Symptom
Rotten egg or sulphur smell from the exhaust
A sharp smell of rotten eggs from the exhaust, usually strongest under load or just after a hard drive. Fuel contains sulphur compounds, and a healthy catalytic converter finishes converting them; the smell means unburnt fuel is reaching the converter in quantities it cannot deal with, so the cause is normally an engine running rich rather than the converter itself. That distinction saves money: the converter is the expensive part and is usually the victim rather than the fault. It is worth acting on — a converter fed a rich mixture long enough is destroyed by it.
Also described as: rotten egg smell, sulphur smell, sulfur smell, smells like eggs, bad smell from exhaust, بوی تخم مرغ گندیده, بوی گوگرد از اگزوز, huele a huevo podrido, olor a azufre, escape huele a huevos, رائحة بيض فاسد, ريحة كبريت, رائحة كبريت من العادم
What this usually means
Fuel contains sulphur compounds, and a healthy catalytic converter finishes converting them; the smell means unburnt fuel is reaching the converter in quantities it cannot deal with. So the cause is normally an engine running rich rather than the converter itself.
That distinction saves money: the converter is the expensive part and is usually the victim rather than the fault. A converter replaced without curing the rich mixture is destroyed by the same fault within months, and the owner concludes that converters are unreliable.
It is worth acting on — a converter fed a rich mixture long enough is destroyed by it, so the smell is an early warning with a real deadline attached.
The rich-mixture population is the same short list as any over-fuelling complaint: a leaking injector, excessive fuel pressure, a coolant temperature sensor reading cold so enrichment never tapers, an air mass meter under-reading, or an oxygen sensor no longer correcting. Read the fuel trims, which say directly whether the management can see the problem.
Strongest under load or just after a hard drive is characteristic, because that is when the converter is hottest and the fuel demand highest — so the smell appearing exactly then is consistent with the mechanism rather than a coincidence.
Occasionally it is the fuel itself. A tank of high-sulphur fuel produces the smell with nothing wrong at all, and it clears with the next tankful — which is worth waiting one tank to establish before diagnosing anything.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Smell strongest under load or just after a hard drive | A rich mixture reaching a hot converter — the characteristic pattern | Read fuel trims at idle and under load. Heavily negative trims mean the management is fighting real over-fuelling. | Repair the fuelling fault. The converter is the victim; replacing it first repeats the failure. |
| Trims near zero with a persistent smell | The management cannot see the over-fuelling — the measuring sensor is at fault | Watch upstream oxygen sensor switching and check the coolant temperature signal against ambient on a cold engine. | Renew the sensor found. Fuelling components will not help when the measurement is wrong. |
| Appeared after one particular fill and cleared with the next | High-sulphur fuel — nothing wrong with the car | Wait one tankful before diagnosing. This is cheap and it resolves a real share of these complaints. | No repair. Note the station if it recurs there. |
| Smell with rough running or misfire counts | A misfire delivering unburnt charge to the converter | Read counters per cylinder and act on the named cylinder. | Repair the misfire urgently. This combination destroys converters quickly. |
| Smell with poor performance and a restricted feel | The converter has already been damaged and is now restricting the exhaust | Measure backpressure: under about 1.5 psi at idle and 3 psi at 2,000 rpm on most engines. | Renew the converter AND repair the fuelling fault. Doing only one of the two guarantees a repeat. |
| Smell with a strong fuel smell as well | Substantial unburnt fuel rather than a marginal mixture | Two symptoms together escalate the urgency. Read trims and fuel pressure at once. | Repair before further use. |
-
Smell strongest under load or just after a hard drive
- Likely cause
- A rich mixture reaching a hot converter — the characteristic pattern
- How to confirm it
- Read fuel trims at idle and under load. Heavily negative trims mean the management is fighting real over-fuelling.
- What fixes it
- Repair the fuelling fault. The converter is the victim; replacing it first repeats the failure.
-
Trims near zero with a persistent smell
- Likely cause
- The management cannot see the over-fuelling — the measuring sensor is at fault
- How to confirm it
- Watch upstream oxygen sensor switching and check the coolant temperature signal against ambient on a cold engine.
- What fixes it
- Renew the sensor found. Fuelling components will not help when the measurement is wrong.
-
Appeared after one particular fill and cleared with the next
- Likely cause
- High-sulphur fuel — nothing wrong with the car
- How to confirm it
- Wait one tankful before diagnosing. This is cheap and it resolves a real share of these complaints.
- What fixes it
- No repair. Note the station if it recurs there.
-
Smell with rough running or misfire counts
- Likely cause
- A misfire delivering unburnt charge to the converter
- How to confirm it
- Read counters per cylinder and act on the named cylinder.
- What fixes it
- Repair the misfire urgently. This combination destroys converters quickly.
-
Smell with poor performance and a restricted feel
- Likely cause
- The converter has already been damaged and is now restricting the exhaust
- How to confirm it
- Measure backpressure: under about 1.5 psi at idle and 3 psi at 2,000 rpm on most engines.
- What fixes it
- Renew the converter AND repair the fuelling fault. Doing only one of the two guarantees a repeat.
-
Smell with a strong fuel smell as well
- Likely cause
- Substantial unburnt fuel rather than a marginal mixture
- How to confirm it
- Two symptoms together escalate the urgency. Read trims and fuel pressure at once.
- What fixes it
- Repair before further use.
Codes that produce this symptom (8)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0420 Catalyst System Efficiency Below Threshold — Bank 1 Typical of this code Moderate
- P0430 Catalyst System Efficiency Below Threshold — Bank 2 Typical of this code Moderate
- P0421 Catalyst 1 Efficiency Below Threshold Bank 1 Moderate
- P0422 Catalyst 2 Efficiency Below Threshold Bank 1 Moderate
- P0423 Heated Catalyst Efficiency Below Threshold — Bank 1 Moderate
- P0431 Catalyst 1 Efficiency Below Threshold Bank 2 Moderate
- P0432 Catalyst 2 Efficiency Below Threshold Bank 2 Moderate
- P0433 Heated Catalyst Efficiency Below Threshold — Bank 2 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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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