P066C — Cylinder 2 Glow Plug Control Circuit Low

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 P066C 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 Check for supply voltage at the cylinder 2 glow plug control connector first; then measure resistance from the signal wire to ground — near-zero ohms confirms a short to ground in the harness.
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 P066C does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P066C (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 P066C 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 P066C?

Drivers most often notice: white smoke after a cold start, glow plug warning light staying on, rough idle until warmed up, hard starting in cold weather.

Is it safe to drive with P066C?

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.

Will P066C clear itself?

Sometimes P066C 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 much does it cost to fix P066C?

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.

What does OBD2 code P066C mean?

Cylinder 2 Glow Plug Control Circuit Low (P066C): 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 burned-out glow plug, damaged glow plug wiring or connector, blown fuse in the glow plug circuit.

Diesel engines rely on glow plugs to warm the combustion chambers before fuel can ignite, especially in cold conditions. When the engine control module monitors the glow plug circuit for cylinder 2 and detects that the voltage has dropped below acceptable levels, it stores trouble code P066C. The designation breaks down as follows: "P" indicates a powertrain issue, "06" points to the auxiliary emission control system, "6" identifies the specific circuit, and the final "C" marks it as a circuit low condition. This code tells you the module sees the cylinder 2 glow plug control signal sitting near ground when it should be higher.

The PCM sets P066C by comparing live voltage readings from the glow plug driver circuit against factory-calibrated thresholds. Usually the fault must repeat across multiple drive cycles before the check engine light illuminates. At the moment of detection, the module also saves a freeze frame snapshot — engine speed, load percentage, coolant temperature, and vehicle speed at the time of failure. Cylinder numbering varies by engine orientation: on inline engines count from the front pulley end; on V-configurations refer to the firing-order diagram for your specific make and model, since cylinder 2 may sit on either bank. This matters because some cylinders require intake manifold removal for access, which adds labor time.

Symptoms of a P066C fault

When cylinder 2's glow plug fails or the circuit drops low, the engine runs roughly until it warms up and may produce visible exhaust on startup. The glow plug warning lamp often stays illuminated after the engine is running. In cold weather, starting becomes difficult or impossible if multiple plugs are affected.

Check Engine Light illuminated
Glow plug warning lamp stays on after startup
White smoke from the tailpipe during cold starts
Rough or uneven idle that smooths out once warm
Difficulty starting the engine, particularly in cold temperatures

What triggers P066C

The underlying reasons for a low circuit condition fall into a few categories: loss of proper supply voltage, an unintended ground path, or a failed component pulling the signal down. Corrosion and vibration damage are common culprits in high-heat engine environments.

Insufficient supply voltage reaching the cylinder 2 glow plug control circuit
Signal wire shorted to ground due to chafed or melted insulation
Failed cylinder 2 glow plug control circuit drawing the signal low
Corrosion or moisture intrusion in the glow plug connectors
Faulty glow plug relay or control module
Damaged wiring or loose connectors in the glow plug harness
Burned-out glow plug presenting as an open circuit
Blown fuse protecting the glow plug circuit

How to diagnose P066C

A systematic approach prevents unnecessary parts replacement. Start with non-invasive checks before touching the wiring.

Connect an OBD2 scanner, confirm P066C is stored, and record the freeze frame data — it shows rpm, load, coolant temperature, and vehicle speed when the fault occurred.
Look for related trouble codes that may point to the root cause. Search technical service bulletins for your make and model on the manufacturer website or nhtsa.gov — many diesel glow plug issues have known factory fixes or software updates.
Perform a visual inspection of the wiring harness and connectors feeding the glow plug system. Look for damaged insulation, corrosion, broken wires, or loose contacts.
Measure supply voltage at the cylinder 2 glow plug connector first. Then check resistance from the signal wire to ground — a reading near zero ohms confirms a short to ground in the harness.
Test resistance of each glow plug individually. An open circuit reads infinite resistance.
Probe the glow plug rail for voltage during the pre-glow phase to verify the module is energizing the circuit.
Clear the code, complete a drive cycle that recreates the freeze frame conditions, and verify P066C does not return.

Repair options and costs

Address the cheapest and most common failures before replacing major components.

Replace all glow plugs as a set — $250–$600
Replace the glow plug relay or control module — $150–$450
Replace a single glow plug — $80–$200

Total repair cost for P066C

At independent shop labor rates, the full bill depends on which part actually failed. Replacing one glow plug typically runs $80–$200. Swapping the glow plug relay or control module costs $150–$450. Doing a full set of glow plugs runs $250–$600. Most P066C repairs fall somewhere in that $80–$600 range. Spending time on accurate diagnosis first keeps you from buying expensive parts when the problem is a cheap connector or a single bad plug — the priciest scenario is rarely where you should start.

Can you drive with P066C?

The vehicle remains driveable once the engine is running and up to temperature. Glow plugs only function during the brief pre-heat cycle before and immediately after starting. The real concern is cold-weather starting: if several plugs are dead, the engine may not start on a cold morning or may run very poorly until it warms up. Get this fixed before winter arrives, especially if you live in a cold climate.