P0671 — Cylinder 1 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 P0671 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 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 P0671 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0671 (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 P0671 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

Will P0671 clear itself?

Sometimes P0671 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.

What are the symptoms of P0671?

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

Is it safe to drive with P0671?

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 is the most common cause of P0671?

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

How do I diagnose P0671 myself?

Start with a scan: confirm P0671, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the diesel glow plug system, and verify the suspect component against live data before buying parts.

When your engine control unit logs trouble code P0671, it's telling you there's a problem with the glow plug circuit for cylinder one specifically. Diesel engines rely on these small heating elements to warm the combustion chambers during startup—when air temperatures drop, the fuel needs help igniting properly. The ECU monitors each glow plug's electrical circuit and logs P0671 only after confirming the fault exists under specific operating conditions rather than flagging a single momentary glitch.

Cylinder numbering follows a predictable pattern on inline engines (counting from the front, where the belts are), but V-configuration engines vary by manufacturer. The exact position of cylinder one determines how accessible the affected glow plug is for testing or replacement—some require removing the intake manifold, which adds considerable labor time to the job. When the ECU stores P0671, it also captures freeze frame data: engine load, RPM, coolant temperature, and vehicle speed at the exact moment the fault occurred. That snapshot becomes your roadmap for reproducing the failure condition during diagnosis.

Symptoms

Check Engine Light illuminates on the dashboard
Glow plug warning indicator remains lit after engine startup
Rough or unstable idle that smooths out once the engine reaches normal operating temperature
Visible white exhaust smoke during cold starts
Difficulty starting the engine, particularly in cold weather or after the vehicle sits overnight
Reduced performance under load in colder conditions

Causes

Corroded or moisture-damaged electrical connectors at the glow plug
Open circuit from a burned-out or failed glow plug filament
Faulty glow plug control relay or module that fails to energize the circuit
Damaged wiring insulation causing short circuits or open circuits
Poor ground connection at the glow plug circuit
Defective ECU output driver controlling the cylinder one glow plug circuit
Mechanical failure of the glow plug itself from heat cycling fatigue

How to Diagnose

Plug an OBD2 scanner into the diagnostic port, retrieve code P0671, and record the associated freeze frame data—this tells you the exact engine conditions when the fault triggered.
Pull all stored and pending codes from the ECU. Related codes pointing to other cylinders or the glow plug control system often lead you directly to the root cause rather than chasing symptoms.
Check manufacturer technical service bulletins for your specific make and model—some vehicles have known software calibrations or hardware updates addressing glow plug system faults.
Perform a visual inspection of the wiring harness and connectors feeding the glow plug circuit, looking for chafed insulation, corrosion buildup, or loose terminal fit that could interrupt current flow.
Measure resistance across each glow plug individually with a digital multimeter; an open circuit reads infinite resistance and confirms a failed heating element.
Verify proper voltage at the glow plug bus bar during the pre-glow cycle using a multimeter or test light, then clear P0671, complete a drive cycle matching the freeze frame conditions, and confirm the code does not reappear.

Repair Options

If inspection reveals a single failed glow plug, replacement typically runs between eighty and two hundred dollars for parts and labor. When the control relay or glow plug module itself is defective, expect to pay one hundred fifty to four hundred fifty dollars depending on your vehicle's design and local labor rates. Replacing all glow plugs as a complete set—often the smarter long-term choice if your diesel has high mileage—generally costs between two hundred fifty and six hundred dollars including labor.

How Much Does It Cost to Fix P0671?

Total repair costs span a wide range because the culprit could be anything from a simple connector fix to full glow plug replacement. Most independent shops charge between eighty and two hundred dollars for diagnosing and replacing an individual glow plug. A failed relay or module replacement typically falls in the one hundred fifty to four hundred fifty dollar range. If you opt for a complete set replacement—which often makes sense on older diesel engines where multiple plugs show age-related wear—the job usually lands between two hundred fifty and six hundred dollars. The key is accurate diagnosis before purchasing parts: the most expensive repair is rarely where you should start, and a thorough electrical inspection prevents buying components you don't actually need.

Can You Drive with P0671?

Once your engine warms up and reaches operating temperature, driving typically poses no immediate safety concern. The real risk centers on cold starts—with one or more glow plugs inoperative, your diesel may struggle to fire in freezing weather or fail to start entirely. If you live in a cold climate, addressing P0671 before winter arrives prevents being stranded with a vehicle that won't start on a cold morning.