P0672 Cylinder Two Glow Plug Circuit: Diagnosis And Repair Roadmap
You just pulled code P0672, and it ties to cylinder 2 glow plug circuit open. Your diesel may start after a long crank, stumble at idle, or miss on cylinder 2 only when cold. The frustrating part is the fix is not always “replace one plug,” it can be the harness, relay, or control side. Read this roadmap to test it step by step.
P0672 means the PCM sees an open in the cylinder 2 glow plug circuit, usually from a failed plug or broken/worn wiring. Expect cold-start trouble and rough running until the engine is warm. You’ll get DIY-friendly tests (resistance, continuity, relay checks) plus realistic part and labor cost ranges so you can decide what to do next.
P0672 is the “Cylinder 2 Glow Plug Circuit Open” code, meaning the engine computer detects an open circuit on glow plug #2. Common symptoms include hard cold starts, long cranking, and a cylinder 2 misfire or rough idle. Typical fixes include replacing the #2 glow plug (often $100 to $300) or repairing wiring.
P0672 Cylinder 2 Glow Plug Circuit Meaning
OBD-II code P0672 means the powertrain control module (PCM/ECM) detected an open or otherwise abnormal electrical condition in the cylinder 2 glow plug circuit. “Open” in this context usually points to too much resistance, a broken wire, a failed glow plug element, or poor electrical contact in the feed or ground path for the number 2 plug.
Cylinder 2 glow plugs are used on many diesel engines, where the ECM controls a set of glow plugs to build combustion chamber heat before start and sometimes during warm-up. The ECM expects the circuit to draw current when commanded and to behave within a programmed electrical range for that specific cylinder position.
Cylinder 2 Scope (What The Ecm Is Watching)
Cylinder numbering is engine-dependent, but “cylinder 2” always refers to the second cylinder in the manufacturer’s firing order and wiring map. On a typical 4-cylinder diesel, cylinder 2 is often the front-to-middle cylinder depending on how the manufacturer numbers them, so you should confirm with the underhood label or wiring diagram before assuming which physical glow plug is “#2.”
On V6 and V8 diesels, the glow plug harness can be split by bank, and cylinder 2 might sit on a different bank than cylinder 1. ECM diagnostics usually check for a specific electrical signature per cylinder circuit, so the same code P0672 can still mean the open is in the wiring to the #2 glow plug even if other plugs work fine.
Code Description And What “Open” Usually Indicates
P0672 is commonly described as Cylinder 2 Glow Plug Circuit/Open. In practical terms, the ECM may not be seeing the expected current draw when it energizes the cylinder 2 glow plug, or it may detect voltage that does not match the commanded state.
| What ECM command expects | What triggers P0672 |
|---|---|
| Glow plug circuit current and voltage that match design | Circuit reads open (too high resistance) or behaves outside electrical limits for cylinder 2 |
| Stable control response during glow and, on some engines, after-start warm-up | Abnormal electrical behavior during the test window for cylinder 2 |
Why This Matters To Drivability
A cylinder 2 glow plug circuit open often limits preglow heat for that cylinder, so compression ignition can be harder, especially when the engine is cold. Cold-start weakness is the most common “early clue,” and repeated failures can also lead to rough idle for a short time after start as that cylinder lights later than the rest.
For DIYers, the key point is that P0672 is a circuit fault code, not just “a bad plug.” Treat it like an electrical diagnostic: the fastest wins come from identifying whether the open is inside the glow plug, in the connector, or in the harness that carries power and control to cylinder 2.
Shop tip: On most diesels, you save time by checking the cylinder 2 connector and harness integrity before swapping parts. A lot of P0672 cases come from corrosion, a cracked connector, or a wire rubbed through near the engine cover or valve cover area.
Difficulty rating (meaning-focused): 2/10 to understand and map the circuit, 6/10 to do hands-on electrical testing and safely inspect the harness and glow plug hardware. Professional help is worth it if the engine uses a glow control module or if you find wiring damage that needs proper repair, because one wrong fix can leave you with an intermittent open or repeated glow faults.
P0672 Symptoms And Impact
Cylinder 2 glow plug circuit faults produce hard cold starts, longer crank times, heavy white or gray smoke on startup, and rough or unstable idle until the engine warms. The fault commonly lights the check engine light and can push the engine into reduced-start or limp behavior on some diesel systems.
Common Symptoms
Most owners notice cold-start problems first, especially on mornings below 10 C (50 F) or after extended stops. Cranking takes longer, the engine may cough or produce extra smoke, and idle can be rough for several minutes while the ECU compensates.
Mil And Drive Impact
Illuminated MIL is typical when the control module detects open or short conditions in the cylinder 2 glow plug circuit. The code may appear alone or with other glow-related codes, and freeze-frame data will show ambient temperature and engine rpm at fault occurrence.
Driveability is usually affected primarily during startup and idle; once the engine warms, performance often returns to normal if only one glow plug is out. Persistent faults, damaged wiring, or a failed relay/module can lead to frequent restarts failing, rough running under load, and possible increased wear to the starter and battery from repeated long cranks.
| Repair Item | Typical Cost (approx) | Time | DIY Difficulty |
|---|---|---|---|
| Single glow plug replacement | $100 – $300 | 0.5 – 1.5 hours | Moderate |
| Wiring/connector repair | $50 – $400 | 0.5 – 3 hours | Moderate to Hard |
| Relay or module replacement | $200 – $500 | 1 – 2 hours | Moderate |
Safety: Always disconnect the negative battery terminal and allow the engine to cool before touching glow plugs or high-current wiring, and wear eye protection when working near fuel or hot components. If harness repair requires soldering inside the engine bay, consider professional service to avoid corrosion and insulation failures.
Expert tip: If starting problems begin only when cold and swapping the plug moves the fault to a different cylinder, the fix is almost always a single glow plug or connector, making it a reasonable DIY with basic tools and a torque wrench.
P0672 Diagnostic Workflow
Cylinder 2 glow plug circuit open codes require a methodical path because the fault can be inside the glow plug itself, the wiring between the plug and driver, or the glow control module/relay. The scan tool tells you which circuit is flagged, then live data and electrical checks confirm where the open happens. Start with power and ground confirmation before you condemn a module.
Isolation &Amp;amp;amp; Diagram Reference
Wiring diagrams vary a lot by manufacturer. Some diesels switch glow plug power through a dedicated glow control module that has internal current sensing, while others use a relay plus a controller, so the “open” could be detected at different points. Use the diagram to confirm whether cylinder 2 has its own dedicated output wire or if it shares a feed and only the terminal changes at the plug connector.
In practice, a clean isolation strategy is to test in this order: plug element, then harness between plug and module, then module output.
For example, if the cylinder 2 glow plug measures open at the plug connector with the module disconnected, the wiring is likely fine and the plug or its internal connection is the culprit.
For example, if the plug measures okay but continuity between the plug connector and the module output is open, the harness, splice, or connector interface is the likely failure point.
| Test location | What you’re proving | Common failure outcome |
|---|---|---|
| Cylinder 2 plug circuit at plug connector (module unplugged) | Glow plug element integrity | Open/infinite resistance points to plug or terminal issue |
| Cylinder 2 circuit between plug connector and module output | Harness/connector continuity | Open/infinite resistance points to harness break, splice, or pin damage |
| Module output to ground and feed voltage during glow request | Driver power and ground health | Large voltage drop or missing feed suggests supply/ground problem causing “open” detection |
Insider tip: If you find corrosion at cylinder 2 terminals, treat the harness connector as part of the repair. A “good” resistance reading right after cleaning can go open again once the connector warms and expands, especially on older GM Duramax and Mercedes diesels with high under-hood heat cycles.
P0672 Glow Plug Resistance Tests
Cylinder 2 glow plug electrical faults usually show up first as an open or excessively high resistance in the plug or its feed circuit. Resistance and continuity checks tell you quickly whether the fault is in the glow plug itself, the connector, or the wiring back toward the control/relay.
Ohm &Amp;amp;amp; Continuity Check (Cylinder 2 Plug And Lead)
Glow plug resistance is the most direct check you can do with power removed. Plan on disconnecting the glow plug electrical connector for cylinder 2 and measuring at the glow plug terminals (not at the harness side first, to avoid chasing a downstream wiring issue too early).
For a failed element, resistance typically reads as open (OL) or far above what the same engine’s other glow plugs measure.
For example, if cylinder 1 and cylinder 3 are similar to each other and cylinder 2 is drastically higher or open, cylinder 2 is your problem.
| Meter reading at cylinder 2 | Most likely cause | Next move |
|---|---|---|
| Open/OL on ohms and no continuity | Glow plug element failure or broken internal lead | Replace glow plug and then recheck charging and wiring resistance at connector |
| Much higher Ω than other cylinders | Glow plug degrading, carbon tracking, or partial open | Inspect connector and retest; if still high, replace plug |
| Similar Ω to other cylinders | Connector/wiring or relay/driver feed issue | Proceed to relay coil and power rail checks |
Pro tip: Compare cylinder 2 readings to the neighboring cylinders on the same engine. A “one plug is weird” measurement is far more actionable than chasing an absolute spec value without the exact test chart for your engine.
Relay Coil &Amp;amp;amp; Power Rail Check (Control Side And Feed Side)
After the glow plug itself tests out, the next suspects are the relay (or glow plug controller), the relay coil circuit, and the hot feed to the glow plug harness. A P0672 style open fault can be caused by a relay that never sends voltage to the cylinder 2 feed, or by a broken conductor in the branch.
For relay coil testing, you can usually identify whether the relay is being commanded by measuring voltage at the coil control terminal during the glow event, then comparing it with battery and ground. If the relay input never changes, the issue is upstream (controller/command or wiring), but if the coil gets voltage and the output stays dead, the relay contacts are the likely failure.
Difficulty rating for these tests is DIY-medium (2/5 to 3/5) if you have a decent multimeter and can safely access the glow plug connector area. If you find the relay/controller feed is intermittent or you need to backprobe at a connector with limited access, have a professional handle it, because connector damage and misreads are common when terminals are tight or heat-stressed.
P0672 Wiring &Amp;amp;amp; Connectors Check
Cylinder 2 glow plug circuit faults often come down to a broken conductor, loose terminal, or corrosion at the glow plug lead or controller output. A clean, tight connection is what keeps the circuit’s power and ground path stable during high-current operation. This check focuses on finding the open (or intermittently open) condition by inspecting the wiring and connector integrity around cylinder 2.
Connector Condition
| Finding during inspection | What it usually means | Typical direction for repair |
|---|---|---|
| Infinite/open resistance on cylinder 2 feed | Broken conductor, damaged splice, or failed connector pin | Repair wiring section or replace connector |
| Continuity flickers when harness is moved | Intermittent break inside insulation | Locate rub point, replace harness section |
| Corrosion on pins or dark heat staining | High resistance connection under glow load | Clean, inspect terminals, replace if loose or pitted |
| Loose terminal in connector | Intermittent open circuit | Repair/replace terminal or connector housing |
Difficulty: 2/5 (visual inspection and continuity checks). See a pro: 3/5 if you find open wiring inside the engine harness, or if replacement requires pulling major harness sections on tight-engine-bay platforms.
P0672 Replacement Procedures
Cylinder 2 glow plug replacement is the first repair you do once the circuit tests confirm the plug itself is open. Wiring repair or glow plug relay replacement comes after you rule out the plug and confirm the harness and control side are actually faulty.
Glow Plug Replacement
Glow plug replacement is a moderate DIY job on many diesels, but it can turn into a tough job if the plug is seized. Use a new copper or steel crush washer (if your engine uses one) and a glow plug with the exact part number for your cylinder and engine code.
For example, many GM Duramax, Ford Power Stroke, and Mercedes OM diesel applications use shared glow plug control strategies where the code points to the specific cylinder circuit, so replacing the wrong plug wastes time. For seized plugs, a professional pull-and-rethread is often cheaper than damaging threads in the head.
Harness/relay Replacement
Harness or relay replacement is the right move when the glow plug resistance checks good and wiring continuity from the control side fails, especially for an “open” circuit on cylinder 2. Glow plug relays and control modules are vehicle-specific, so match parts by part number and consult the wiring diagram before unplugging anything you cannot uniquely identify.
Torque guidance: Glow plug torque varies widely by engine and plug design, so use the OEM spec from your exact service manual. If you lack the spec, a reasonable approach is “snug to spec, not by feel,” because over-tightening can strip threads in the cylinder head or distort the glow plug seat.
| Repair path | Do this when tests show… | Typical parts cost range (rough) | Time estimate |
|---|---|---|---|
| Glow plug only | Cylinder 2 plug circuit is open at the plug | $100 to $300 for glow plug replacement | 1 to 2.5 hours |
| Harness repair | Cylinder 2 wiring is open, intermittent, or heat damaged | Varies widely by complexity | 1.5 to 4 hours |
| Glow control module/relay | Cylinder 2 output does not actuate when commanded, after wiring is verified | Module replacement can run $200 to $500 | 1 to 3 hours |
Best practice: Take connector photos before unplugging, label connectors by cylinder if your engine layout is tight, and keep the connector mating surfaces clean. A clean reassembly prevents recurring “open circuit” codes that come from poor terminal contact after a repair.
P0672 Costs, Time, And Safety
Cylinder 2 glow plug circuit open fixes can run from roughly $100 to $300 for a glow plug swap, while wiring repair work varies a lot. Engine control module replacement, when it is truly the cause, is often quoted around $200 to $500, but diagnostic time drives the final bill.
Parts &Amp;amp;amp; Labor Estimates
Labor time depends on how fast the harness and relay diagnostics point to the glow plug itself versus a broken wire, corroded connector, or failed control driver. Common DIY-friendly cases take less time because the fault is clear, but intermittent opens and deep connector damage add billable hours quickly.
For example, many shops quote a minimum diagnostic time first, then re-check current draw or continuity after each correction. Shop rates vary by region and vehicle, so these numbers are time and parts ballparks, not guarantees.
| Likely Cause (Code P0672) | Typical Parts Cost | Typical Labor Time | Rough Total Range |
|---|---|---|---|
| Cylinder 2 glow plug | $30 to $150 | 0.8 to 1.5 hr | $100 to $300 |
| Connector or wiring repair (open) | $20 to $250 (pins, pigtail, splice materials) | 1.0 to 3.0 hr | $150 to $600+ |
| Glow plug control module / driver | $150 to $450 | 1.0 to 2.5 hr | $200 to $500+ |
| Multiple faults (relay, fuse, grounds) | $10 to $300 | 1.5 to 4.0 hr | $250 to $900+ |
Safety &Amp;amp;amp; Driving Impact
Glow plug circuit work is safe in the sense that it is low-voltage at the plugs themselves, but diesel glow systems can still involve hot components, sharp metal, and tight engine-bay access. Wear eye protection and disconnect the battery before touching connectors or harness sections, especially if you need to move any control-module wiring.
For driving impact, an open on cylinder 2 usually means harder cold starts, extra white smoke during warm-up, and rough idle until the remaining cylinders stabilize. Extended running with a persistent open can increase fuel dilution and raise oil contamination risk because incomplete combustion is most common during repeated cold starts.
Pro tip: If the engine cranks slowly, you see severe misfire, or the exhaust smells strongly of unburned fuel, stop troubleshooting and get it diagnosed. Those symptoms can point beyond the glow circuit (fuel pressure, compression issues, or an intake restriction).
DIY difficulty is typically 2/5 for direct glow plug replacement on many common four-cylinder diesels, but it rises to 4/5 when the harness is buried or you suspect a control module fault. Professional service is the better move if the plug ohms good yet the circuit still reads open, or if you find damaged wiring that requires splicing beyond a simple connector repair.
Quick Summary
P0672 means the Cylinder 2 glow plug circuit is open, so you will inspect the plug, wiring, and related control components.
| Topic | What to Expect |
|---|---|
| Meaning of P0672 | Cylinder 2 glow plug circuit open |
| Likely Fix Area | Glow plug, wiring harness, connectors, and possibly glow plug control module |
| Typical Cost Range | Glow plug often $100 to $300, control module often $200 to $500, wiring varies |
| Best Next Step | Test Cylinder 2 glow plug and circuit continuity before replacing parts |
Frequently Asked Questions
What Does Obd2 Code P0672 Mean For Cylinder 2 Glow Plug?
P0672 means the PCM detected an open circuit fault in the glow plug circuit for cylinder 2. In plain terms, the control side is not seeing the expected electrical path or control signal for that specific glow plug.
What Usually Causes Code P0672 To Set On A Diesel?
Common causes include a failed cylinder 2 glow plug, an open or damaged wiring harness/connector, or a control related fault. If the harness or connector has corrosion or a broken wire, the PCM can interpret it as an open circuit.
How Much Does It Typically Cost To Fix P0672?
Repair cost varies a lot because the underlying cause can be simple or involved. One breakdown shows glow plug replacement often runs about $100 to $300, wiring harness repairs can vary widely, and control module replacement (if needed) is often cited around $200 to $500.
Can I Drive With A P0672 Code, Or Will It Cause A No-start?
It can still run, but drivability and cold starts are the first things that get worse. Since P0672 affects cylinder 2 glow plug operation, hard starting is more likely, especially when the engine is cold.
What Tests Should I Do First To Diagnose P0672 Correctly?
Start by inspecting the cylinder 2 glow plug connector and wiring for damage or corrosion, then verify the glow plug circuit condition before replacing parts. A smart approach is to confirm the specific “open circuit” behavior on cylinder 2, rather than assuming the glow plug is bad every time.
What Common Mistake Makes P0672 Come Back After A Glow Plug Replacement?
Replacing only the glow plug without fixing an open in the harness or connector is the big one. If the wire, terminal, or connection still has an open, the PCM will keep flagging cylinder 2 as faulty and the code will return.
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