P066F — Cylinder 3 Glow Plug Control Circuit High

P066F — Cylinder 3 Glow Plug Control Circuit High. A fault in the diesel glow plug system. The car is usually drivable while you plan the repair. Typical causes: damaged glow plug wiring or connector, faulty glow plug relay or control module, blown fuse in the glow plug circuit.

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 P066F 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 3 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 P066F does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P066F (parts and labor, USD, independent shop rates): Replace a full set of glow plugs: $250–$600; Replace the glow plug relay/module: $150–$450; Replace one glow plug: $80–$200. Overall, fixing P066F 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 are the symptoms of P066F?

Drivers most often notice: hard starting in cold weather, rough idle until warmed up, Check Engine Light on, white smoke after a cold start.

How much does it cost to fix P066F?

Replace a full set of glow plugs — $250–$600; Replace the glow plug relay/module — $150–$450; Replace one glow plug — $80–$200. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

How do I diagnose P066F myself?

Start with a scan: confirm P066F, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the cylinder 3 glow plug control, and verify the suspect component against live data before buying parts.

What is the most common cause of P066F?

The most common causes are: damaged glow plug wiring or connector, faulty glow plug relay or control module, blown fuse in the glow plug circuit.

Will P066F clear itself?

Sometimes P066F 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.

When your vehicle's computer detects a problem with the heating elements that help diesel engines start in cold weather, it logs code P066F. This specific fault means the control circuit for cylinder 3's glow plug is reading a voltage that is higher than it should be — essentially, the system is seeing "high" when it expects normal operating voltage. The powertrain control module monitors this circuit constantly while the engine runs, and when readings stay outside acceptable limits across multiple drive cycles, P066F gets stored and the check engine light illuminates.

Glow plugs serve a critical purpose in diesel engines, particularly during cold starts. Unlike spark plugs in gasoline engines, glow plugs produce heat to warm the combustion chamber so diesel fuel can ignite properly. Each cylinder has its own glow plug, and the control module energizes them in sequence before and during startup. Cylinder numbering follows the engine's firing order — on inline engines, count from the front pulley end; on V-configurations, consult your model's firing-order diagram. When the module detects unexpectedly high voltage on cylinder 3's control circuit, it means something is pulling that signal line toward battery voltage when it should be switched off or at a lower potential.

The module saves freeze frame data alongside P066F, capturing engine speed, load, temperature, and vehicle speed at the exact moment the fault occurred. This snapshot is your best diagnostic starting point because it shows under what conditions the problem appeared. The "circuit high" condition typically stems from one of three root causes: a wire shorted to a power source, a broken ground path, or a failed component that is dragging the signal line up to battery voltage levels.

Common Symptoms of P066F

Engine struggles to start, especially when temperatures drop
Rough running or misfiring that smooths out once the engine reaches operating temperature
Check Engine Light remains illuminated after startup
White or gray exhaust smoke visible during cold starts
Glow plug warning indicator stays lit on the dashboard

What Causes P066F?

The signal wire for cylinder 3 is shorted directly to battery positive voltage
An open or failed ground return path in the cylinder 3 glow plug control circuit
The cylinder 3 glow plug control module or relay is defective and pulling the signal high
Damaged wiring insulation or corroded connectors in the glow plug harness
A burned-out or internally shorted cylinder 3 glow plug
Corrosion or moisture intrusion at the glow plug electrical connectors
A blown fuse protecting the glow plug power circuit

How to Diagnose P066F

Connect an OBD2 scanner and verify that P066F is stored as an active or pending code. Record the freeze frame data — this shows rpm, load, coolant temperature, and vehicle speed when the fault was detected, which helps recreate the failure conditions.
Scan for any related diagnostic trouble codes. P066F sometimes appears alongside other glow plug or intake air temperature codes, and these can point you toward the actual root cause rather than just the symptom.
Check manufacturer technical service bulletins using your vehicle identification number on the automaker's website or nhtsa.gov. Some P066F failures have documented factory fixes, including software recalibrations that resolve the issue without any parts replacement.
Visually inspect the entire glow plug wiring harness for cylinder 3, looking for chafed insulation, melted wire jackets near hot engine components, corroded pins in the connector, or any signs of previous repair work that may have been done incorrectly.
With the cylinder 3 glow plug connector unplugged, measure voltage at the harness side of the connector. If you still read high voltage with the component disconnected, the wiring harness itself has a short to power. If the voltage drops to normal range, the glow plug or its control module is the culprit.
Measure resistance across each individual glow plug — a properly functioning plug reads between 0.5 and 2 ohms depending on the design. An open circuit reading or infinite resistance indicates a failed glow plug that needs replacement.
Clear the code, then complete a drive cycle that replicates the conditions from your freeze frame data, including similar engine temperature and load conditions. Confirm that P066F does not return before considering the diagnosis complete.

Repair Options and Costs

The repair approach depends entirely on which component failed, and working from simplest to most complex saves money:

Replace a single cylinder 3 glow plug — $80 to $200
Replace the glow plug control relay or module — $150 to $450
Replace the complete glow plug set as a preventive measure — $250 to $600

How Much Does It Cost to Fix P066F?

Depending on which component triggers P066F, expect to pay between $80 and $600 total (parts and labor at independent shop rates in the United States). Single glow plug replacement sits at the lower end of that range, while replacing a full set of plugs pushes toward the upper limit. The glow plug relay or control module replacement falls in the middle. An accurate diagnosis before purchasing parts is essential — the cheapest component to fix is also the most common cause, so rule out wiring issues and single-plug failures before committing to expensive module replacement.

Can You Drive with P066F?

The severity classification for this fault is short, meaning you can continue driving once the engine is warmed up and running normally. The practical risk appears during cold starts: if multiple glow plugs have failed, or if cylinder 3 is severely affected, the engine may fail to start on a cold morning or crank noticeably slower than normal. Addressing this fault before cold weather arrives is strongly recommended, but immediate roadside breakdown is unlikely.