If your car failed the NCT on emissions, your diesel’s exhaust smoke was too dense, or your petrol car’s carbon monoxide (CO) or hydrocarbon (HC) readings were too high. Most failures come from carbon build-up, a blocked diesel particulate filter (DPF), a dirty air filter or a faulty sensor. Try the cheap fixes first, then book the retest within 21 days.
Key facts
- The full NCT costs €60 and a retest costs €40. Retests that don’t need test equipment are free, but an emissions retest does need it.
- You must book the retest within 21 days of the first test and complete it within 30 days. Otherwise you pay for a full test again.
- A “Major” fail gives you 30 days to fix the problem. A “Dangerous” fail means the car mustn’t be driven on the road.
- Diesels are tested for smoke. Petrol cars are tested for CO, HC and lambda (the balance of air and fuel).
- Bring the car to the test with the engine at normal operating temperature.
Source: NCTS frequently asked questions, checked November 2026.
What does the NCT emissions test check?
The NCT emissions test measures the gases (petrol) or smoke particles (diesel) coming out of your exhaust and compares them with limits for your car. NCTS says the limits depend on the car’s year of manufacture, so an older car is held to a different standard than a newer one.
Diesel cars: the smoke test
The tester revs the engine hard several times while a meter in the exhaust measures how dense the smoke is. Dense, dark smoke means fuel isn’t burning completely, which is usually down to carbon on the injectors, a clogged air filter, a struggling DPF or a sticking exhaust gas recirculation (EGR) valve.
Petrol cars: CO, HC and lambda
Petrol cars are tested at idle and at a higher engine speed. On NCTS’s sample test report for a petrol car, the limits are:
- CO: no more than 0.5% at low idle and 0.3% at high idle
- HC: no more than 200 parts per million at high idle
- lambda: between 0.97 and 1.03
Lambda shows whether the engine is running rich (too much fuel) or lean (too much air). Older cars can have different limits.
Engine warning lights and OBD
Since 2021, the NCT also plugs into your car’s on-board diagnostics (OBD) to read stored fault codes. NCTS says that, for now, an OBD fault code results in an “Advisory Pass” and doesn’t need a re-inspection on its own, though it strongly recommends getting the fault fixed. Warning lights for safety systems such as ABS and electronic braking are checked separately and can fail the test.
Either way, an engine warning light is worth sorting before the test. The fault behind it, such as a failing sensor, a DPF problem or a sticking EGR valve, is often the same thing that pushes the emissions reading over the limit.
Why did my car fail the NCT on emissions?
The wording on your Vehicle Inspection Report points to the most likely cause. Use this table to decide what to try first and when to go straight to a garage.
| What failed | Likely causes | Try first | It’s a garage job if… |
| Diesel smoke over the limit | Carbon build-up, dirty injectors, clogged air filter, partly blocked DPF, sticking EGR valve | New air filter, a long drive at speed, a fuel treatment | The reading was far over the limit, or it fails again after the clean-up |
| Visible or coloured smoke | Overloaded DPF, injector fault, burning oil or coolant | Check the colour. If it’s black or grey, follow the steps below | The smoke is blue or white, or the DPF warning light stays on |
| Petrol CO or HC too high | Misfire, worn spark plugs, dirty air filter, carbon deposits, tired catalytic converter | Spark plugs, air filter, and a fully warm engine for the test | Readings stay high after the basics |
| Lambda out of range | Faulty lambda (oxygen) sensor, air or exhaust leak | Listen for exhaust blows | Almost always: the sensor or leak needs diagnosing |
| OBD fault codes recorded | A stored fault, from a sensor to a DPF or EGR problem | Have the codes read | Always: find and fix the fault |
| DPF missing or tampered with | The DPF has been removed or altered | – | Always: refit a working DPF |
Can I drive my car after failing the NCT?
It depends on how the fault was graded. With a “Major” fail, you have 30 days to get the car repaired and back to the test centre, and you won’t get a certificate until it passes.
A “Dangerous” fail is different. NCTS says it’s illegal and unsafe to drive a car with a dangerous defect, and you could face penalty points and a court appearance if you’re stopped by the Gardaí. If your car was failed dangerous, arrange for it to be recovered or repaired where it is.
How to get a diesel ready for the NCT emissions retest
Start with the cheapest checks and work up. Many diesels that fail on smoke pass after a new air filter, the right oil and a proper run to clear carbon. If yours still smokes after that, it needs a diagnosis.
- Check the air filter. A clogged filter starves the engine of air, so the fuel doesn’t burn completely and you get dark smoke. It’s a cheap part and often a quick job.
- Check the engine oil. Most diesels with a DPF need a low-ash (low-SAPS) oil. The wrong oil leaves ash that the DPF can’t burn off. If an oil change is due, do it before the retest.
- Give the car a proper run. Short journeys leave soot in the engine and the DPF. A run of 30 minutes or more at a steady speed on a motorway or dual carriageway heats the exhaust enough for the DPF to burn off trapped soot (this is called regeneration). If you can, drive straight to the test centre so the engine arrives hot.
- Use a fuel treatment to clear carbon. Carbon on injectors, valves and pistons stops fuel from burning completely, and that shows up as smoke. Dipetane is a 100% hydrocarbon fuel treatment that helps fuel burn more completely, so existing carbon deposits break down and pass out through the exhaust. It can help cut smoke by up to 50%. Add 1 litre for every 200 litres of fuel (50 ml per 10 litres). It starts working straight away, but a badly clogged engine can take 1–3 days of driving to clear, so treat the fuel a few days before the retest. If the test is the same day, give the car 30 minutes of hard driving on a motorway or dual carriageway first. More on using a fuel treatment before an emissions test.
- Have the fault codes read. If the car still smokes, or a warning light is on, ask a garage to run a diagnostic and check the injectors, EGR valve, turbo and sensors.
- Ask about a forced regeneration or a DPF clean. A garage can force the DPF to regenerate using diagnostic equipment, or remove it and clean it professionally. Either costs far less than a new filter. Read more about how to keep your DPF clear.
Petrol car failed on emissions? What to check
Petrol emissions failures are more often mechanical than diesel ones: a misfire, a failing lambda sensor, an air leak or a worn-out catalytic converter. Work through these:
- Make sure the engine is fully warm when it’s tested. A cold catalytic converter can’t do its job.
- Replace worn spark plugs and a dirty air filter.
- Listen for exhaust blows or air leaks, which upset the lambda reading.
- Have any stored fault codes read.
- If the catalytic converter or lambda sensor has failed, it needs replacing.
Carbon deposits on valves and injectors can push HC and CO readings up, and a fuel treatment that helps fuel burn more completely may help. It won’t fix a failed sensor or catalytic converter.
NCT retest: how long you have and what it costs
You must book the retest within 21 days of your first test and have it done within 30 days. Miss either deadline and you’ll need a full NCT at the full price.
| Item | Cost |
| Full NCT | €60 |
| Retest that needs test equipment (including emissions) | €40 |
| Visual-only retest (for example, replaced wipers) | Free |
| Garage diagnostic scan | Around €20–€30 |
| DPF clean or repair | Around €300 to over €1,000 |
NCT fees: NCTS. Garage costs: typical Irish prices from odo.ie, 2026. Garage prices vary with the car and the garage.
Pre-NCT check: getting your car ready for the emissions test
A few days of preparation make a real difference to the emissions result. Before your NCT:
- Fix any warning lights, or at least have the fault codes read.
- Check the oil level, and that it’s the grade your handbook specifies.
- Replace the air filter if it’s dirty or overdue.
- Give the car at least one long drive at motorway or dual carriageway speed in the days before.
- If you use a fuel treatment, add it a few days before the test so it has time to work.
- Drive to the test centre on a longer route so the engine arrives at normal operating temperature, as NCTS asks.
- Unplug anything connected to the OBD port, such as a tracker or dongle.
When to see a mechanic
A long drive and a fuel treatment can help when the cause is carbon and soot. They can’t fix worn or broken parts. Go to a garage if:
- the smoke is blue (burning oil) or thick and white with a sweet smell (coolant), which points to worn seals, piston rings or a head gasket
- the reading was far over the limit
- a warning light stays on
- the DPF is cracked or blocked beyond what a drive can clear
- the turbo, EGR valve, lambda sensor or catalytic converter has failed
Never remove or tamper with a DPF. A car without its DPF no longer meets its type approval, so it isn’t legal on Irish roads, and it won’t pass the emissions test.
Northern Ireland, vans and lorries
Cars registered in Northern Ireland take the MOT through the Driver and Vehicle Agency (DVA), not the NCT. If you’re in the North or in Great Britain, see our guide to what to do if your car failed its MOT on emissions, and our comparison of NCT, MOT and DOE tests.
Vans, lorries and buses in Ireland take the Commercial Vehicle Roadworthiness Test (CVRT) instead of the NCT, and it has its own smoke test.
FAQs
Does the NCT test diesel emissions?
Yes. Diesel cars get a smoke test, where the tester revs the engine and measures how dense the exhaust smoke is. The limit depends on the car’s year of manufacture. Carbon build-up, a dirty air filter or a struggling DPF are the usual reasons a diesel fails.
Is the engine warning light an NCT fail?
NCTS says OBD fault codes currently lead to an “Advisory Pass” rather than a fail, though it strongly recommends fixing them. Safety warning lights such as ABS are checked separately and can fail the test. The fault behind an engine warning light can also push the emissions reading over the limit, so get it checked before your test.
How much is an NCT retest after an emissions fail?
€40. An emissions retest needs test equipment, so it isn’t one of the free visual-only retests. A full NCT costs €60, and you’ll have to pay that again if you don’t book the retest within 21 days or complete it within 30.
How long do I have to get an NCT retest?
You must book the retest within 21 days of the first test and have it completed within 30 days. With a Major fail, that gives you about a month to get the car repaired.
Will a fuel treatment help my car pass the NCT?
It can help if the cause is carbon and soot, because cleaner combustion means less smoke. It won’t fix a mechanical fault, and nothing can guarantee a pass. Dipetane is added to fuel like other additives, but it contains only hydrocarbons already found in fuel, with no detergents, solvents or metals.
How long before the NCT should I add Dipetane?
Ideally a few days before, with plenty of driving in between. It works from the first tank, but a heavily clogged engine needs 1–3 days to clear. If you only have a few hours, give the car 30 minutes of hard driving on a motorway or dual carriageway right before the test.
Got an NCT coming up, or failed on emissions? Find your nearest Dipetane stockist
