Failed your NCT on emissions? What to do next

An NCT emissions failure means one of three things: the exhaust gas from a petrol engine, or the smoke from a diesel, measured over the limit for your vehicle; the emission control system, such as the catalytic converter, the diesel particulate filter (DPF) or the EGR valve, is missing, modified, leaking or obviously faulty; or the inspector did not run the emissions test at all, because the engine warning light was on or the oil or coolant level was wrong¹. Each is a major fault: the vehicle has failed, and you have 30 days to have it repaired and re-tested^1,2. It does not automatically mean the catalytic converter or DPF needs replacing. The readings on your report usually point to the cause, and the cause has to be fixed first, or a new part can fail the same way³.

What the NCT emissions test checks

The NCT follows the Road Safety Authority’s NCT Manual, which sets the test method and the pass and fail limits for passenger vehicles with up to 8 passenger seats. The limits depend on the vehicle’s date of first registration¹.

Before the test: warning lights, oil and coolant

Before the probe goes into the exhaust, the inspector checks the engine. If the engine warning light shows a fault, the oil warning light stays on, the oil is below the minimum or above the maximum mark, the coolant is low or there are obvious engine faults, the emissions test is not carried out and the vehicle fails¹. So a vehicle with a lit engine warning light fails the emissions section whatever its exhaust is like.

One allowance helps diesel owners. DPF regeneration can put diesel into the engine oil and push the level over the “max” mark. On a vehicle with a DPF that is not a reason for failure, unless the level is above the “X” mark some dipsticks have, or the dashboard shows too much fuel in the oil¹.

Petrol vehicles

On a petrol vehicle first registered from 1 January 1994, with the engine at normal operating temperature, the inspector holds it at 2,500 rpm (or a speed the manufacturer specifies) for at least 30 seconds and measures carbon monoxide (CO), hydrocarbons (HC) and lambda. If they are outside the limits, the engine is held at that speed for 3 minutes and they are measured again. Then the engine returns to idle and CO is measured once more¹. The vehicle fails if a reading is over these limits¹:

First registered	At idle	At 2,500 rpm
Before 1 October 1986	CO 4.5%, HC 1,000 ppm	Not measured
1 October 1986 to 31 December 1993	CO 3.5%, HC 750 ppm	Not measured
1 January 1994 to 1 July 2002	CO 0.5%	CO 0.3%, HC 200 ppm, lambda 0.97 to 1.03
After 1 July 2002	CO 0.3%	CO 0.2%, HC 200 ppm, lambda 0.97 to 1.03

Where the vehicle manufacturer’s own limits are higher than these, the manufacturer’s figures are used¹. What the three readings measure: CO is a poisonous gas that a working catalytic converter turns into carbon dioxide; HC is fuel that left the engine unburned; lambda is the balance of air and fuel, where a reading below 0.97 means the engine is running too rich and above 1.03 too lean⁴.

Diesel vehicles

Diesels have a smoke test. With the engine fully warm, the inspector presses the accelerator to the floor from idle several times while a smoke meter measures how much light the exhaust smoke blocks, a figure in m^-1, and the result is the average of the readings¹. The vehicle fails if that average is higher than the figure on the manufacturer’s plate on the vehicle, or higher than these limits¹:

First registered	Smoke limit
1 January 1980 to 1 July 2008	2.5 m-1 (non-turbo) or 3.0 m-1 (turbo)
After 1 July 2008	1.5 m-1
After 1 September 2015, with a Euro 6 engine	0.7 m-1

A diesel making so much black smoke that it could damage the smoke meter is failed without the normal test, and so is one whose engine cannot reach 90% of the maximum speed its manufacturer specifies¹.

The emission control equipment

On petrol and diesel vehicles alike, the inspector checks that the emission control system fitted by the manufacturer is present, complete, properly connected and not bypassed. A catalytic converter, DPF or EGR valve that was fitted when the vehicle was made but is now missing is a major fault. So are leaks that would affect the readings, and an idle speed outside the manufacturer’s recommendation. A minor leak, such as at a joint or a pin hole, is not a reason for failure on its own¹.

Fault codes, hybrids and older vehicles

The NCT is bringing in on-board diagnostics (OBD) checks in phases. For now, a stored fault code gives an “Advisory Pass”, not a fail^1,2. The engine warning light is a different matter: if it shows a fault, the vehicle fails, as above¹. Hybrid vehicles are treated as electric vehicles and have no exhaust emissions test. Petrol vehicles registered before 1 January 1980 have no gas test, and diesels registered before then have a visual smoke check instead of a measured one¹.

Start with your Vehicle Inspection Report

After the test you are given a Vehicle Inspection Report that lists every reason for failure and whether it is a minor, major or dangerous fault^2,5. Its emissions section shows each reading next to its limit: on a petrol vehicle, CO at low idle, and CO, HC and lambda at high idle⁶. Keep it. The readings tell you and your garage where to look. And if a replacement catalytic converter is ever the subject of a warranty claim for an emissions failure, the manufacturer asks to see the failed emissions report before it will consider it³.

Petrol: what the readings point to

On the report	What it usually means	Check first
CO too high, HC low	The catalytic converter is not converting all the gas that passes through it3	The catalytic converter. If HC is above about 60 ppm, it has probably been contaminated with unburned fuel, so find where the fuel is coming from first3
HC over 200 ppm	A definite fault: the engine is over-fuelling, and that must be put right before a replacement catalytic converter is fitted3	Misfires (spark plugs and leads), the lambda sensor, the fuel injection system and the MAP sensor3
Lambda outside 0.97 to 1.03	A problem with the air and fuel mixture. It does not always mean the lambda sensor needs replacing3	Below 0.97 the engine is running rich, above 1.03 lean4: look for the fuelling or air fault behind it
Engine warning light on	No emissions test; a fail on its own1	Have the fault codes read

Two more clues. A catalytic converter with a blue or purple tint on its shell, or a melted and warped core, has overheated because unburned fuel reached it and ignited inside, and that is always a sign of a fault in how the engine runs³. And the fault code P0420, for an inefficient catalytic converter, does not prove the converter is at fault: a lambda sensor fault, an air leak in the exhaust or retarded ignition timing can set it too³. More in our P0420 guide and what causes a faulty catalytic converter.

Diesel: what a smoke failure points to

A smoke failure points first to how the engine is burning its fuel. The checks the manufacturer sets out before a DPF is replaced make a good list for a garage to work through: the EGR valve and its pipe, the air-flow meter, engine and turbo wear, leaking injectors, the glow plugs, the air filter, the sensors, the DPF pressure pipes, and the engine oil: most vehicles with a DPF need a low-ash oil, and a high oil level is a sign of fuel from failed regeneration attempts⁷. Our guides to why DPFs block and DPF fault codes go through them.

How the vehicle arrives matters too. The NCT Manual says the engine must be at normal operating temperature, warmed up by normal driving rather than by idling, and that an engine left idling for any length of time will show a high smoke reading¹. NCTS also asks for the vehicle to be at normal operating temperature when it arrives⁸. So drive it to the test centre, and don’t leave it ticking over while you wait.

Removing the DPF is not a way round a smoke failure. A DPF, EGR valve or catalytic converter that was fitted when the vehicle was made and is now missing fails the NCT¹.

What to do next

1. Check what kind of fail it is. Every emissions reason for failure in the NCT Manual is a major fault¹: the vehicle has failed and gets no NCT certificate until it passes a re-test². A “fail dangerous” result is different: the inspector puts a sticker on the vehicle, and it must not be used on the road at all. Driving it is an offence that can mean penalty points and a court appearance^2,5. Whatever the result, road traffic law and your insurer require the vehicle to be roadworthy at all times².
2. Have the cause diagnosed. Give your garage the report and ask it to read the fault codes before any part is ordered. The manufacturer’s fitting advice for catalytic converters and for DPFs starts with the same rule: find and fix the root cause of the failure before fitting a replacement^3,7.
3. Fix the cause, then replace the part if it is still needed. If the readings and the garage point to the catalytic converter or DPF, see the next section for choosing the right one. Never use exhaust paste in front of a catalytic converter: it hardens, breaks off in chunks and damages the core³.
4. Book the re-test within 21 days. The re-test must be booked within 21 days of the test and carried out within 30 days of it; after that the vehicle needs a full test again^2,5. A full test costs €60 and a re-test €40. Only re-tests that need no test equipment, such as a check that a wiper blade has been replaced, are free², and an emissions re-test needs the equipment. The re-test covers the items that failed⁵, and NCTS says the wait for a re-test is usually under 3 days².
5. Arrive warm, with your documents. Drive to the re-test so the engine is at normal operating temperature, and bring the vehicle’s registration certificate (or registration book or licensing certificate) and your driving licence⁸.

Choosing a replacement catalytic converter or DPF

A replacement has to suit your engine and the Euro emissions standard your vehicle was built to, not just bolt on. For vehicles approved to Euro 5 or Euro 6, EU law goes further: national authorities must prohibit the sale or fitting of a new replacement catalytic converter, DPF or other pollution control device that is not type approved⁹.

In our range, type-approved parts are marked Type Approved and have an H at the end of the part number. Every part lists the vehicles and years it is catalogued for, most with the engine code, so check it against your vehicle. Most come with a fitting kit, all carry a 2-year warranty (3 years on premium silicon-carbide DPFs), and they are backed by our fitment guarantee.

We deliver to the Republic of Ireland for €30.99 per order, whatever the size of the part. Orders placed before 4pm on a working day are normally dispatched the same day, and the courier usually takes 2 to 3 working days, well inside the 30 days you have for the re-test. Prices are in euro and are export prices, with no UK VAT: Irish VAT at 23% and any customs duty are collected by the courier before or on delivery, with its own clearance fee, and we show an estimate of the Irish VAT under each price. The details are on our Delivery page.

Browse catalytic converters, DPFs and combined catalytic converter and DPF units, or choose your vehicle from the vehicle list, for example the Volkswagen Golf, Skoda Octavia, Ford Focus, Toyota Corolla, Toyota Yaris or Nissan Qashqai. Or enter your vehicle below to see the parts catalogued for it.

Frequently asked questions

Can I drive my vehicle after failing the NCT on emissions?

An emissions failure on its own is a major fault, not a dangerous one, so it does not bring a “fail dangerous” sticker^1,2. The vehicle gets no new NCT certificate until it passes the re-test, it must be kept roadworthy, and the NCT is enforced by An Garda Síochána, who have access to NCT records².

How much is an NCT re-test for emissions?

€40, because it needs the test equipment. Re-tests that need no test equipment are free. If the re-test is not done within 30 days of the original test, a full test is needed and charged again, currently €60².

Does the NCT check the DPF?

Yes. The inspector checks that the emission control system is present, complete and not bypassed, and a DPF that was fitted when the vehicle was made but is now missing is a major fault¹. A failing DPF can also show up as a smoke reading over the limit or as a lit engine warning light, and either fails the vehicle^1,7.

Does the NCT read fault codes?

It is bringing in OBD checks in phases, and for now a stored fault code gives an Advisory Pass rather than a fail². But if the engine warning light shows a fault, the emissions test is not carried out and the vehicle fails¹. Clearing the codes before the test does not fix the fault behind them: see our P0420 guide.

Is my van tested at an NCT centre?

Not if it is a light goods vehicle: one with seats for only two passengers, no rear side windows and no rear side doors that open. Those are inspected at an approved VTN centre, not at an NCT centre². The NCT Manual covers passenger vehicles with up to 8 passenger seats¹.

Will a new catalytic converter pass the NCT?

Only if whatever caused the old one to fail has been fixed. Emissions failures normally come from a fault in the vehicle, and an HC reading over 200 ppm means the engine is over-fuelling, which must be put right before a replacement is fitted³. Fit a part catalogued for your engine, type approved where the law requires it⁹, and book the re-test once the cause is fixed.