General guidance on NCT emissions

jaydev · 19-09-2018 10:38pm #1

Hi all,

I wondered if one of the emissions experts could try to post a guide that details the NCT emissions failure reasons - most popular of course, based on something that explains what lean and rich is vs. something else.

I've researched a fair amount and am kind of understanding but because everyone's question is unique to their car it is quite hard to use any of those existing ones to help.

Maybe I am asking too much. But I thought something like explaining what is generally wrong when CO is high / low on low idle and same for high idle then in the context of HC and Lambda readings being above or below range.

For example my failure readings were:

Engine temp 84deg
Low Idle CO=.12% HC=24ppm - which was within range
High Idle CO=2.06% HC=123ppm Lambda=0.94 - so lambda was too low (just) but CO was way above the limit.

I'm reading that means rich. I've also read comments about taking car for a good run to burn off carbon (not exactly sure what that does) and even someone suggested switching 10w40 oil for 5w30 to reduce clogging.

Was thinking it might be useful for someone to create a quick sticky for emissions failures as it seems loads of people are asking for some basic guidance.

Anyway thanks for any help with explanations, I appreciate all the knowledge shared here.

JimPa · 19-09-2018 10:43pm

Yeah I've always thought it would be a good idea to have an emissions sticky here. Maybe even ask a competent mechanic to put it together in return for a bit of free advertising in the sticky. The questions just keep coming up

crosshair1 · 19-09-2018 11:03pm

Deal with lambda issues first.

Unfortunately with most petrol cars now at least 10yrs old and aftermarket catalytic convertors appearing to become less and less efficient, in a lot of cases nct emission failures arent worth getting involved in.

jaydev · 19-09-2018 11:11pm

OK - so have new air filter, plugs and oil filter ready to go at weekend. Then I'll get the lambda level tested again.
Anything else I can or should do other than that?

rex-x · 19-09-2018 11:19pm

Emissions failures with regards the nct are not super complicated.

The test is broken down into:

Lambda: This is the air to fuel ratio, it should always (under test conditions) be 1. They allow .97-1.03 but really it should be 1.
A reading below 1 indicates a rich mixture, above 1 indicates a lean mixture. Lambda is NOT affected by combustion so if you have
a misfire etc it wont affect the lambda reading.

Low idle:
High idle:

These are just the revs the test is performed at, idle and about 3k rpm in neutral respectively.

The CO reading relates to the gas emissions and it is the job of the cat to reduce this. The lambda directly affects this reading. Too rich and the CO numbers will rise rapidly and also the cat will not be able to do its job and will eventually fail.
Equally if the mixture is too lean the cat will fail from the high temps (leaner is hotter).

The HC reading in layman's terms tells us how much petrol is going down the exhaust i.e. how good was our combustion? A misfire, low compression, poor timing etc will affect this reading.

If we have a car with results like this: High Idle CO=1.57% HC=23ppm Lambda=1.00 then this indicates a failed cat.

Results like this: High Idle CO=2.06% HC=123ppm Lambda=0.94 tell us the mixture is too rich and combustion is poor. This could be caused by a failed lambda sensor (upstream sensor only) a failed map sensor perhaps (if the sensor reads the air amount as being 10% greater than actually then the fuelling will be 10% more causing a rich condition etc) or any other number of issues. There are many factors than can affect emissions and a good mechanic is always your best bet!

Dipetane is good stuff but as a rule if you have a wrong lambda reading Dipetane wont help you. If you have slightly high CO or slightly high HC then maybe Dipetane will help by cleaning the cat and injectors slightly accompanied by a spirited drive. If you find your car failing the test slightly by a high CO reading the getting the cat nice and hot right before the test can get you over the line as they work better when warm.

crosshair1 · 19-09-2018 11:22pm

Fitting the service items will probably not have any affect, the engine is not being fuelled correctly for whatever reason.
When the fault is rectified it will most likely need a new cat to bring co in line.

jaydev · 19-09-2018 11:27pm

crosshair1 wrote: »

Fitting the service items will probably not have any affect, the engine is not being fuelled correctly for whatever reason.
When the fault is rectified it will most likely need a new cat to bring co in line.

Thanks for the info.
I have them to fit anyway so will do that. Had read that an old air filter could cause rich running but I guess not by how much needed.
So will swap out the Lambda sensor as that's the cheapest of the possible parts mentioned and we'll go from there.

Appreciate the help.

rex-x · 19-09-2018 11:33pm

jaydev wrote: »

Thanks for the info.
I have them to fit anyway so will do that. Had read that an old air filter could cause rich running but I guess not by how much needed.
So will swap out the Lambda sensor as that's the cheapest of the possible parts mentioned and we'll go from there.

Appreciate the help.

The best suggestion id have for you is don't go swapping parts because you could go broke!. A good lambda sensor will be over 150 (cheap ones just don't work). A good mechanic will get your issue resolved for you by reading live data on lambda to see its function, map sensor etc plus visual inspection.

jaydev · 19-09-2018 11:36pm

Ok fair point - don't wanna go throwing cash down the hole. There's a place that claim to be good at emissions work nearby so I'll give them a go, they should have reliable test gear.

Thanks.

rex-x · 19-09-2018 11:49pm

Emissions problems can be hard to fix and unfortunately not all garages are capable of fixing them without blindly throwing parts at the problem. Not much advice I can give about that though, a good mechanic is hard to come by especially in a main dealer but thats another story...

TallGlass · 19-09-2018 11:50pm

Servicing the car is always good as a baseline before the test itself so you can rule out them items needing changing. I always give the oil a change, filter changed, check over air filter, sparks etc.. see if they need doing, but an annual test is a good time to change them out, even if they can last for years. Again personal choice.

I'm no expert on emmisions, but the big deal or headache with them failing. Is there is so many inputs that could cause the issue.

For me a good starting point is a car diagnostic tool and checking for any errors codes etc..

Before the test try give the car a good run and have the cat reasonably hot.

And look after the cat, the amount of times I cringe at drivers blasting into puddles and pools of water, the cats under there roasting hot and that water is going to rapidly cool the metal.

Like the poster above said, you can read the results and narrow it down to a certain part, but it's a pick and mix, of pulling out replacing and pure luck. Use the results to zoom in on an area, visually have a look, if something looks bad chances are it is. If it looks okay, then get the car hooked up and see what the management system is saying.

And just to add, YouTube is a good resource. Put in your car and check is it a common problem, results look the same? Do some research on the issues your having and check the common issues. All cars have there own banks of common issues that are easily solvable. Think simple at first and don't overthink issues.

samih · 20-09-2018 11:06am

When you have a lambda value of 0.94 is not just slightly out: It's out by several country miles. A lambda value of 0.998 is stightly out and has already a large effect on emissions and 0.98 classifies as a country mile out. The regulations just set the range to 0.97-1.03 for not to fail on a slight exhaust leak for example but if the lambda is not 1.00(0) there is something wrong with your car and that needs to be fixed.

Only when the lambda is 1.00(0) and CO/HC are still above the limits should you start considering a new catalytic converter and in ideal situation the HC should be low single digits and the CO close to 0.

samih · 20-09-2018 11:23am

I had a 124 series Merc back in the day and it was running beautifully with emissions like Lambda 1.00, HC 3, CO 0.02 on the NCT at high idle.

Then one day the air flow meter (MAF/AFM/AMM) failed and the car was literally pushing straight fuel out of exhaust. The problem happened outside shops and unfortunately for me my wife was driving at the time and drove the car home for about 10 kilometers. When I started the engine at home black smoke was coming out of the exhaust and the car was running like a pig.

I disconnected the failed component and, in my case, as the Merc also had a map sensor (quality redundancy there) it was running fine without it just with a slightly worse throttle response. The next day I got a new AFM and the performance returned to normal. However on the next NCT the lambda was again 1.00 but the CO had increased from 0.02 to over 1 and the HC from from 3 to 40 i.e. the car had similar emissions to a car without a catalytic converter fitted.

The conclusion was that the cat melted and failed due to the relatively short drive home where the excess fuel was burning in the cat and had probably heated it up red or even yellow hot. Apart from a failed AFM for example failed spark plug or coil pack could result in extra unburned fuel pouring of the engine and destroying a perfectly good cat in the process. That's why it's really important not to just go replacing the cat without actually fixing the underlying issue first.

Also, based on my experince, if the car starts to misfire etc. it's a good plan to walk home and get the car towed to a suitable garage in order not to destroy the cat. The standard cats tend to be much higher quality than the aftermarket jobbies and should never be replaced until everything else is ruled out.

rex-x · 20-09-2018 3:15pm

Almost every modern or semi modern car nowadays cuts all fuel to a cylinder when a misfire is detected to prevent that

Lisa-ballivor · 07-10-2022 1:10am

Car failed on emissions test .. co 1.18 lambda 1.08. Hc 7 .. 132 polo 1.2 ltr .. .. gave it a complete service .. bottle of cataclean .. brought it to emissions garage and he done a new bank 1 o2 sensor ..he tested the emissions and said it was now in pass level .. retested at nct center and it failed again. .this time co down to pass at .0.10 . But lambda even higher 1.38 . Any one with similar story ? . Also drove hard to the nct retest

Atlantic Dawn · 07-10-2022 1:54am

https://www.boards.ie/discussion/comment/119711392#Comment_119711392

One thing that can mess it up again is if you park up at the NCT centre idling with the engine running, that usually results in higher results too. Better to park it up and leave it there when you arrive.

Lisa-ballivor · 07-10-2022 7:31am

Really. Always lead to believe .is drive like you stole it on the way to nct , have it nice and hot .. and be on time instead of the car sitting waiting ages

barneygumble99 · 07-10-2022 2:13pm

The way a petrol emission test works is car is brought up to between 2500-3000 rpm for a period of 30 seconds max or if the readings stay within the limits for 10 continuous seconds. If they haven’t passed after 30 seconds, it’s now goes into a 3 and a half minutes countdown at the same revs. If it hasn’t passed after 3 and a half minutes it goes into another 30 seconds , after that it stops for a few seconds before going into another 30. During any of those phases of the emissions come below the limit for 10 continuous seconds the test ends, and it begins the low idle test for 30 seconds or 10 seconds if the CO reading is passing.

So for high idle the car is at minimum 2500 rpm for 5 minutes. So it doesn’t matter if the car is hot or stone cold at the start of the test, if the emissions system is working correctly it has lots of time to pass. You go out and start your car from cold and hold the throttle at 2500 rpm for 5 minutes and see would you put your hand on the manifold or cat. It will be as hot as if you’d driven from Belfast to cork.

Personally I think a hard drive every so often, doesn’t even have to be Italian tune up, is a good help to keep a car performing. The hotter the car , the better for its emission system and its economy.

jj880 · 07-11-2022 8:18pm

Failed myself today on emissions. High idle lambda reading 1.04. Just above the 1.03 pass limit. Car is 2008 Corsa 1.2 petrol.

Got a brand new silencer box put on a couple months ago but the rest of the exhaust is maybe 4 years old now. I asked the NCT tester what he thought I should do and he said because I was so close to passing on the high idle that some air might be getting into the exhaust in the older middle and front portion and to get that replaced for the retest.

Is this good advice?

Should I give the car a good fast run to the retest and be exactly on time?

Thanks.

User1998 · 07-11-2022 9:50pm

https://www.boards.ie/discussion/comment/119846645#Comment_119846645

Bring it to an exhaust specialist or a good mechanic. Auto Exhausts in Dublin are very good

Batman1irish · 15-11-2023 11:23pm

Hi did you ever get problem fixed with emissions I have a 13 polo 1.2 to sheets with very similar readings

angrybeaver · 01-03-2024 9:27am

Hi All, hoping someone can advise. My wife’s 2014 Kia Sportage 1.6 petrol failed the nct on emissions yesterday.

high idle

lambda 0.98

CO 0.69 vol% max limit allowed 0.20 vol%

she took it herself and we never did a thing to it. Passed on all else only a wheel nut. She does little mileage on it and wouldn’t be a hard driver.

Is Dipetane and the Italian tune up worth a shot?

thanks

General guidance on NCT emissions

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