Symptom
White smoke from exhaust on cold start
Thick white smoke when starting a cold diesel, clearing as the engine warms. This is unburnt fuel rather than steam — the combustion chamber was too cold to ignite it properly, which usually means glow plugs are not doing their job. Distinct from the thin white vapour any exhaust produces in cold weather, which is harmless condensation.
Also described as: white smoke, white smoke on startup, smoking when cold, unburnt fuel smoke, diesel white smoke, smoke until warm, دود سفید غلیظ از اگزوز, دود سفید که نمیرود, humo blanco en frio, humo blanco al arrancar, humo blanco escape, دخان أبيض على البارد, دخان أبيض عند التشغيل, دخان أبيض من الشكمان
Some causes mean you should stop driving
2 of the 40 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
This is unburnt fuel rather than steam — the combustion chamber was too cold to ignite it properly, which usually means glow plugs are not doing their job. That distinction is the whole diagnosis, and it is easy to make: unburnt diesel smells strongly of fuel and hangs in the air; steam has no smell and dissipates.
Distinct from the thin white vapour any exhaust produces in cold weather, which is harmless condensation and has its own entry. Thick, smelly and slow to clear is this; thin, odourless and gone within a minute is that.
Glow plugs are the first candidate and they are measurable rather than inferable. Each has a low resistance and they should agree closely across the set; one failed plug in four is routinely tolerated in mild weather and only shows in a hard frost, which is why this complaint is seasonal. The control module and relay fail as often as the plugs, so confirm the circuit energises for the pre-heat period before condemning four plugs — and know before starting that a seized plug shearing on removal turns a small job into a cylinder head.
Low compression produces the same picture and should be considered where the plugs test good: a cold cylinder that cannot reach ignition temperature will not fire until the engine warms.
And injection timing or poor atomisation puts fuel in at the wrong moment or in the wrong form, which burns incompletely when cold and adequately when hot.
White smoke that persists once the engine is warm is a different fault entirely — that is coolant, and it belongs with the head gasket question.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Thick white smoke from cold, smelling strongly of fuel, clearing as it warms | Unburnt fuel — the chamber is too cold to ignite it, usually the glow plug circuit | Smell it: unburnt diesel is unmistakable and steam has no smell. Measure each glow plug's resistance and compare across the set. | Renew the plugs as a set with the relay or control module where that is the failure. |
| Only in hard frost, fine in mild weather | One or two failed plugs the engine tolerates until it is properly cold | The seasonal pattern is itself evidence. Resistance across the set shows the outliers. | Renew as a set — fitting only the failed ones leaves the rest at the same age. |
| Plugs and relay test good, smoke persists cold | Low compression on one or more cylinders, or injection timing and atomisation | Take a cold compression test, and read injector correction values from a cold start. An outlier names the cylinder. | Repair the injector or the compression fault on its own findings. |
| Smoke continues once the engine is thoroughly warm | Not this fault — coolant entering the cylinders | Warm persistence is the discriminator, along with a sweet smell and a falling coolant level. | Pursue as a head gasket question. See the coolant-loss entry in this vocabulary. |
| Glow plug lamp flashing rather than simply lit | On most diesels the coil lamp doubles as the engine fault lamp — this may not be about the plugs at all | Read the codes. A flashing lamp is that vehicle's check engine light. | Diagnose the stored code before fitting glow plugs. |
| Thin, odourless white vapour that clears within a minute | Condensation — the exhaust working correctly in cold weather | No smell, thins to nothing, does not return once warm. This is its own entry in this vocabulary. | No repair. |
-
Thick white smoke from cold, smelling strongly of fuel, clearing as it warms
- Likely cause
- Unburnt fuel — the chamber is too cold to ignite it, usually the glow plug circuit
- How to confirm it
- Smell it: unburnt diesel is unmistakable and steam has no smell. Measure each glow plug's resistance and compare across the set.
- What fixes it
- Renew the plugs as a set with the relay or control module where that is the failure.
-
Only in hard frost, fine in mild weather
- Likely cause
- One or two failed plugs the engine tolerates until it is properly cold
- How to confirm it
- The seasonal pattern is itself evidence. Resistance across the set shows the outliers.
- What fixes it
- Renew as a set — fitting only the failed ones leaves the rest at the same age.
-
Plugs and relay test good, smoke persists cold
- Likely cause
- Low compression on one or more cylinders, or injection timing and atomisation
- How to confirm it
- Take a cold compression test, and read injector correction values from a cold start. An outlier names the cylinder.
- What fixes it
- Repair the injector or the compression fault on its own findings.
-
Smoke continues once the engine is thoroughly warm
- Likely cause
- Not this fault — coolant entering the cylinders
- How to confirm it
- Warm persistence is the discriminator, along with a sweet smell and a falling coolant level.
- What fixes it
- Pursue as a head gasket question. See the coolant-loss entry in this vocabulary.
-
Glow plug lamp flashing rather than simply lit
- Likely cause
- On most diesels the coil lamp doubles as the engine fault lamp — this may not be about the plugs at all
- How to confirm it
- Read the codes. A flashing lamp is that vehicle's check engine light.
- What fixes it
- Diagnose the stored code before fitting glow plugs.
-
Thin, odourless white vapour that clears within a minute
- Likely cause
- Condensation — the exhaust working correctly in cold weather
- How to confirm it
- No smell, thins to nothing, does not return once warm. This is its own entry in this vocabulary.
- What fixes it
- No repair.
Codes that produce this symptom (40)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0380 Glow Plug/Heater Circuit A Typical of this code Moderate
- P052F Glow Plug Control Module 1 System Voltage Typical of this code Moderate
- P0540 Intake Air Heater A Circuit Typical of this code Low
- P0543 Intake Air Heater A Circuit Open Typical of this code Low
- P064C Glow Plug Control Module 1 Typical of this code Moderate
- P066A Cylinder 1 Glow Plug Control Circuit Low Typical of this code Moderate
- P066C Cylinder 2 Glow Plug Control Circuit Low Typical of this code Moderate
- P066E Cylinder 3 Glow Plug Control Circuit Low Typical of this code Moderate
- P0670 Glow Plug Control Module 1 Control Circuit/Open Typical of this code Moderate
- P067A Cylinder 4 Glow Plug Control Circuit Low Typical of this code Moderate
- P06B9 Cylinder 1 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P06BA Cylinder 2 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P06BB Cylinder 3 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P06BC Cylinder 4 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P06BD Cylinder 5 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P06BE Cylinder 6 Glow Plug Circuit Range/Performance Typical of this code Moderate
- P2604 Intake Air Heater A Circuit Range/Performance Typical of this code Moderate
- P2605 Intake Air Heater B Circuit/Open Typical of this code Moderate
- P2606 Intake Air Heater B Circuit Range/Performance Typical of this code Moderate
- P2607 Intake Air Heater B Circuit Low Typical of this code Moderate
- P2608 Intake Air Heater B Circuit High Typical of this code Moderate
- P2609 Intake Air Heater System Performance Typical of this code Moderate
- P263E Glow Plug Control Module 1 Over Temperature Typical of this code Moderate
- P26FB Exhaust Gas Recirculation Cooler Bypass Control Stuck Closed Bank 1 Typical of this code Moderate
- U0106 Lost Communication With Glow Plug Control Module 1 Typical of this code Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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