P066B — Cylinder 1 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 P066B 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 1 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 P066B does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P066B (parts and labor, USD, independent shop rates): Replace one glow plug: $80–$200; Replace the glow plug relay/module: $150–$450; Replace a full set of glow plugs: $250–$600. Overall, fixing P066B 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

What does OBD2 code P066B mean?

Cylinder 1 Glow Plug Control Circuit High (P066B): a powertrain (engine/transmission) fault affecting the diesel glow plug system. The car is usually drivable while you plan the repair. Most often caused by damaged glow plug wiring or connector, faulty glow plug relay or control module, corroded glow plug connectors.

What are the symptoms of P066B?

Drivers most often notice: white smoke after a cold start, glow plug warning light staying on, hard starting in cold weather, Check Engine Light on.

How do I diagnose P066B myself?

Start with a scan: confirm P066B, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the cylinder 1 glow plug control, and verify the suspect component against live data before buying parts.

What is the most common cause of P066B?

The most common causes are: damaged glow plug wiring or connector, faulty glow plug relay or control module, corroded glow plug connectors.

Is it safe to drive with P066B?

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.

When your engine control unit detects that the voltage on the cylinder 1 glow plug control circuit sits higher than it should, it logs OBD2 trouble code P066B. Diesel engines rely on glow plugs to warm the combustion chambers during cold starts, and the ECU constantly monitors each plug's control circuit for proper operation. When that monitored voltage approaches battery level instead of the expected lower signal, the module flags P066B because it means the circuit is not behaving as designed. The fault specifically targets cylinder 1, which on most inline engines is the first cylinder counting from the front belt end — on V-configurations the numbering varies by manufacturer, so consulting your model's firing-order diagram is essential before working on the vehicle.

The glow plug system works by applying battery voltage to heating elements inside each cylinder for a few seconds after you turn the key. The ECU controls a relay or module that energizes the plugs based on coolant temperature and engine load. When P066B is stored, the control module has seen a "high" condition — meaning the signal wire leading to cylinder 1's glow plug is being pulled up toward battery voltage when it should be switched off or at a lower potential. This can happen if the ground return for that circuit opens up, if the signal wire accidentally contacts a power source, or if the internal switching element inside the glow plug itself fails in a way that drags the line high. Because the ECU typically requires this condition to persist across multiple drive cycles before lighting the Check Engine lamp, the fault is usually genuine rather than a fleeting sensor glitch.

Common symptoms of P066B

Noticeable white exhaust smoke, especially during the first few minutes after a cold start
Glow plug indicator remaining illuminated on the dashboard longer than normal or staying on constantly
Difficulty starting the engine when ambient or engine temperatures are low
Illumination of the Check Engine warning light on the instrument cluster
Rough or uneven idle that smooths out once the engine reaches normal operating temperature
Reduced overall starting reliability in colder climates where glow plugs do the most work

What causes P066B?

An open or interrupted ground return path within the cylinder 1 glow plug control circuit
The signal wire for cylinder 1 shorted directly to battery positive voltage
A failed cylinder 1 glow plug control element that pulls the circuit high even when commanded off
A burned-out or internally broken glow plug that creates an abnormal reading
Physical damage to the glow plug wiring harness, including chafed insulation or broken strands
A blown fuse protecting the glow plug bus bar or control module
Corrosion or moisture intrusion at the cylinder 1 glow plug connector causing high-resistance contact

How to diagnose P066B

Plug in an OBD2 scan tool and confirm that P066B is an active or stored fault. Capture the freeze frame data attached to the code — this snapshot records engine speed, calculated load, coolant temperature, and vehicle speed at the exact moment the module detected the problem. Those conditions often point directly to the root cause.
Scan for any other trouble codes that appeared alongside P066B. Related electrical faults or sensor codes frequently reveal whether the issue is wiring-related, module-related, or truly confined to the glow plug itself. Also search for manufacturer TSBs using your VIN — some vehicles have known software calibrations or harness updates that address recurring P066B occurrences.
Perform a thorough visual inspection of the entire glow plug wiring harness, paying special attention to the connector at cylinder 1. Look for melted insulation, bent pins, corrosion buildup, and any signs that the harness has been rubbing against hot engine components.
With the cylinder 1 glow plug disconnected from its harness, use a multimeter to probe the signal pin at the harness side. If you still measure high voltage (close to battery potential) with the plug unplugged, the harness itself has a short to power. If the voltage drops to normal operating range once the plug is out of the picture, the glow plug or its internal switching element is the culprit.
Measure the resistance of the cylinder 1 glow plug across its terminals. A properly functioning plug shows a low resistance reading — typically between 0.5 and 2 ohms depending on design. An open or burned-out plug displays infinite resistance or an overload condition on your meter.
Verify that battery voltage is present at the glow plug rail during the pre-glow phase using a circuit tester. This confirms the supply side of the system is working before you focus on signal-level faults.
After any repairs, clear the code, complete a drive cycle that replicates the conditions stored in the freeze frame, and recheck with your scanner to confirm P066B does not return.

Repair options and costs for P066B

Begin with the most probable and least expensive possibilities before spending on major components:

Replace a single cylinder 1 glow plug — parts and labor run $80–$200 at most independent shops. This is by far the most common fix when the plug itself is defective.
Replace the glow plug relay or control module — typically costs $150–$450 depending on make, model, and whether the part requires dealer-only programming.
Replace all glow plugs as a complete set — averaging $250–$600 when done together, this is often the best long-term solution on high-mileage diesels, especially if multiple plugs show signs of wear.

Overall, addressing P066B typically falls somewhere between $80 and $600 in total out-of-pocket expense. An accurate diagnosis before purchasing parts prevents the frustration of swapping expensive modules only to discover the real culprit was a single bad plug or a damaged wire.

Can you drive with P066B?

The vehicle remains drivable once the engine is running and fully warmed up. The primary concern is cold-start reliability — with one or more glow plugs disabled, a diesel may struggle to fire or fail to start altogether on chilly mornings. Addressing P066B before cold weather arrives is the practical choice, though short trips in moderate temperatures are generally safe with the code present.