OBD-II Code P0672: Cylinder 2 Glow Plug Circuit Failure

What Does P0672 Mean?

P0672 signifies the Powertrain Control Module (PCM) detected a fault in the electrical circuit for the cylinder #2 glow plug. A glow plug acts as a miniature heater within the diesel engine's cylinder, warming it to enable fuel ignition during a cold start. This code points to an open or shorted electrical condition, meaning the circuit is broken or operating outside manufacturer voltage or resistance parameters.

Technical definition: The official SAE/OBD-II definition for P0672 is "Cylinder 2 Glow Plug Circuit/Open". This indicates the engine control module registered the cylinder 2 glow plug electrical circuit drawing an incorrect amount of current, or none at all, when activated. The module monitors each circuit individually and sets this code when feedback voltage or resistance falls outside the expected range.

Can I Drive With P0672?

Yes, But With Caution. Yes, you can drive with a P0672 code, but it is not recommended for extended periods in cold climates. You will experience hard starting, rough idling, and increased emissions. Ignoring it strains your starter and battery, and unburnt fuel damages the Diesel Particulate Filter (DPF) over time, causing a $2,000+ repair. Address the issue promptly to avoid being stranded.

Common Causes

• Failed Cylinder 2 Glow Plug (Very Common) — The glow plug burned out or suffered an internal failure, creating an open circuit. Glow plugs have a finite lifespan and are the most frequent culprit, showing an out-of-spec resistance reading.
• Damaged Wiring or Connector (Common) — The electrical connector attached to the glow plug or the main wiring harness is loose, frayed, corroded, or broken. Constant engine vibration, heat, and moisture degrade these components.
• Failed Glow Plug Control Module (GPCM) (Less Common) — The dedicated module controlling glow plug operation has an internal fault affecting the cylinder 2 driver. This is a known issue on Duramax and Power Stroke platforms.
• Failed Glow Plug Relay or Fuse (Less Common) — A main relay or fuse providing power to the glow plug system failed. While this typically triggers multi-cylinder codes (like P0670), it sometimes presents as a single cylinder fault initially.
• Poor Ground Connection (Rare) — A corroded or loose ground strap for the engine block causes high resistance in the glow plug circuit. The circuit requires a solid ground to complete the electrical path.
• Faulty Powertrain Control Module (PCM/ECM) (Very Rare) — The specific driver circuit within the main engine computer that commands the GPCM failed, or the software contains a glitch requiring a dealer update.

Symptoms

• Hard Starting in Cold Weather — The engine cranks excessively before starting, or fails to start entirely as ambient temperatures drop below 40°F (4°C).
• White Smoke from Exhaust on Startup — Visible white smoke emits from the tailpipe for a short period after a cold start, caused by unburnt fuel from the cold cylinder.
• Rough Idle After Startup — The engine runs unevenly, shakes, or misfires for the first few minutes after starting cold, then smooths out as it warms up.
• Flashing Glow Plug Light — On vehicles from manufacturers like Volkswagen, the glow plug indicator flashes continuously to signal a system fault.
• Decreased Fuel Economy — An engine failing to combust efficiently during its warm-up phase consumes more fuel, dropping MPG.
• Check Engine Light On (also visible on scanner) — The Malfunction Indicator Lamp (MIL) illuminates on the dashboard, storing the P0672 code.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace the Cylinder 2 Glow Plug — Parts: $25-$60, Labor: $90-$150, ~1.5 hr book time (DIY)
• Replace All Glow Plugs — Parts: $150-$350, Labor: $250-$450, ~3.5 hr book time (Intermediate)
• Repair or Replace Wiring Harness/Connector — Parts: $15-$75, Labor: $100-$300, ~1.5 hr book time (Intermediate)
• Replace the Glow Plug Control Module (GPCM) — Parts: $200-$350, Labor: $85-$150, ~1 hr book time (Professional)
• Replace the Powertrain Control Module (PCM/ECM) — Parts: $800-$1,200, Labor: $200-$300, ~1.5 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: For the Glow Plug Control Module (GPCM), a used OEM unit from a low-mileage vehicle is a cost-effective alternative to new aftermarket parts, which are unreliable for models like Duramax. Always buy glow plugs new.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the part number matches exactly.
• Source from a reputable salvage yard with a return policy.
• Avoid parts from vehicles with high mileage or known electrical issues.

Decision logic:

• If The failed part is the glow plug itself → Always buy new. It is a low-cost wear item.
• If The failed part is the GPCM and the vehicle is a high-value truck → Favor a new OEM part or a reputable remanufactured unit to avoid compatibility issues.
• If The failed part is the GPCM and budget is the primary concern → A used OEM module from a low-mileage donor is a viable option, but carries a higher risk of premature failure.

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

Worst-case if a used part fails: $200-$400 if a used GPCM fails after installation, requiring repeat labor costs.

What Happens If You Wait — Timeline

1. 0-1 month: Check Engine Light illuminates with code P0672. In temperatures above 40°F (4°C), there are no noticeable symptoms. In colder weather, the engine cranks for an extra second before starting. (MPG impact: 0-1%% · Added cost: $0)
2. 1-4 months: Hard starting in cold weather becomes consistent, requiring multiple key cycles. A puff of white smoke is visible from the exhaust for up to a minute after starting. The starter and battery are under increased strain. (MPG impact: 1-5%% · Added cost: $50-$100 in wasted fuel and accelerated battery wear.)
3. 4-12 months: The engine runs noticeably rough and misfires for the first several minutes after a cold start. Fuel efficiency drops as unburnt fuel coats the Diesel Particulate Filter (DPF). (MPG impact: 5-10%% · Added cost: $300-$800 if the starter motor fails from excessive cranking.)
4. 12+ months: The DPF becomes heavily clogged with soot. This forces frequent regeneration cycles, eventually leading to permanent DPF damage. Severe exhaust backpressure damages turbocharger seals. (MPG impact: 10-15%% · Added cost: $2,000-$5,000+ for a mandatory DPF replacement.)

Cost of Not Fixing It

• 0-3 months: Hard starting in cold weather causes excessive strain on the starter motor and battery. Noticeable white smoke on startup and a drop in fuel economy. (Added cost: $300-$800 for a new starter if it fails due to repeated, long cranking cycles.)
• 3-12 months: Unburnt fuel from the non-firing cylinder clogs the Diesel Particulate Filter (DPF), forcing more frequent regeneration cycles and reducing DPF lifespan. (Added cost: $300-$1,500 for professional DPF cleaning if it becomes heavily loaded with soot.)
• 12+ months: Catastrophic failure of the Diesel Particulate Filter (DPF) due to excessive soot loading. Severe exhaust backpressure damages the turbocharger. (Added cost: $2,000-$7,000+ for DPF replacement and turbocharger repairs.)

Diagnosis Steps

1. Scan for Codes & Review Freeze Frame Data
   Use an OBD-II scanner to confirm P0672. Check for other glow plug codes (P0670, P0671). Multiple codes strongly suggest a shared problem like a GPCM or relay. Review freeze frame data for engine temperature when the code set.
   Tools: OBD-II Scanner (Beginner)
2. Visual Inspection
   Locate the cylinder #2 glow plug. Inspect its electrical connector and visible wiring for corrosion, melting, or frayed insulation. A burnt or loose connector is a common, easily identifiable cause.
   Tools: Flashlight, Inspection Mirror (Beginner)
3. Test Glow Plug Resistance
   Disconnect the cylinder 2 glow plug connector. Set a multimeter to Ohms (Ω). Place one probe on the plug's terminal and the other on a clean engine ground. A healthy plug reads 0.1 to 2.0 Ohms. A reading of 'OL' (Over Limit) confirms the plug failed.
   Tools: Multimeter, Socket set (Intermediate)
4. Check Voltage at the Connector
   Have a helper turn the ignition 'ON' (engine off). Using a multimeter set to DC Volts, test the wiring connector that attaches to the glow plug. It must show battery voltage (11-14V) for a few seconds as the glow plugs are commanded on. Zero voltage points to a wiring or GPCM issue.
   Tools: Multimeter, Helper (Intermediate)
5. Swap Test the Glow Plug
   If you lack a multimeter, swap the suspect cylinder 2 glow plug with a known good one from another cylinder. Clear codes and start the engine. If the code changes to the new cylinder, the plug is faulty. If P0672 returns, the wiring or module failed.
   Tools: Socket Set, Torque Wrench, OBD-II Scanner (Intermediate)
6. Test the Glow Plug Control Module (GPCM)
   If the plug and wiring pass, test the GPCM. Verify it receives correct power, ground, and PCM command signals using a vehicle-specific wiring diagram. Incorrectly probing the GPCM causes permanent damage.
   Tools: Multimeter, Vehicle-specific wiring diagram, Back-probe pins (Advanced)
7. PRO TIP: Perform a Voltage Drop Test
   Connect the multimeter's positive lead to the battery positive and negative lead to the glow plug connector's power pin while active. A reading above 0.5V indicates excessive resistance in the power-side wiring. Repeat for the ground side.
   Tools: Multimeter with back-probe pins (Advanced)
8. PRO TIP: Analyze with a Current Clamp
   Use an oscilloscope with a low-amp current clamp around the cylinder 2 glow plug wire. A good plug shows a 20-25A initial spike, settling to 5-10A. An open circuit shows zero current.
   Tools: Oscilloscope, Low-Amp Current Clamp (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: Below 40°F (4°C) (During a cold start when the glow plug cycle is commanded.)
• Glow Plug Control Module Command: ON (Key-on, engine-off (pre-heat cycle) or initial cranking.)
• Battery Voltage: 10.5V - 12.5V (During the pre-heat cycle, as the glow plugs place a heavy load on the battery.)
• Engine RPM: 0 RPM (pre-heat) or < 200 RPM (cranking) (The fault is typically detected before the engine starts successfully.)

Related Codes

• P0670 — Indicates 'Glow Plug Control Module Circuit Malfunction.' Diagnose this first, as it points to a system-wide failure of the module, power, or ground.
• P0671, P0673, P0674, P0675, P0676, P0677, P0678 — Identical faults for other cylinders. Multiple codes point to a shared cause like the GPCM or wiring harness, rather than simultaneous plug failures.
• P0380 — Glow Plug/Heater Circuit 'A' Malfunction. Indicates a system-wide or bank-wide fault rather than a specific cylinder.

Climate & Environmental Factors

• Cold Climate: P0672 is most noticeable during cold starts (below 40°F / 4°C) when the engine struggles to ignite without all glow plugs functioning.
• High Humidity / Road Salt: Regions using heavy road salt experience higher corrosion rates on electrical connectors and ground points, increasing circuit resistance.
• Altitude: At higher altitudes, less dense air makes cold starting difficult, forcing the PCM to command longer glow plug cycles and accelerating wear.

How to Talk to a Mechanic About This Code

Say this: "I have a P0672 code on my diesel truck and it's having trouble starting cold. I'd like to book a diagnostic appointment. Please start by testing the resistance of the #2 glow plug and checking for voltage at the connector. I'd also like you to check the resistance of the other glow plugs from the harness to see if any others are failing."

This signals you have done your research and guides the technician toward a logical diagnostic path. It prevents them from immediately blaming an expensive control module and encourages testing the cheapest failure points first.

Avoid saying:

• 'My check engine light is on, can you fix it?'
• 'It won't start right, just do whatever it takes.'
• 'My friend said it's probably the computer, can you replace it?'

Questions to ask before authorizing the repair:

• What was the resistance reading on the cylinder 2 glow plug?
• Did you get battery voltage at the connector when the glow plugs were commanded on?
• If the glow plug is bad, what is the cost to replace all of them at once versus just the one?
• If the glow plug and wiring are good, what is the next diagnostic step and its associated cost?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended if the vehicle is under warranty or if initial diagnosis by an independent shop points to a complex module or programming failure.
  Best for: Vehicles still under powertrain or emissions warranty., Complex, recurring electrical issues or manufacturer-specific problems., When a PCM/GPCM replacement and programming is confirmed necessary.
  Downsides: Highest labor rates, often 1.5-2x more than independent shops., May recommend replacing an entire harness or module when a simpler repair is possible. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most P0672 scenarios. A good independent diesel shop handles glow plug, GPCM, and wiring issues efficiently and cost-effectively.
  Best for: Out-of-warranty vehicles, especially common US-brand diesel trucks., Straightforward glow plug and wiring harness repairs., Cost-conscious owners who want a trusted relationship with a mechanic.
  Downsides: Quality varies greatly; look for shops specializing in diesel repair with ASE-certified technicians., May lack expensive, dealer-level programming tools for the newest models. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for P0672 diagnosis and repair. This is not a simple bolt-on repair and requires diesel-specific knowledge typically absent at chain shops.
  Best for: Simple services like oil changes, tires, and battery replacement.
  Downsides: Technicians often lack specialized diesel engine training., High pressure to upsell; may misdiagnose the issue and recommend unnecessary parts., Unlikely to be equipped for the risks of glow plug replacement (e.g., breakage). (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the total estimated repair cost for the P0672 fault and any other known issues exceeds 40-50% of the truck's current private-party market value, seriously consider selling the vehicle as-is or trading it in.

• Car worth $15000, fix is $1200: Fix it. The repair cost is well below the threshold and will restore key functionality.
• Car worth $8000, fix is $3500: Borderline. This repair is a significant percentage of the truck's value. Get a second opinion before proceeding.
• Car worth $4000, fix is $2500: Walk away. The repair cost is over half the vehicle's value. It is not an economically sound investment.

What Scan Tool You Need for This Code

Minimum: A basic OBD-II code reader that can read and clear powertrain codes. For P0672, this is enough to confirm the code and see if it returns after a repair.

A basic scanner cannot command the glow plug system to turn on for testing purposes, nor can it read manufacturer-specific codes. It offers no live data to see what the GPCM is doing.

Budget: BlueDriver Pro (~$100) — Pairs with a smartphone app to read/clear codes, view freeze frame data, and stream live data from the GPCM. It helps you see if the module is attempting to command the glow plugs on.

Mid-range: Foxwell NT510 Elite / HD500 (~$150) — Offers manufacturer-specific diagnostics with bidirectional control to perform a 'glow plug test,' manually commanding the GPCM to activate the circuit so you can test for voltage without a helper.

Professional: Autel MaxiCOM MK808 / MK906BT (~$500-1200) — Provides full bidirectional control to test the GPCM and individual circuits. It accesses all vehicle modules, reads OEM-specific codes, and graphs live data with high precision.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the diagnostic trouble codes.
2. Start the engine from cold and allow it to complete a full warm-up cycle.
3. Perform a vehicle-specific drive cycle to run all readiness monitors.

Drive cycle (~30 minutes): A diesel drive cycle requires a cold start (engine off for 8+ hours), followed by a warm-up period. Drive at a steady highway speed (55 mph) for 10-15 minutes, followed by mixed city driving with several stops.

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

Before emissions retest: drive at least 100 miles to fully set monitors.

Watch out for:

• Simply clearing the code does not fix the underlying problem and the code will return.
• Disconnecting the battery clears the code but resets all readiness monitors, requiring a lengthy drive cycle before an emissions test.
• Not allowing the engine to cool down completely (cold soak) before starting the drive cycle prevents certain monitors from running.

Will This Fail Emissions / State Inspection?

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

• California: An active P0672 code causes an automatic failure of the Smog Check. All OBD-II readiness monitors must be complete, requiring driving over 100 miles after repair.
• New York: A vehicle with an illuminated Check Engine Light and an active P0672 code fails the mandatory annual OBD-II emissions inspection.
• Texas: In the 17 counties requiring emissions testing, an active P0672 code results in an inspection failure. Readiness monitors must be in a 'Ready' state.

Most Commonly Affected Vehicles

• Ford F-250/F-350 Super Duty with 6.0L or 6.4L Power Stroke (2003-2010) — The GPCM is notoriously prone to failure due to its mounting location on the engine, exposing it to intense heat and vibration.
• Chevrolet / GMC Silverado/Sierra 2500/3500 HD with Duramax LLY, LBZ, LMM, LML Engines (2004-2016) — The GPCM on LLY (2004.5-2005) engines is a very common failure point. Replacing glow plugs on the driver's side is highly labor-intensive.
• Volkswagen Jetta/Golf/Passat with 2.0L TDI (2009-2014) — Glow plugs on these engines frequently seize in the cylinder head. Per VW TSB 2029514, a faulty glow plug relay supplies over-voltage, causing repeated burnouts.
• Dodge / Ram 2500/3500 with 6.7L Cummins (2007-2024) — Most Cummins engines use a single intake grid heater instead of individual glow plugs. A failing grid heater bolt falling into the engine causes catastrophic damage.
• Mercedes-Benz Sprinter (2500/3500) with OM642/OM647/OM651 Engines (2002-2018) — The V6 (OM642) and 5-cylinder (OM647) diesel engines are well-known for failures of both the glow plugs and the glow plug control module.
• Jeep Grand Cherokee with 3.0L EcoDiesel (2014-2019) — Failures of individual glow plugs and the control module are frequently reported on the VM Motori-built EcoDiesel engine.
• BMW 335d / 535d / X5 xDrive35d with M57/N57 Diesel (2008-2016) — The glow plug control unit is a common failure item located under the intake manifold, making replacement a labor-intensive job.

Manufacturer-Specific Notes

• Ford: On 6.0L/6.4L engines, the GPCM mounts directly to the valve cover, exposing it to extreme heat and vibration, causing high failure rates.
• Volkswagen/Audi: TDI glow plugs seize easily due to carbon buildup. TSB 2029514 notes a faulty relay supplies incorrect voltage, causing repeated burnouts.
• Chevrolet/GMC: Duramax LLY (2004.5-2005) GPCMs fail frequently. Avoid aftermarket replacements, as they cause data bus communication errors.
• Dodge/Ram: 6.7L Cummins engines use an intake grid heater instead of individual glow plugs. A broken grid heater bolt falling into the intake causes catastrophic engine damage.

Real Owner Stories

2004.5 Chevy Silverado (LLY Duramax) with persistent P0672

Owner bought the truck with a known P0672 code. The check engine light was on, but the truck was drivable.

What they tried:

1. Replaced the #2 glow plug, but the code returned.
2. Tested the wiring harness from the GPCM connector to the glow plug, showing good continuity.
3. Replaced the GPCM with aftermarket units, causing major data bus errors.
4. Installed a high-quality remanufactured OEM GPCM.

Outcome: The remanufactured OEM GPCM resolved the P0672 code. The issue was an internal failure of the original module's driver for cylinder 2.

Lesson: On LLY Duramax trucks, a single glow plug code is often caused by a failing GPCM. Avoid cheap aftermarket modules; opt for OEM or quality remanufactured units.

2006 Ford F-250 (6.0L Power Stroke) with confusing test results

Owner had a P0672 code and previously replaced the #2 glow plug, wiring harness, and GPCM, but the code returned.

What they tried:

1. Tested the resistance of all glow plugs from the GPCM connector.
2. Glow plug #2 had normal resistance (1.1 Ohms).
3. Glow plug #5 had an open circuit (no reading).

Outcome: The P0672 code was triggered, but the actual failed component was the glow plug in cylinder #5. Replacing #5 fixed the issue.

Lesson: Don't assume the code always points to the correct cylinder fault. Test the resistance of ALL glow plugs from the harness connector.

2006 VW Jetta TDI with a rough wiring harness

A P0672 code appeared on a Jetta with a visibly poor glow plug wiring harness held together with tape.

What they tried:

1. Swapped the glow plug from cylinder #2 with cylinder #4.
2. Cleared the codes and ran the engine. The P0672 code for cylinder #2 returned.

Outcome: Since the code did not follow the glow plug, the diagnosis pointed definitively to a problem in the wiring harness for cylinder #2.

Lesson: The 'swap test' is a powerful diagnostic step. If the fault code does not move with the component, the problem is in the wiring or controller.

How to Prevent This Code From Triggering

• Use High-Quality Diesel Fuel & Additives (Every fill-up) — Lower quality fuel leads to carbon deposits. Excessive carbon coats the glow plug tip, causing it to overheat or seize in the cylinder head.
• Apply Anti-Seize During Installation (Whenever replacing glow plugs) — Applying high-temperature anti-seize compound to the threads (NOT the tip) prevents seizing in the aluminum cylinder head, reducing the risk of breakage during future removal.
• Maintain a Healthy Battery and Charging System (Annually) — Glow plugs draw high current. A weak battery provides low voltage, forcing the system to work harder and tricking the control module into setting false circuit codes.
• Clean Harness Connectors (Every 60,000 miles or if working in the area) — Clean main GPCM and individual glow plug connectors with electrical contact cleaner and apply dielectric grease. This prevents moisture intrusion and high resistance.
• Address Hard Starting Immediately (As it occurs) — Repeated long cranking cycles put immense strain on the starter motor and battery, leading to premature failure.

Frequently Asked Questions

Should I replace all my glow plugs if only one has failed?

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 prevents repeating the same labor-intensive job.

What are the most common mistakes when diagnosing P0672?

The most frequent error is immediately replacing the expensive Glow Plug Control Module (GPCM) without performing basic tests. Always test the glow plug resistance (0.1-2.0 Ohms) and verify voltage at the connector (~12V) first.

Can a bad glow plug cause engine damage?

A failed glow plug itself will not damage the engine. The primary risk occurs during removal; if a seized tip breaks off into the cylinder, it causes catastrophic piston and valve damage.

Can I test a glow plug without a multimeter?

Yes, by swapping the suspected bad plug from cylinder 2 with a known good one from another cylinder. Clear the codes and run the engine. If the trouble code follows the plug to the new cylinder, the plug is faulty.

What's the difference between a glow plug and a spark plug?

A glow plug is a heating element used in a diesel engine to warm the cylinder, allowing fuel to ignite from compression. A spark plug is used in a gasoline engine to create a high-voltage spark that directly ignites the air-fuel mixture.

Can I fix a P0672 code myself?

If the cause is a bad glow plug on an accessible cylinder, it is a manageable DIY repair for intermediate mechanics. However, access is difficult on some V-engines, and you must stop immediately if the plug requires excessive force to turn.

Why does my truck only have trouble starting when it's cold?

Diesel engines rely on the heat generated by high compression to ignite fuel. A cold engine block absorbs this heat, preventing ignition until glow plugs provide the necessary initial warmth.

Can a bad battery cause a P0672 code?

Yes, a weak battery causes low system voltage during the heavy-draw glow plug activation cycle. The control module interprets this low voltage as a circuit fault and sets a false P0672 code.

How long does it take to replace a glow plug?

The time varies from 30 minutes for an easily accessible plug to over two hours for one buried under other components. On a Duramax, a driver-side plug requires removing the inner fender liner and steering shaft.

What does a P0672 code mean on a Duramax?

On a Duramax, P0672 points to a fault in the glow plug circuit for cylinder #2, the front-most cylinder on the driver's side. Common causes are the glow plug itself or a failed Glow Plug Control Module (GPCM).

Key Takeaways

• P0672 indicates an electrical fault in the cylinder #2 glow plug circuit, which prevents the cylinder from heating up for cold starts.
• Test the glow plug's resistance first; a healthy plug reads between 0.1 and 2.0 Ohms, while a failed plug reads 'OL' (Over Limit).
• Check the glow plug connector for 11-14V during the initial key-on pre-heat cycle to rule out wiring or control module failures.
• Replace all glow plugs simultaneously if one fails, as they share identical lifespans and replacing the set prevents repeating a $250+ labor charge.