OBD-II Code P0671: Cylinder 1 Glow Plug Circuit Malfunction

What Does P0671 Mean?

The P0671 code means your vehicle's Powertrain Control Module (PCM) detected an electrical fault in the glow plug circuit for cylinder #1. Glow plugs are vital heating elements in each cylinder of a diesel engine; they heat the combustion chamber to ensure a quick, efficient start when the engine is cold. This code specifically indicates the circuit for the first cylinder's glow plug is open, shorted, or has an out-of-spec voltage reading, preventing it from heating correctly.

Technical definition: The official SAE/ISO definition for P0671 is "Cylinder 1 Glow Plug Circuit/Open". This indicates the powertrain control module (PCM) detected a loss of continuity (an open circuit), a short to ground, or an abnormal voltage reading in the cylinder 1 glow plug electrical circuit. The PCM monitors this circuit and sets the fault if it detects a voltage variation of more than 10% from the expected value or if the current draw is outside its specified range. The Glow Plug Control Module (GPCM) commands the plugs on for 1 to 120 seconds depending on oil temperature and altitude, but disables them if the engine oil temperature exceeds 55°C (131°F).

Can I Drive With P0671?

Yes, But With Caution. Yes, you can drive with a P0671 code, but avoid extended driving. While the immediate risk is low, you will experience hard starting in cold weather, straining your battery and starter motor. More critically, a faulty glow plug prevents the Diesel Particulate Filter (DPF) from regenerating on modern diesels. This leads to a clogged DPF, turning a minor issue into a $2,000-$5,000 catastrophic expense.

Common Causes

• Defective Cylinder 1 Glow Plug (Very Common) — The internal heating element of the glow plug burns out over time, creating an open circuit. Alternatively, its internal resistance shifts out of specification under the heat and high current of actual operation.
• Damaged or Corroded Wiring/Connectors (Common) — Engine heat and vibration make the wiring harness brittle, causing it to fray or break. The connector at the glow plug frequently suffers from corrosion, moisture intrusion, and poor contact.
• Faulty Glow Plug Control Module (GPCM) (Common) — The GPCM controls the on/off cycle for all glow plugs. An internal failure of the driver circuit for cylinder 1 stops the plug from receiving power, or the module fails its own diagnostic self-test.
• Failed Glow Plug Relay (Common) — The relay acts as a high-current switch, delivering battery power to the glow plugs. Failure cuts power to the entire system or to an individual bank of cylinders.
• Corroded or Faulty Ground Connection (Less Common) — The glow plug system draws massive current and relies on a solid ground connection to the chassis or engine block. Corrosion or a loose ground strap creates high resistance, causing intermittent faults.
• Blown Fuse (Rare) — A blown high-amperage fuse cuts all power to the circuit. This is a secondary failure caused by a short circuit in the wiring or a shorted glow plug that must be fixed before replacing the fuse.
• Faulty Powertrain Control Module (PCM) (Very Rare) — It is extremely rare for the main engine computer to fail. Only consider this after exhaustively testing wiring, relays, and the GPCM.

Symptoms

• Check Engine Light (MIL) is On — The PCM illuminates the Malfunction Indicator Lamp on the dashboard to alert the driver as soon as the fault is detected.
• Difficult or No Start in Cold Weather — The engine cranks for an extended time before starting, or fails to start entirely when ambient temperatures drop below 40°F (4°C).
• Rough Idle on Cold Start — For the first 30 to 90 seconds after starting, the engine runs unevenly and shakes due to incomplete combustion in the cold cylinder.
• White or Grey Smoke from Exhaust on Startup — Unburned fuel from the unheated cylinder vaporizes in the hot exhaust, creating a noticeable plume of white or grey smoke immediately after a cold start.
• Hesitation or Reduced Power When Cold — The engine hesitates or feels sluggish during acceleration until it warms up completely, as the PCM adjusts fuel delivery to the affected cylinder.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Replace Cylinder 1 Glow Plug — Parts: $20-$60, Labor: $120-$450, ~2.5 hr book time (Intermediate)
  Ford F-250/F-350 (6.0L Power Stroke, 2004.5-2007): OEM Motorcraft ZD-13 / 4C3Z-12A342-A (Alt: Bosch 80043, Standard Motor Products GP110)
  Chevrolet/GMC (6.6L Duramax LLY, 2004.5-2005): OEM ACDelco 63G / GM 97326305 (Alt: Bosch 0250203011, Denso DG-191)
  Volkswagen Jetta/Golf (TDI, ~2009-2014): OEM VW/Audi 03L905061L (Alt: Bosch GP151, NGK 91440)
• Repair or Replace Wiring/Connector — Parts: $15-$75, Labor: $100-$250, ~2 hr book time (Intermediate)
• Replace Glow Plug Control Module (GPCM) — Parts: $150-$450, Labor: $100-$200, ~1 hr book time (Intermediate)
  Ford F-250/F-350 (6.0L Power Stroke, 2003-2007): OEM Motorcraft DY-876 / YC3Z-12B533-AA (Alt: Alliant Power AP63406, Dorman 904-222)
  Chevrolet/GMC (6.6L Duramax LLY, 2004.5-2005): OEM ACDelco/Beru 97304030 (Alt: Standard Motor Products GPM102, Dorman 904-108)
  Volkswagen Jetta/Golf (TDI, ~2005-2010): OEM VW/Audi 038907281D (Alt: Bosch 0281003032, Meyle 1008000021)
• Replace Glow Plug Relay — Parts: $30-$80, Labor: $50-$120, ~0.5 hr book time (DIY)
  Ford F-250/F-350 (7.3L Power Stroke, 1994-2003): OEM Motorcraft DY-861 / F81Z-12B533-AC (Alt: Alliant Power AP63405, Standard Motor Products GPR109)
• Professional Diagnosis — Parts: $0, Labor: $130-$200, ~1.5 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: For Glow Plug Control Modules (GPCM), a used OEM unit from a low-mileage vehicle is a reliable, cost-effective alternative to expensive new OEM parts, especially for models where new parts are discontinued. Reputable rebuild services are also a strong option. Used glow plugs themselves are never recommended.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the donor vehicle was not scrapped for electrical or engine failure.
• Match the part number exactly; modules are specific to year and engine code.
• Purchase from a reputable salvage yard that offers a warranty (30-90 days is typical).

Decision logic:

• If The part is a glow plug → Always buy new. They are wear items and not worth the risk of reusing.
• If The part is a GPCM and a quality new aftermarket or OEM unit is available for <$250 → Buy new for the longer warranty and reliability.
• If The part is a GPCM and new OEM is discontinued or >$400 → A used OEM module or a professional rebuild service is a sensible choice.

Warranty tradeoff: Used GPCMs typically come with a 30-90 day warranty. New aftermarket modules offer 1-year to limited lifetime warranties. New OEM parts carry a 1-2 year warranty.

Worst-case if a used part fails: $200-$500 if a used module fails after installation, requiring repeat labor costs and the purchase of another part.

What Happens If You Wait — Timeline

1. 0-1 month: Code P0671 is stored, and the Check Engine Light is on. In warm climates, there are no noticeable symptoms. In cool weather (below 40°F/4°C), you notice a slightly longer crank time. (MPG impact: 0-1%% · Added cost: $0)
2. 1-3 months: Hard starting in cold weather becomes consistent. The engine has a rough, shaky idle for the first 30-90 seconds and emits white smoke from the exhaust. DPF regeneration cycles are inhibited by the ECU, starting the clogging process. (MPG impact: 1-3%% · Added cost: $20-$50 in wasted fuel and minor battery/starter wear.)
3. 3-6 months: The Diesel Particulate Filter (DPF) becomes progressively clogged due to failed regeneration attempts. The DPF warning light illuminates. The vehicle enters a limited power or 'limp mode'. Fuel economy drops noticeably. (MPG impact: 5-15%% · Added cost: $200-$600 for a forced DPF regeneration at a shop.)
4. 6+ months: Catastrophic DPF failure. The filter is completely blocked with hardened soot and ash. The engine fails to start or has severely limited power. Extreme backpressure causes damage to turbocharger seals. (MPG impact: 15-25%+% · Added cost: $2,500-$10,000 for DPF replacement. This turns a sub-$100 glow plug repair into a multi-thousand dollar catastrophe.)

Cost of Not Fixing It

• 0-1 month: Hard starting in cold weather, increased strain on starter and battery, reduced fuel economy. (Added cost: $50-$200)
• 1-6 months: Failed DPF regeneration cycles progressively clog the Diesel Particulate Filter. The engine enters limp mode. (Added cost: $300-$800 (for forced regeneration or professional cleaning))
• 6+ months: Catastrophic DPF failure requiring full replacement. Excess backpressure causes turbocharger damage. (Added cost: $2000-$8000+ (for DPF replacement and related engine repairs))

Diagnosis Steps

1. Verify the Code
   Use an OBD-II scanner to confirm P0671 is the active code. Document any other codes present (e.g., P0670, P0380, or other cylinder faults like P0672) and freeze frame data, then clear the codes to see which ones return immediately.
   Tools: OBD-II Scanner (Beginner)
2. Visually Inspect the Circuit
   Thoroughly inspect the wiring harness leading to the cylinder 1 glow plug. Look for brittle or frayed wires, melted connectors, or green/white corrosion on the pins. Trace the harness back to the GPCM and check the main engine-to-chassis ground strap for tightness.
   Tools: Flashlight, Mirror (Beginner)
3. Test the Glow Plug Resistance
   Disconnect the wire from the #1 glow plug. Set a digital multimeter (DVOM) to Ohms. Place one probe on the glow plug's terminal and the other on the engine block (ground). A good glow plug has a resistance between 0.6 and 2.0 Ohms. An infinite reading (OL) indicates an open circuit. A zero reading indicates a short circuit. Replace the plug if it fails.
   Tools: Digital Multimeter (DVOM), Socket/Wrench Set (Intermediate)
4. Check for Voltage at the Connector
   Have a helper turn the ignition to the 'ON' position (do not start). Use a DVOM set to DC volts to test the glow plug's electrical connector. Place the positive lead on the connector's terminal and the negative lead on a good engine ground. You should see battery voltage (approx. 12V) for several seconds. No voltage points to a problem upstream (wiring, relay, or GPCM).
   Tools: Digital Multimeter (DVOM), Helper (Intermediate)
5. Test the Glow Plug Relay and Fuses
   Locate the glow plug relay/module and associated high-amperage fuses in the under-hood fuse box. Visually inspect the fuse. Test the relay by swapping it with an identical one from a non-essential system (like the horn) or by performing a voltage drop test across its main terminals when activated; a drop of more than 0.5V indicates a bad relay.
   Tools: Owner's Manual, Digital Multimeter (DVOM) (Intermediate)
6. Test the Glow Plug Control Module (GPCM)
   Using a vehicle-specific wiring diagram, verify the GPCM has proper power and ground inputs. Check for a command signal from the PCM. If all inputs are good but there is no voltage output on the wire for cylinder 1 during the cycle, the GPCM is faulty and requires replacement.
   Tools: Digital Multimeter (DVOM), Vehicle-Specific Wiring Diagram (Advanced)
7. Advanced: Perform a Voltage Drop Test
   To find hidden resistance, connect the DVOM's negative lead to the battery's negative terminal and the positive lead to the glow plug's body to test the ground side. To test the power side, connect the positive lead to the battery's positive terminal and the negative lead to the glow plug wire's terminal. Activate the circuit. A reading above 0.2V on the power side or 0.1V on the ground side indicates excessive resistance.
   Tools: Digital Multimeter (DVOM) (Advanced)
8. Advanced: Perform a Current Ramp Test
   Using a DC amp clamp connected to an oscilloscope or graphing multimeter, clamp the wire leading to the cylinder 1 glow plug. Activate the glow plug cycle. A healthy plug shows a sharp spike in current draw to around 15-20 amps, tapering off as it heats up. A low or zero current draw confirms an open circuit.
   Tools: DC Amp Clamp, Oscilloscope or Graphing Multimeter (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Oil Temp: Below 40°F / 4°C (During a cold start attempt)
• Glow Plug Command: ON (Key-On, Engine-Off (pre-heat cycle))
• Battery Voltage: 10.5-12.5V (During glow plug activation (heavy load))
• Engine RPM: 0 (The fault is most often detected before the engine starts)

Related Codes

• P0670 — This is a general code for a Glow Plug Control Module Circuit Malfunction. P0670 indicates a fault with the module's overall function. Seeing both codes together strongly suggests the GPCM is the root cause.
• P0672, P0673, P0674, etc. — These codes are identical to P0671 but for different cylinders. Seeing multiple codes at once makes it unlikely that several plugs failed simultaneously. Suspect a common point of failure like the GPCM, relay, main fuse, or a bad ground.
• P0380 — This is an older, generic code for 'Glow Plug/Heater Circuit 'A' Malfunction.' On vehicles that show both, P0380 points to a primary system fault (like the relay) causing the specific cylinder code like P0671.
• P0562 — This code means 'System Voltage Low.' Low battery voltage causes the GPCM to malfunction and set erroneous glow plug circuit codes. Test your battery and charging system before diving into the glow plug circuit.

Climate & Environmental Factors

• Cold Climates: Glow plugs and their control systems are used most heavily in cold weather, leading to frequent thermal cycling and a higher failure rate. Hard starting symptoms are most apparent at temperatures below 40°F (4°C).
• Humidity and Moisture: High humidity or exposure to road salt accelerates corrosion on wiring connectors, ground straps, and GPCM pins, leading to high resistance and circuit faults.
• High Altitude: At higher altitudes, the air is less dense, making compression ignition more difficult. The glow plug system stays on longer to compensate, shortening the lifespan of the glow plugs.

How to Talk to a Mechanic About This Code

Say this: "I have a P0671 code on my [Vehicle Year, Make, Model] and I'd like to schedule a diagnostic. I believe it's a fault in the cylinder 1 glow plug circuit. I'd like you to test the glow plug, the wiring, and the control module to pinpoint the failure."

This signals you've done your research and understand the possible causes. It focuses the technician's diagnostic time on the correct components and prevents them from starting from scratch or suggesting unrelated services.

Avoid saying:

• 'My check engine light is on, can you look at it?'
• 'My truck is hard to start.'
• 'Just fix whatever is wrong.'

Questions to ask before authorizing the repair:

• What was the resistance reading on the cylinder 1 glow plug?
• Did you see proper voltage at the glow plug connector when the circuit was activated?
• If you're recommending replacing all the glow plugs, what was the condition or resistance of the other plugs?
• If you're recommending a new GPCM, how did you verify the module was the point of failure and not the wiring to it?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended for complex, newer, or German vehicles.
  Best for: Vehicles under warranty, Complex German brands (BMW, Mercedes, Audi) where DPF regeneration is tied to the glow plug system, Diagnosing manufacturer-specific quirks, like misleading ECU reports on Sprinters
  Downsides: Highest labor rates, Prefers to replace entire assemblies (like a full harness) instead of repairing a single wire (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most common diesel trucks, provided the shop is a reputable diesel specialist.
  Best for: Out-of-warranty American diesel trucks (Ford, GM, Ram), Owners looking for a better value than the dealer, Shops with a specific focus on diesel repair
  Downsides: Quality and expertise vary widely; must vet the shop's reputation and diesel experience (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID. This is not a job for a general-purpose chain shop.
  Best for: Simple services like oil changes and tires
  Downsides: Technicians generally lack the specific diesel expertise required for an accurate diagnosis, High pressure to sell parts leads to replacing components without proper testing (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 50% of the vehicle's private-party value, seriously consider selling or trading it in rather than repairing it.

• Car worth $8000, fix is $450: Fix it. This is a routine repair cost that is a small fraction of the truck's value.
• Car worth $8000, fix is $4500: Walk away. The initial P0671 was ignored, leading to DPF failure. The repair cost is over 50% of the vehicle's value.

What Scan Tool You Need for This Code

Minimum: A code reader that reads and clears codes and displays live data (Engine Temp, Battery Voltage).

A $20 reader just gives you the code. It cannot show you if the GPCM is commanding the plugs on or what the voltage is during the cycle, which is essential for diagnosis. You will be replacing parts blindly.

Budget: BlueDriver Pro (~$99) — Reads codes, provides freeze frame data, and graphs live data so you can watch battery voltage during the key-on cycle. A good starting point for a DIY diagnosis.

Mid-range: Foxwell NT510 Elite (~$180) — Offers manufacturer-specific diagnostics and basic bidirectional controls. It allows you to command the glow plug relay or module on/off, which is a huge step up for testing the circuit.

Professional: Autel MaxiCOM MK808 / MK808S (~$500) — Full bidirectional control. You command the GPCM to activate, letting you test for voltage at the plug connector without a helper. You run active tests on the relay and module itself, definitively isolating the point of failure.

Rent vs buy: For a one-time code read, use the free service at an auto parts store. To properly diagnose P0671 yourself, you need to see live data and test components, making buying at least a budget pick a worthwhile investment for any diesel owner.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the P0671 code.
2. Start the engine from cold and let it idle for 2-3 minutes.
3. Perform a complete drive cycle to allow readiness monitors to run.

Drive cycle (~30 minutes): From a cold start, drive in mixed city/highway conditions for 15-20 minutes, including several steady-speed cruises and stop-and-go periods. Allow the vehicle to idle for 5 minutes. Then, shut down and let it cool completely.

Readiness monitors affected: Comprehensive Component Monitor (CCM), Diesel Particulate Filter (DPF) Monitor, Exhaust Gas Sensor Monitor

Before emissions retest: drive at least 50 miles to fully set monitors.

Watch out for:

• Simply clearing the code with a scanner without a drive cycle fails to reset the readiness monitors.
• The code returns immediately if the underlying electrical fault was not properly fixed.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An active P0671 code causes an automatic failure of the smog check. All readiness monitors must be 'Ready' to pass, requiring a proper drive cycle after the repair.
• New York: The NYS inspection includes an OBD-II scan. The presence of a Check Engine Light and the P0671 code results in an emissions test failure.
• Texas: In the 17 counties requiring emissions testing, an illuminated Check Engine Light is an automatic failure. The inspection station detects if codes were recently cleared and rejects the vehicle until readiness monitors are set.

Most Commonly Affected Vehicles

• Ford F-250/F-350 Super Duty (with 6.0L & 7.3L Power Stroke) (2003-2007) — On 6.0L engines, the glow plug harnesses and GPCM are common failure points. On the older 7.3L, the glow plug relay is a very frequent issue.
• Chevrolet / GMC Silverado / Sierra 2500HD/3500HD (with 6.6L Duramax) (2004-2016) — Glow plug control modules (GPCM) are a notorious failure point, especially on 2004-2005 LLY models, often requiring a rebuild as new parts are unavailable. Later models are more prone to individual glow plug failures.
• BMW 335d, X5 xDrive35d (E90, E70) (2009-2013) — On the M57 diesel engine, the glow plug control unit is a much more common failure than the plugs themselves. A faulty controller prevents DPF regeneration.
• Audi / Volkswagen A3, A6, Golf, Jetta (TDI) (1999-2015) — The glow plug wiring harness is a known weak point that degrades over time. A failed glow plug is often identified visually by a black, sooty tip.
• Dodge / Mercedes-Benz Sprinter (2002-2009) — Extremely prone to glow plug and control module failures. The ECU frequently reports an incorrect cylinder; a P0671 code is often caused by a shorted plug in cylinder #2.
• Hyundai / Kia Tucson, Santa Fe, Sorento (CRDi) (2011-2018) — Issues stem from corroded ground wires for the glow plug control system, triggering multiple glow plug codes simultaneously.
• Jeep Grand Cherokee (3.0L CRD) (2007-2008) — These models use a Mercedes-Benz diesel engine (OM642) and frequently suffer from failures of the glow plug control module, a part shared with Sprinter vans.
• Ford Ranger (T6, International) (2011-2022) — Diesel variants of the global Ford Ranger commonly log P0671 for standard glow plug or wiring faults.

Manufacturer-Specific Notes

• Ford: On 6.0L PowerStroke engines, the glow plug control module (GPCM) and fuel injection control module (FICM) are separate units mounted near each other and are often confused. The glow plug wiring harnesses are notoriously brittle and break during service.
• General Motors (Chevrolet/GMC): On Duramax engines, cylinder numbering is odd on the passenger side (1, 3, 5, 7) and even on the driver's side (2, 4, 6, 8). The GPCM on 2004-2005 LLY engines is a well-documented failure point; reputable rebuild services are better than unreliable aftermarket replacements.
• Volkswagen / Audi: On many TDI engines, a failed glow plug is visually identified. A good plug has a clean, light grey tip, while a failed one is black and covered in soot. Some 2.0L models had a faulty glow plug relay that supplied excessive voltage, burning out plugs prematurely.
• Mercedes-Benz / Dodge: On Sprinter vans, the ECU's fault reporting is misleading. It commonly stores a P0671 'open circuit' code for cylinder 1 when the actual problem is a shorted glow plug in a different cylinder. Always test all glow plugs.

Real Owner Stories

2006 Ford F-250 6.0L Power Stroke - Brittle Harness

Owner had a P0671 code and a Check Engine Light but no issues starting the truck.

What they tried:

1. Tested the resistance of the #1 glow plug and got a reading of 0.02 ohms, indicating a fault.
2. Broke the brittle harness connector while removing it.

Outcome: Replaced the single glow plug and the passenger-side harness. Cleared the code with a scanner, and the Check Engine Light did not return.

Lesson: On high-mileage or older trucks, especially the 6.0L Power Stroke, plastic wiring harnesses become very brittle. Expect them to break and have a replacement harness on hand before starting the job.

2011 GMC Sierra 2500HD 6.6L Duramax - Misdiagnosis

P0671 code appeared. Owner replaced all 8 glow plugs as a preventative measure, but the code would not clear.

What they tried:

1. Replaced all glow plugs with a non-OEM brand.
2. After the code returned, they were advised to check the new plugs and wiring.

Outcome: The issue was either a faulty new glow plug out of the box or an underlying wiring/GPCM issue. The owner swapped the #1 and #3 glow plugs to see if the code followed the plug or stayed at cylinder #1.

Lesson: Don't assume a new part is a good part. When a code returns immediately after a repair, verify the new part is functional. Using OEM parts prevents compatibility issues.

Mercedes-Benz Sprinter - Unusual Root Cause

Vehicle presented with a P0671 code for an open circuit on cylinder 1.

What they tried:

1. Initial diagnosis focused on testing the glow plug and wiring for cylinder 1, which appeared to be fine.

Outcome: Testing revealed the actual fault was a shorted glow plug in cylinder #2. The ECU incorrectly reported the fault as an open circuit on cylinder #1. Replacing the glow plug in cylinder #2 resolved the P0671 code.

Lesson: On Mercedes-Benz/Dodge Sprinters, the ECU's fault reporting is misleading. A code for one cylinder is often caused by a different type of fault on another. Test all glow plugs in the system.

2007 GMC Sierra 3500 Duramax - Intermittent Issue

Owner reported a P0671 code that only appeared when the truck sat for several days. There were no noticeable symptoms.

What they tried:

1. The owner sought advice before a trip, concerned about the severity.

Outcome: Forum members advised that with no symptoms in warm weather, the issue was not critical for an immediate trip. The problem was a glow plug at the beginning of its failure, only detected during a cold-start self-test.

Lesson: An intermittent P0671 code without symptoms is a warning. It becomes a hard fault eventually and must be addressed before cold weather arrives to prevent a no-start situation or DPF regeneration issues.

How to Prevent This Code From Triggering

• Use High-Quality Diesel Fuel (Every fill-up) — Lower quality fuel has impurities that lead to incomplete combustion and increased carbon buildup on glow plug tips, insulating them and reducing heating efficiency.
• Test Glow Plugs Before Winter (Annually) — A simple resistance test identifies a failing plug before it leaves you stranded in cold weather. This allows for replacement in a controlled, non-emergency situation.
• Clean Glow Plug Threads and Bores During Replacement (During replacement) — Carbon buildup in the glow plug bore is a primary reason plugs seize and break during removal. Using a glow plug reamer or wire brush to clean the bore ensures proper heat transfer for the new plug and makes future removal easier.
• Use a Torque Wrench for Installation (During replacement) — Over-tightening a glow plug damages it or strips the threads in the aluminum cylinder head. Under-tightening leads to poor electrical ground and heat transfer. Always use the manufacturer's specified torque.
• Consider Using Anti-Seize Sparingly and Correctly (During replacement) — Applying a small amount of high-temperature, electrically conductive anti-seize to the threads aids future removal. If you use it, you must reduce the torque value by 25-30% to avoid over-tightening.

Frequently Asked Questions

Can I drive with a P0671 code?

You can, but you shouldn't. You risk being stranded by a no-start condition in cold weather. More importantly, you risk clogging your Diesel Particulate Filter (DPF), leading to a multi-thousand dollar repair.

I replaced the glow plug, but the P0671 code came back. What's next?

The fault lies elsewhere. Re-inspect your work, ensuring the connector is clean and tight. Next, perform a voltage drop test on the power and ground wires to find hidden resistance, or test the Glow Plug Control Module (GPCM).

Should I replace all glow plugs if only one is bad?

Yes, it is highly recommended. Glow plugs wear at a similar rate, so if one fails, the others are near the end of their service life. Replacing them all at once saves significant future labor costs.

My multimeter says the glow plug resistance is okay, but the code persists. Why?

A simple cold resistance test misses failures that only occur under heat and load. The fault is likely intermittent. Use a current ramp test or a voltage drop test to observe the active circuit.

Can a bad glow plug damage my engine?

Direct engine damage is unlikely from a single bad glow plug. The primary risks are excessive strain on the starter and battery from prolonged cranking. The most expensive secondary risk is costly damage to the DPF from failed regeneration cycles.

Why does my scanner say P0671, but the problem is with another cylinder's glow plug?

This is a known diagnostic quirk on some vehicles, particularly Mercedes-Benz Sprinters. The engine computer incorrectly reports a fault for one cylinder when the actual short circuit is in another. Always test all glow plugs, not just the one flagged by the code.

What is the hardest part about fixing a P0671 code?

The biggest challenge is physical access, as glow plugs are often buried under other components. Old glow plugs frequently seize in the cylinder head and snap during removal. This turns a simple job into a major repair requiring specialized extraction tools.

What does a bad glow plug look like?

You can often visually identify a failed glow plug. A healthy, working glow plug has a light grey or whitish-colored tip. A failed plug typically has a black, sooty tip caked with carbon.

Key Takeaways

• P0671 flags an electrical failure in the cylinder 1 glow plug circuit, preventing the cylinder from heating properly during cold starts.
• A burnt-out glow plug causes over 70% of P0671 codes, with brittle wiring harnesses and failed Glow Plug Control Modules (GPCMs) making up the rest.
• Expect hard starting, a rough idle for the first 60 seconds, and white exhaust smoke when ambient temperatures drop below 40°F (4°C).
• Ignoring P0671 prevents Diesel Particulate Filter (DPF) regeneration, turning a $150 glow plug repair into a $3,000+ DPF replacement within 6 months.