P066D Code: Cylinder 2 Glow Plug Control Circuit High – Diagnose & Clear with iCarsoft CR Ultra P

If you drive a diesel vehicle and struggle with hard cold starts, no-start conditions, rough idle, or an illuminated check engine light, the P066D fault code signals a critical glow plug system failure. P066D stands for Cylinder 2 Glow Plug Control Circuit High, an OBD‑II diesel-specific code triggered when the Powertrain Control Module (PCM) detects abnormally high voltage in the glow plug control circuit for cylinder 2. This fault disables preheating, cripples cold‑weather performance, and can leave you stranded. Basic code readers only confirm the code, but the iCarsoft CR Ultra P professional diagnostic scanner lets you pinpoint wiring damage, a faulty glow plug control module (GPCM), or a defective glow plug and clear P066D quickly and safely at home.

This code is especially common in:

• Chevrolet Silverado 2500HD 6.6L Duramax (2011–2015)
• Ram 2500 6.7L Cummins (2013–2018)

What Is the P066D Fault Code?

Official DTC Definition:
Cylinder 2 Glow Plug Control Circuit High

Glow plugs preheat the combustion chamber to enable reliable diesel ignition in cold temperatures. The PCM regulates voltage to the glow plug for cylinder 2 via the glow plug control module. When the circuit voltage exceeds the factory threshold (typically >14.5V), the PCM logs P066D and disables glow plug operation to protect electrical components.

• Cylinder 2: The second cylinder in the firing order
• Glow Plug Control Circuit: Wiring and module that powers the glow plug
• Circuit High: Voltage is abnormally elevated
• Effect: Hard/no starts in cold weather, rough warm‑up, reduced power

Key Symptoms of P066D

• Check engine light (MIL) illuminated
• Extended cranking or failure to start when cold
• Rough idle until engine warms up
• White smoke from exhaust during cold start
• Reduced fuel economy
• Glow plug warning light illuminated
• Intermittent starting issues
• Poor performance in low temperatures

Common Causes of P066D

• Damaged or shorted wiring in the cylinder 2 glow plug circuit (most common)
• Faulty glow plug control module (GPCM)
• Defective or burned‑out glow plug for cylinder 2
• Corroded or loose electrical connectors
• Poor ground connection for the glow plug circuit
• Faulty glow plug relay
• Voltage surge from a failing alternator
• Internal PCM driver malfunction (very rare)

Real‑World Repair Scenario

Why iCarsoft CR Ultra P Is the Best Tool for P066D

Generic OBD scanners can’t test glow plug circuits or monitor real‑time voltage. The iCarsoft CR Ultra P delivers dealership‑level diagnostics with professional features:

Learn more: iCarsoft CR Ultra P Official Product Page

Step‑by‑Step: Diagnose & Clear P066D with iCarsoft CR Ultra P (With Exact Values)

1. Plug iCarsoft CR Ultra P into the OBDII port under the dashboard.
2. Turn ignition to ON (engine off).
3. Select your vehicle or use Auto VIN for automatic detection.
4. Go to Engine System > Glow Plug System and run Full System Scan.
5. Confirm P066D: Cylinder 2 Glow Plug Control Circuit High.
6. Enter Live Data to monitor cylinder 2 glow plug voltage:
   • Normal operating range: 11.5–14.5V
   • P066D fault condition: >14.5V (consistent high reading)
7. Use Active Test to activate the glow plug and check for proper response.
8. Inspect wiring, connectors, GPCM, and glow plug; replace faulty components.
9. Use Clear Fault Codes to erase P066D.
10. Perform PCM Reset and cold‑start test to confirm normal operation.

FAQ: P066D & iCarsoft CR Ultra P

Why You Must Fix P066D Immediately

Ignoring P066D leads to:

• Complete no‑start conditions in cold weather
• Premature wear on starter motor and battery
• Increased fuel consumption and emissions
• Potential damage to the glow plug control module
• Unexpected breakdowns in winter conditions

Final Thoughts

P066D – Cylinder 2 Glow Plug Control Circuit High – is a serious but highly fixable diesel code, especially common in GM Duramax and Ram Cummins engines. With the iCarsoft CR Ultra P, you can diagnose voltage issues, test components, repair circuits, and clear P066D at home without expensive dealership labor.

Don’t let a faulty glow plug circuit leave you stranded in the cold. Get the iCarsoft CR Ultra P and restore reliable cold‑weather diesel performance today.

Disclaimer: This guide is for informational purposes only. Always consult your vehicle’s service manual or a certified automotive technician before performing diesel glow plug or electrical repairs. iCarsoft is not responsible for damage from improper tool use or vehicle maintenance.