P066D — Cylinder 2 Glow Plug Control Circuit High

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P066D is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the diesel glow plug system for damage, corrosion, chafing or poor contact.
Subsystem check With the cylinder 2 glow plug control disconnected, measure signal voltage at the harness connector: a high reading with the component unplugged indicates a short to power in the harness; if it drops to normal, the component itself is faulty.
Subsystem check Measure resistance of each glow plug — an open plug reads infinite resistance.
Subsystem check Check voltage at the glow plug rail during the pre-glow phase.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P066D does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P066D (parts and labor, USD, independent shop rates): Replace a full set of glow plugs: $250–$600; Replace one glow plug: $80–$200; Replace the glow plug relay/module: $150–$450. Overall, fixing P066D usually costs between $80 and $600. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

Is it safe to drive with P066D?

Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

What are the symptoms of P066D?

Drivers most often notice: white smoke after a cold start, Check Engine Light on, glow plug warning light staying on, rough idle until warmed up.

What is the most common cause of P066D?

The most common causes are: burned-out glow plug, blown fuse in the glow plug circuit, faulty glow plug relay or control module.

Will P066D clear itself?

Sometimes P066D clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

How do I diagnose P066D myself?

Start with a scan: confirm P066D, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the cylinder 2 glow plug control, and verify the suspect component against live data before buying parts.

When your vehicle's engine control unit detects that the electrical circuit responsible for heating the second cylinder's glow plug is showing a voltage reading that sits too high, it logs trouble code P066D. This diagnostic code falls squarely in the powertrain category because it involves engine management systems. The glow plug system matters because diesel engines rely on these heating elements to warm the combustion chamber during cold starts, allowing fuel to ignite properly when temperatures are low. When the control module sees circuit voltage near battery level instead of the expected operating range, it flags the fault and illuminates the check engine light.

The "circuit high" portion of P066D tells you that the powertrain control module is receiving a voltage signal from the cylinder 2 glow plug circuit that exceeds normal parameters. In practical terms, this usually means one of three situations: a power feed has shorted into the signal wire, a ground return has opened and left the circuit floating high, or a failed component is pulling the line up to battery voltage. On inline diesel engines, cylinder numbering runs from the front of the block (the belt end) toward the transmission. V-configuration engines vary by manufacturer, so consulting your specific firing order diagram is essential before any repair work. Accessing cylinder 2 varies significantly between platforms—some engines let you reach it easily from above, while others require removing the intake manifold, which adds substantial labor time to any repair.

To set P066D, the control module monitors live data from the glow plug control circuit across multiple drive cycles, comparing actual voltage against factory-calibrated thresholds. The moment readings fall outside acceptable limits, the module stores the code and captures a freeze frame snapshot—essentially a photograph of engine operating conditions including RPM, load percentage, coolant temperature, and vehicle speed at the exact moment of failure. Reviewing that freeze frame first gives any diagnosis a solid starting point and prevents chasing phantom problems.

Symptoms

Persistent white exhaust smoke during cold starts that clears once the engine reaches operating temperature
Check engine light illuminated on the dashboard
Glow plug warning indicator remaining lit after the pre-heat cycle completes
Noticeable engine roughness and stumble during idle that smooths out as the engine warms
Difficulty starting or extended cranking time, especially in cold ambient temperatures
Reduced performance under load due to incomplete combustion in cylinder 2

Common Causes

The most frequent culprit behind P066D involves corrosion or deterioration at the cylinder 2 glow plug connector itself. Exposure to heat cycling and engine bay contaminants gradually degrades the electrical connection.

Corroded, damaged, or loose electrical connector at cylinder 2 glow plug
Open ground return wire in the cylinder 2 glow plug control circuit
Signal wire between the control module and cylinder 2 shorted directly to battery positive
Failed cylinder 2 glow plug with internal short pulling the circuit high
Blown fuse protecting the glow plug relay circuit
Damaged wiring harness with exposed conductors chafing against metal surfaces
Faulty glow plug relay or integrated control module driving excessive voltage onto the circuit

How to Diagnose P066D

Before starting any physical inspection, connect a professional-grade OBD2 scanner to confirm P066D is actively stored and record all available freeze frame data. This snapshot reveals the exact operating conditions when the fault occurred, which helps you reproduce the failure during testing.

Retrieve any related diagnostic trouble codes that accompanied P066D. Codes stored in other modules often point toward the root cause—whether that's a power supply issue, a CAN bus communication problem, or a failed sensor upstream. Many manufacturers publish technical service bulletins specifically addressing persistent OBD2 codes, so searching your vehicle's TSB database by VIN can reveal known issues with documented solutions or software updates.

Perform a thorough visual inspection of the entire glow plug wiring harness. Focus especially on the cylinder 2 connector and surrounding harness where it routes near hot exhaust components. Look for cracked insulation, melted wire jackets, corrosion buildup at the terminals, and any signs that the harness has been rubbing against engine hardware. Poor electrical connections create resistance that mimics voltage irregularities.

With cylinder 2's glow plug connector unplugged, measure voltage at the harness side of the connector using a digital multimeter. If you see battery voltage present with the component disconnected, the harness itself contains a short to power somewhere between the connector and the control module. If voltage drops to near zero with the plug removed, the glow plug or relay internal to the assembly has failed.

Check individual glow plug resistance using the multimeter's resistance function. A healthy diesel glow plug typically reads between 0.5 and 2 ohms depending on design. An infinite reading or open circuit indicates a burned-out element that requires replacement. Testing each plug in the engine provides baseline data for comparison.

Monitor voltage at the glow plug positive terminal during the pre-glow sequence with a scope or high-impedance voltmeter. The control module should pulse ground to each plug in sequence during startup. Absence of pulsing voltage confirms the relay or module has failed. After completing repairs, clear all codes and drive the vehicle through a complete drive cycle including cold startup conditions matching the original freeze frame. P066D should not reappear.

Repair Options

Depending on what your diagnosis reveals, several repair paths address P066D:

Replace all glow plugs as a set — recommended when multiple plugs show wear or age: parts and labor $250–$600
Replace single cylinder 2 glow plug if isolation confirms isolated failure: parts and labor $80–$200
Replace glow plug relay or integrated control module if circuit testing shows no fault in wiring or plugs: parts and labor $150–$450

Cost Breakdown

For most P066D repairs, expect to pay between $250 and $600 for a full glow plug set replacement including labor at an independent shop. Single plug replacement typically runs $80 to $200. Glow plug relay or control module replacement falls in the $150 to $450 range depending on your vehicle's architecture. Overall, fixing P066D generally costs between $80 and $600 depending on which component has actually failed. Spending time isolating the exact faulty part before purchasing components prevents unnecessary expense—the most expensive repair scenario is rarely the first one to investigate.

Driving Safety Verdict

Severity: Short. Your vehicle remains driveable once the engine reaches normal operating temperature, but cold-weather starting becomes unreliable when cylinder 2's glow plug fails completely. If you live in a cold climate or anticipate winter driving, address P066D promptly. In mild weather, you can delay repair briefly, but continuing to run with multiple failed plugs accelerates wear on your battery and starter motor during difficult cold starts.