P068D — Cylinder 7 Glow Plug Control Circuit High #

Please note: The code description for P068D may vary depending on the vehicle manufacturer.
Known description variations:

• P068D – Cylinder 7 Glow Plug Control Circuit High (Generic / GM / Ford / FCA)
• P068D – Cylinder 7 Glow Plug Circuit High Input (Mercedes-Benz)
• P068D – Cylinder 7 Glow Plug Relay Circuit High (BMW)

Summary of the P068D Code #

P068D is a Diagnostic Trouble Code (DTC) that indicates the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an unusually high voltage in the glow plug control circuit for cylinder 7. The glow plug is an electrically heated element inside a diesel engine’s cylinder head, designed to aid cold starting by pre-heating the combustion chamber. When “circuit high” is detected, it means that the measured voltage is above the normal or expected range. This issue can result in poor cold starting, increased emissions, or even potential engine misfires. While not immediately catastrophic, ignoring P068D can lead to long-term engine performance issues, especially in cold weather conditions.

Common Symptoms for Code P068D #

If your vehicle triggers the P068D code, you may notice several warning signs. These symptoms are typically most evident during cold starts or in colder climates, given the glow plug’s role in aiding combustion before the engine reaches operating temperature. Common symptoms include:

• Hard or extended engine cranking during cold starts
• Rough idle after starting, especially in low temperatures
• Illuminated Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL)
• Reduced engine performance or hesitation
• Increased exhaust smoke during start-up

Likely Causes of P068D (in Order of Frequency) #

The most probable reasons behind a cylinder 7 glow plug circuit high condition are usually related to electrical faults or component failures. In order of how frequently they’re encountered, likely causes include:

1. Failed glow plug relay or control module (responsible for regulating voltage to the glow plugs)
2. Faulty glow plug in cylinder 7 (open circuit or internal short)
3. Damaged wiring or connectors in the cylinder 7 glow plug circuit (corrosion, loose, or broken wires)
4. Poor electrical ground connection for the glow plug or its circuit
5. Faulty Engine Control Module (ECM/PCM) (rare, usually after other causes are ruled out)

Step-by-Step Diagnostic Guide for P068D #

A methodical approach is essential for accurately diagnosing the root cause of P068D. Here’s a step-by-step process you or your technician should follow:

• 1. Confirm the code: Use an OBD-II/EOBD scan tool to verify the presence of P068D and check for related codes (especially other glow plug circuit DTCs).
• 2. Inspect visual wiring and connectors: Examine the glow plug harness for cylinder 7 for signs of corrosion, damage, or loose connections.
• 3. Test the glow plug: With the ignition off, disconnect the glow plug for cylinder 7. Measure its resistance with a multimeter; compare the reading to OEM specifications.
• 4. Check control relay/module: Test the glow plug relay or control module for output voltage and proper switching function.
• 5. Examine circuit voltage: With the ignition on, back-probe the cylinder 7 glow plug connector to measure voltage. An abnormally high reading confirms the circuit high condition.
• 6. Verify ECM/PCM operation: If all hardware checks out, perform a scan for ECM/PCM software updates or faults. Replace only if other components test OK.
• 7. Clear codes and retest: After repairs, clear codes and perform multiple cold starts to confirm resolution.

Possible Repairs for P068D #

The appropriate repair for a P068D code depends on the root cause identified during diagnosis. Typical fixes include:

• Replace the cylinder 7 glow plug if it has failed or is out of specification
• Repair or replace damaged wiring/connectors in the glow plug circuit
• Replace the glow plug relay or control module if faulty
• Clean and secure ground connections related to the glow plug system
• Update or replace the ECM/PCM (only after all other repairs have been attempted)

Is It Safe to Drive with P068D? #

Driving with a P068D code is generally safe for short distances or in warm weather, but it’s not recommended for prolonged use, especially in cold climates. The main risk is hard starting, rough running, and increased emissions during cold starts. Over time, repeated cold-start misfires can increase wear on engine components and lead to more severe issues. For the health of your engine and to avoid future complications, address this code promptly.

Vehicles Most Commonly Affected by P068D #

While P068D can appear on any modern diesel vehicle equipped with OBD-II diagnostics, it’s most often found in:

• Chevrolet and GMC Duramax diesel trucks (2011+)
• Ford Super Duty diesels (Power Stroke 6.7L, 2011+)
• Ram trucks with Cummins diesel engines (2013+)
• Mercedes-Benz Sprinter (OM642 and OM651 engines)
• BMW diesel models (N57 and B57 engines)

Common Mistakes & Best Practices with P068D #

Misdiagnosing or ignoring the P068D code can lead to unnecessary repairs and additional headaches. Here’s what to watch out for and how to address it smartly:

• Replacing the glow plug without testing the relay or wiring first
• Overlooking corroded or loose connectors—always inspect before replacing parts
• Neglecting to clear codes and verify the repair with multiple cold starts
• Assuming the ECM/PCM is faulty before ruling out all other hardware causes
• Failing to check for OEM technical service bulletins (TSBs) related to glow plug circuit issues

Quick FAQ for P068D #

Q: Will my diesel start if I have a P068D code?
A: In mild climates, your engine may start with minimal issues. In colder weather, you’ll likely experience hard starting, rough idle, or excessive smoke until the engine warms up.

Q: Can I replace just one glow plug, or should I change them all?
A: If only the cylinder 7 glow plug has failed and the others test within OEM specifications, you can replace just the faulty one. However, if your vehicle has high mileage, replacing all glow plugs as preventive maintenance can be a wise investment.