks far longer than normal. The check‑engine light flashes, and once the engine finally fires, it may idle unevenly for a few seconds before smoothing out. In some cases the vehicle will lose a noticeable amount of torque on acceleration, especially when the engine is cold. These signs appear because the diesel‑engine control strategy relies on a timed glow‑plug pulse for cylinder 10; an open circuit prevents the plug from heating, delaying combustion and forcing the PCM to compensate with richer fuel and higher idle speed.

Symptoms

• Hard start or extended cranking – the starter must turn the engine longer before ignition occurs.
• Rough idle on initial start‑up – the engine may shake or stall for a few seconds, then settle.
• Reduced low‑end power – acceleration feels sluggish until the PCM adjusts timing.
• Check‑engine illumination – a flashing or steady MIL appears, often accompanied by a stored P0680 code.
• Intermittent loss of power – the fault may clear after a warm‑up, then return when the engine cools down.

Why Cylinder 10 Glow‑Plug Circuit Problems Occur

Open or Corroded Wiring Harness

The glow‑plug circuit runs from the PCM to the plug through a high‑temperature wire bundle. Heat‑induced insulation breakdown, corrosion at connector pins, or a broken splice creates an open circuit that the PCM flags as P0680.

Faulty Cylinder 10 Glow Plug

A cracked ceramic element or burned electrode inside the plug eliminates the heating current. The PCM still commands voltage, but no resistance is measured, producing the “circuit/open” condition.

PCM Output Driver Failure

Inside the PCM, a transistor or driver circuit supplies the high‑current pulse to the glow plug. If this component burns out, the PCM cannot deliver voltage, and the diagnostic routine records an open circuit.

Software or Calibration Issue

Incorrect mapping of glow‑plug timing or a corrupted calibration table can cause the PCM to misinterpret a healthy circuit as open. Re‑programming the module often resolves this subtle fault.

Secondary Possibilities (Redirected)

While a blown fuse or a malfunctioning relay could interrupt power, the primary fault usually resides in the PCM‑controlled circuit. A diagnostic scan will confirm whether the issue is truly a wiring/open‑circuit condition or a broader control‑module communication problem.

Diagnostic and Repair Procedures

1. Retrieve and Confirm the Code

Connect a professional OBD‑II scanner, read the stored codes, and verify that P0680 is present without conflicting codes that point to other systems.

1. Inspect Visual Wiring Condition

Locate the glow‑plug harness near the engine block. Look for cracked insulation, melted sections, or water intrusion. Clean corroded pins with a contact‑cleaner brush.

1. Perform Continuity Test

With the ignition off, disconnect the cylinder 10 glow‑plug connector. Use a multimeter to check continuity from the PCM pin to the plug terminal. An open reading confirms a wiring break.

1. Measure Voltage at the Plug

Re‑connect the plug, crank the engine, and measure voltage at the plug terminal. The PCM should supply 8–12 V for a brief pulse. Zero or erratic voltage indicates a PCM driver fault.

1. Swap the Glow Plug (If Wiring Is Good)

Replace the cylinder 10 plug with a known good unit. Re‑test voltage and observe if the fault clears. If the code persists, the PCM is the likely source.

1. PCM Communication Test

Using the scan tool, command a live data stream for the glow‑plug control module. A lack of response or abnormal data points to a communication failure within the PCM.

1. Re‑program or Update PCM Software

If the hardware checks out, apply the latest PCM calibration file from the manufacturer. Re‑flash the module and clear the code. Verify that the fault does not return after a drive cycle.

1. Replace the PCM/ECU When Repair Is Not Viable

When the output driver is damaged or internal board corrosion is evident, replacement is the most reliable solution. A new PCM must be programmed to the vehicle’s VIN and calibrated for the engine’s glow‑plug strategy.

Cost Estimates

• Wiring repair or connector cleaning: $50‑$120 labor.
• Glow‑plug replacement (diesel plug only): $30‑$80 part, $40‑$70 labor.
• PCM re‑programming: $120‑$180 labor, plus possible software access fees.
• PCM replacement (including programming): $600‑$900 for the module, $200‑$300 labor.

When Replacement Makes More Sense Than Repair

If continuity testing reveals a broken internal driver, or if repeated repairs fail to clear the P0680 code, installing a new PCM is the most dependable path forward. Modern control modules are tightly integrated with security, immobilizer, and emission‑control networks; a faulty board can cause intermittent failures that are costly to troubleshoot repeatedly.

Flagship One specializes in VIN‑matched control modules, delivering a plug‑and‑drive solution that includes factory‑level programming and a comprehensive warranty. Replacement PCM units vary depending on production date and software version, so the correct module is matched by VIN before programming. This approach eliminates dealer‑only re‑coding delays and ensures that your vehicle’s control strategy is restored to OEM specifications.

Preventive Maintenance

• Regularly Inspect Glow‑Plug Wiring – During routine service, check the harness for signs of wear, especially after high‑temperature operation or exposure to road salt.
• Replace Glow Plugs at Manufacturer Intervals – Diesel engines often require glow‑plug replacement every 80,000–100,000 mi to avoid premature failure.
• Maintain Clean Connectors – Use dielectric grease on plug terminals to prevent moisture ingress, which can cause corrosion and open circuits.
• Stay Current with PCM Software Updates – Manufacturers release calibration patches that address known glow‑plug timing issues; applying these updates reduces the likelihood of false‑open detections.
• Monitor for Unusual Cranking Times – Early detection of extended start periods can prompt a quick scan before the fault escalates to a full PCM failure.