Symptom
Emissions test failure
The vehicle failed on measured emissions or on a stored fault. Many codes with no driveability symptom at all still cause an automatic failure.
Also described as: failed emissions, failed mot, failed smog, smog fail, emissions fail, failed inspection, در معاینه فنی رد شد, تست الایندگی قبول نشد, no paso la itv, no pase la itv, fallo en emisiones, reprobo verificacion, reprobe verificacion, verificacion de emisiones, no paso revision tecnica, رسبت في فحص العوادم, ما نجحت بالفحص الدوري, فشل اختبار الانبعاثات
What this usually means
Many codes with no driveability symptom at all still cause an automatic failure, and that is the first thing to establish: did it fail on measured gas, or on a stored fault? They are different problems with different repairs, and the certificate says which.
A failure on a stored fault is the easier half. The lamp being lit is itself a failure in most jurisdictions regardless of what the car emits, so the repair is the code rather than the emissions — and a car that drives perfectly can fail on an evaporative fault it will never feel.
A failure on measured gas divides by which gas. High carbon monoxide and hydrocarbons together point at a rich mixture or incomplete combustion — fuelling, ignition or a failing catalyst. High oxides of nitrogen point at combustion temperature — a failed exhaust gas recirculation valve is the classic, since its whole purpose is to lower peak temperature. High hydrocarbons alone point at unburnt fuel: misfire, an ignition fault, or oil being burnt.
Preparation matters more than people expect and is free. The catalyst must be at working temperature, so a car tested from cold, or after a queue, often fails a test it would pass after a proper drive. An overdue service, a fouled air filter and old oil all raise readings. So does a marginal thermostat, because an engine that never reaches temperature never leaves its enrichment.
Do not clear codes on the way to a retest. That resets the readiness monitors, and the vehicle is then rejected as untestable rather than failed — which costs a second appointment.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Failed with the engine management lamp lit, gas readings fine | The stored fault itself — a lit lamp is an automatic failure in most jurisdictions | Read the code. A car that drives perfectly can fail on an emissions fault it will never feel. | Repair the code, then allow the readiness monitors to complete before retesting. |
| High carbon monoxide and hydrocarbons together | A rich mixture or incomplete combustion — fuelling, ignition, or a catalyst no longer converting | Read fuel trims and lambda sensor switching. Compare upstream and downstream sensor activity to assess the catalyst. | Repair the fuelling or ignition fault first. A catalyst rarely fails without a cause upstream of it. |
| High oxides of nitrogen | Combustion temperature too high — most often exhaust gas recirculation not working | Command the valve and watch its position feedback, or inspect it. Carbon holding it shut is the usual finding. | Clean or renew the valve and its cooler passages. Check crankcase ventilation, since oil mist binds the soot. |
| High hydrocarbons alone | Unburnt fuel reaching the exhaust — a misfire, an ignition fault, or oil being burnt | Read misfire counters and inspect the plugs. Blue smoke points at oil rather than fuel. | Repair the misfire or oil consumption. Both destroy catalysts, so this failure is also a warning. |
| Tested from cold or after a long queue | The catalyst was not at working temperature — the car may pass after a proper drive | Note the conditions of the test. This is a genuine and common reason for a marginal failure. | Drive for twenty minutes at sustained speed before retesting. Free, and it resolves a real share of marginal failures. |
| Service badly overdue, or the engine slow to warm | Ordinary condition — old oil, a fouled filter, or a thermostat leaving the engine in enrichment | Check the service history and watch live coolant temperature reach the operating figure. | Service it and renew the thermostat if it is not reaching temperature. Cheap, and it moves readings measurably. |
-
Failed with the engine management lamp lit, gas readings fine
- Likely cause
- The stored fault itself — a lit lamp is an automatic failure in most jurisdictions
- How to confirm it
- Read the code. A car that drives perfectly can fail on an emissions fault it will never feel.
- What fixes it
- Repair the code, then allow the readiness monitors to complete before retesting.
-
High carbon monoxide and hydrocarbons together
- Likely cause
- A rich mixture or incomplete combustion — fuelling, ignition, or a catalyst no longer converting
- How to confirm it
- Read fuel trims and lambda sensor switching. Compare upstream and downstream sensor activity to assess the catalyst.
- What fixes it
- Repair the fuelling or ignition fault first. A catalyst rarely fails without a cause upstream of it.
-
High oxides of nitrogen
- Likely cause
- Combustion temperature too high — most often exhaust gas recirculation not working
- How to confirm it
- Command the valve and watch its position feedback, or inspect it. Carbon holding it shut is the usual finding.
- What fixes it
- Clean or renew the valve and its cooler passages. Check crankcase ventilation, since oil mist binds the soot.
-
High hydrocarbons alone
- Likely cause
- Unburnt fuel reaching the exhaust — a misfire, an ignition fault, or oil being burnt
- How to confirm it
- Read misfire counters and inspect the plugs. Blue smoke points at oil rather than fuel.
- What fixes it
- Repair the misfire or oil consumption. Both destroy catalysts, so this failure is also a warning.
-
Tested from cold or after a long queue
- Likely cause
- The catalyst was not at working temperature — the car may pass after a proper drive
- How to confirm it
- Note the conditions of the test. This is a genuine and common reason for a marginal failure.
- What fixes it
- Drive for twenty minutes at sustained speed before retesting. Free, and it resolves a real share of marginal failures.
-
Service badly overdue, or the engine slow to warm
- Likely cause
- Ordinary condition — old oil, a fouled filter, or a thermostat leaving the engine in enrichment
- How to confirm it
- Check the service history and watch live coolant temperature reach the operating figure.
- What fixes it
- Service it and renew the thermostat if it is not reaching temperature. Cheap, and it moves readings measurably.
Codes that produce this symptom (276)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0030 O2 Sensor Heater Control Circuit — Bank 1 Sensor 1 Typical of this code Moderate
- P0031 O2 Sensor Heater Control Circuit Low — Bank 1 Sensor 1 Typical of this code Moderate
- P0032 O2 Sensor Heater Control Circuit High — Bank 1 Sensor 1 Typical of this code Moderate
- P0036 O2 Sensor Heater Control Circuit — Bank 1 Sensor 2 Typical of this code Moderate
- P0037 O2 Sensor Heater Control Circuit Low — Bank 1 Sensor 2 Typical of this code Moderate
- P0038 O2 Sensor Heater Control Circuit High — Bank 1 Sensor 2 Typical of this code Moderate
- P0040 O2 Sensor Signals Swapped — Bank 1 Sensor 1 / Bank 2 Sensor 1 Typical of this code Moderate
- P0041 O2 Sensor Signals Swapped — Bank 1 Sensor 2 / Bank 2 Sensor 2 Typical of this code Moderate
- P0050 O2 Sensor Heater Control Circuit — Bank 2 Sensor 1 Typical of this code Moderate
- P0051 O2 Sensor Heater Control Circuit Low — Bank 2 Sensor 1 Typical of this code Moderate
- P0052 O2 Sensor Heater Control Circuit High — Bank 2 Sensor 1 Typical of this code Moderate
- P0053 O2 Sensor Heater Resistance — Bank 1 Sensor 1 Typical of this code Moderate
- P0059 O2 Sensor Heater Resistance — Bank 2 Sensor 1 Typical of this code Moderate
- P0130 O2 Sensor Circuit — Bank 1 Sensor 1 Typical of this code Moderate
- P0133 O2 Sensor Circuit Slow Response — Bank 1 Sensor 1 Typical of this code Moderate
- P0134 O2 Sensor Circuit No Activity Detected — Bank 1 Sensor 1 Typical of this code Moderate
- P0135 O2 Sensor Heater Circuit — Bank 1 Sensor 1 Typical of this code Moderate
- P0137 O2 Sensor Circuit Low Voltage — Bank 1 Sensor 2 Typical of this code Moderate
- P0138 O2 Sensor Circuit High Voltage — Bank 1 Sensor 2 Typical of this code Moderate
- P0150 O2 Sensor Circuit — Bank 2 Sensor 1 Typical of this code Moderate
- P0153 O2 Sensor Circuit Slow Response — Bank 2 Sensor 1 Typical of this code Moderate
- P0154 O2 Sensor Circuit No Activity Detected — Bank 2 Sensor 1 Typical of this code Moderate
- P0155 O2 Sensor Heater Circuit — Bank 2 Sensor 1 Typical of this code Moderate
- P0157 O2 Sensor Circuit Low Voltage — Bank 2 Sensor 2 Typical of this code Moderate
- P0158 O2 Sensor Circuit High Voltage — Bank 2 Sensor 2 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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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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