P066C — Cylinder 2 Glow Plug Control Circuit Low #

Note: The description for DTC P066C may vary depending on the vehicle’s manufacturer.

• Cylinder 2 Glow Plug Control Circuit Low (Ford, GM, Chrysler, Mercedes-Benz, Nissan, Toyota)
• Cylinder 2 Glow Plug Circuit Low Input (BMW, Volkswagen, Audi)
• No. 2 Glow Plug Control Circuit Low (Mazda, Isuzu, Peugeot)

Summary of code P066C #

DTC P066C indicates that the Engine Control Module (ECM) has detected a low voltage or resistance issue in the glow plug control circuit for cylinder 2. Glow plugs, used in diesel engines, are heating elements designed to preheat the combustion chamber for easier starting and smoother operation, especially in cold conditions. The control circuit manages the electrical current supplied to each glow plug. When the voltage in cylinder 2’s circuit drops below the expected threshold, the ECM registers P066C and typically illuminates the Check Engine Light (CEL). This code may be triggered by electrical faults, a failed glow plug, or problems within the control module itself. Prompt diagnosis is essential to prevent starting difficulties and potential engine damage in cold weather.

Frequent symptoms for code P066C #

When P066C is present, you might notice issues related to cold starting and engine performance. Some symptoms are subtle, while others are more pronounced, particularly during chilly mornings or in winter.

• Hard starting, especially in cold weather
• Rough idle after start-up
• Increased white or gray exhaust smoke at start
• Reduced engine performance until warm
• Illuminated Check Engine Light

Probable causes for code P066C #

The most common reasons behind a P066C code usually involve an electrical issue within the glow plug circuit for cylinder 2. However, other factors may contribute. Here’s what technicians typically find, ranked from most to least frequent:

1. Faulty glow plug in cylinder 2
2. Open or shorted wiring or connector in the glow plug circuit
3. Defective glow plug relay or control module
4. Poor electrical ground connection
5. Corroded or loose terminal at the glow plug or relay
6. ECM or PCM (Powertrain Control Module) malfunction (rare)

Step-by-step diagnosis for code P066C #

Diagnosing P066C involves a combination of visual inspection, simple electrical tests, and sometimes component replacement. Here’s a methodical approach to pinpoint the fault:

• 1. Verify code and reset: Confirm P066C with a scan tool, clear codes, and perform a cold start to see if it returns.
• 2. Check for related codes: Look for other glow plug or circuit codes that may indicate a broader issue.
• 3. Visual inspection: Examine the wiring, connectors, and harness at cylinder 2’s glow plug and at the relay/module for corrosion, damage, or disconnection.
• 4. Test glow plug: Remove cylinder 2’s glow plug and test its resistance with a multimeter. A healthy plug typically has 0.5–2 ohms; higher or infinite resistance means it’s faulty.
• 5. Check circuit voltage: With the key ON (engine cold), measure voltage at the glow plug terminal. It should match battery voltage during pre-glow. Low or zero voltage suggests an open circuit or relay fault.
• 6. Inspect relay/module: Test the glow plug relay for correct operation. Swap with a known good relay if possible.
• 7. Check ground: Ensure the cylinder 2 glow plug ground is clean, tight, and free from corrosion.
• 8. Test at ECM: If all else checks out, back-probe the ECM’s glow plug control output for cylinder 2 to ensure it’s sending the proper signal.

Possible repairs for code P066C #

Repairing P066C focuses on correcting the detected fault in the glow plug control circuit for cylinder 2. Based on the diagnostic findings, the following repairs are commonly effective:

• Replace the faulty glow plug in cylinder 2
• Repair or replace damaged wiring or connectors in the glow plug circuit
• Replace a failed glow plug relay or control module
• Clean and secure ground connections
• Clean or replace corroded terminals
• Reprogram or replace the ECM/PCM (only if verified as faulty, which is rare)

Is it safe to drive with code P066C? #

Limited: You can usually drive with a P066C code if the outside temperatures are mild, as the engine may still start and run. However, in cold weather, the engine may become hard to start, misfire, or produce excessive smoke, which can eventually lead to unburned fuel damaging the diesel particulate filter (DPF) or other exhaust components. It’s best to address the issue promptly, especially if you rely on the vehicle in winter or in regions with low temperatures.

Vehicles most commonly affected by code P066C #

P066C is most frequently encountered in modern diesel vehicles equipped with individual glow plug monitoring. The following vehicle makes and models are commonly reported:

• Ford F-Series (F-250, F-350 Super Duty with Power Stroke diesel)
• Chevrolet Silverado and GMC Sierra (Duramax diesel engines)
• Ram 2500/3500 (Cummins diesel)
• Mercedes-Benz Sprinter (OM651, OM642 engines)
• Volkswagen Touareg, Amarok (V6 TDI)
• Nissan Titan XD (Cummins diesel)
• Toyota Hilux, Land Cruiser (D-4D diesel)

Common mistakes and best practices for code P066C #

Incorrect troubleshooting for P066C often leads to unnecessary part replacements or missed root causes. Here’s how to avoid common pitfalls and ensure a correct repair:

• Assuming all glow plugs or relays are faulty without testing each component individually
• Overlooking the importance of a proper ground connection
• Skipping a thorough visual inspection for simple wiring or connector faults
• Failing to clear codes and retest after repairs
• Not using a multimeter to confirm resistance and voltage values

Express FAQ for code P066C #

Below are quick answers to common questions about P066C:

• Q: Can a single faulty glow plug cause hard starting in all conditions?
  A: Typically, a bad glow plug only affects starting in cold weather or when the engine is cold. In warm weather, the effect is minimal.
• Q: Will P066C clear itself if I replace the glow plug?
  A: Sometimes, but you should always clear the code with a scan tool and verify the repair by performing a cold start cycle.