How to pass NCT emmision test after failing

Kyledonegal

Hi guys.. I see a lot of posts regarding nct emmision tests. Some of these dont include all of the relevent information, or are just not helpful.

Im not am expert but i am a vauxhall/opel owner. And everyone one diesel/petrol just always give bother.

1) how to diagnose. This is important as it will limit you expense on buying various parts not needed.

Check for a hole/leak in your exhuast pipe. Can do this by jacking the car up. Use paper, place it near the pipe and run it from top to bottom. The paper will ripple if there is a leak and it will also leave a black spot on the paper. Remember to have the paper about 1-2 inches away from thr pipe as it can get hot.

Large hole you can nearly detect by eye when reving the engine. Have someone look under the car and youll see hot carbon come from the pipe or small amount of black smoke.

The reason you start here is because its the one of the easiest to diagnose. Plus if theres a leak you will never pass until its fixed not matter what you do.

1.1 exhaust is good. Move to the lamdbas. These would generally prompt an engine fault code. These are sensors that montior gasses/liquids in your exhaust system. Generally there are 2 sensors. Bank 1 and bank 2. Most cars you can diagnose engine mgt codes by yourself. E.g vauxhall and opel. Place foot to floor on brake and accelerator. Turn key 2 click and hold pedels down. After 10 seconds the engine code blinks. Count the blinks. 10 blinks = 0 remember. So 10,1,5,9 blinks will be error code P0159.

This help you determine which if any lamdba os bad. The first bank or second. If its neither of these move on.

1.2 so now we have determined no leaks in the exhaust or bad sensors. Normally people here will tell you to get a new catalyic converter. Which can be super expensive.

Heres a few tips and tricks now to hopefully prevent you needing a new cat.

Carbon deposits build up in your car. Just like a chimney. So.. not to promote driving over the speed limit. Having car rev high on 1,2,3 gear will give it a good clear out. 4 is most effective. But dangerous as where can you do this? Again not promoting dangerous driving.

Get a good straight and floor the car. Letting the rev in every gear rev nearly to the max. This will give it a good clear out. You made need to do this 2 or 3 times depending on how long your airstrip is lol.

Get a new air filter 10 euro
Get a new fuel filter 10 euro
Change the spark plugs. I recommend OE or something like ngk platinum. 25 euro
Get 1 litre of dipetane. Totally safe to mix with fuel. 10 euro reduces emissions by 15-50 percent. Must put into a 3/4 to full tank and youll only use 250 ml dipetane.
Do it 2 days before test.

Before you got for your test. You want you car idleing for at least 30 mins. And the a good 30 mim run. Give her a good spin high on revs when you can. Engine should be fully warm going in.

Show up on time not 10 mins early. As they never take you early and you want to reduce the time your car is sitting after the warm up you just gavr it.

Plus dont leave the car running outside when arriving. What happens is your not reving it. Emissions build up again as its be idling. They go in a and test emissions first. All those emissions will get u caught.

Turn car off and hope the wait time is less that 15 mins before they take your car.

Also common is maf sensor, will throw up a fault code.
Not common, fuel filter.. most people think it is. But ive seen this changed so many times and didnt impact enmissions. But this is a service part and should be changed now and then. If never changed could be the cause. More so in diesel.
Air housing - again another fault code should appear. Not common.
Something more serious i the engine. Will need to be found by a good mechanic. Not often.. youll generally have other problrems appearing and emissions wont be the clue. It generally causes bigger and more noticable problems

If all this fails. Sorry its time to buy a cat

Anyone who has any additional tips/tricks or anything ive missed please share

rex-x

This is well intentioned but not very good advice

scannerd

Yes nice write up but the main issue with emissions (before ever changing cat) is having the air fuel ratio spot on for your car, there is quite a but more to getting that right then you have covered / can cover in a general post as it varies by model / mileage etc.

jca

All was going well until it just turned out to be an ad for snake oil...

Kyledonegal

rex-x wrote: »

This is well intentioned but not very good advice

Thats fine you feel this way rex, but you may want to elaborate. If you feel anything herr is incorrect you should highlight this for people to know

Kyledonegal

scannerd wrote: »

Yes nice write up but the main issue with emissions (before ever changing cat) is having the air fuel ratio spot on for your car, there is quite a but more to getting that right then you have covered / can cover in a general post as it varies by model / mileage etc.

Totally true. Intake contamination. Maf sensors, fuel pump, air housing filter, and so on can impact emissions. But as always emissions is a rule out 1 by 1. Its very difficult to find the culprit at times. Above hopefully covers the most common and also the easiest culprits to root out. Anything else would need to checked by a good mechanic. And when i said good, not one that will have you change 5 parts before he gets it right...just becuase he didnt complete a proper root cause analysis

blackbox

Lambda sensors can be faulty/worn out without showing a fault code. Usually fault codes are only triggered by heater circuit fault.

If you are burning a load of oil, no amount of Dipetane is going to fix it.

.

Kyledonegal

blackbox wrote: »

Lambda sensors can be faulty/worn out without showing a fault code. Usually fault codes are only triggered by heater circuit fault.

If you are burning a load of oil, no amount of Dipetane is going to fix it.

.

More often or not the lamdba sensor is bad... you do get fault code per lamdba. This does not meam 100% lamdba is bad.

Bank 1 error code could mean something more serious in the engine is wrong. Youll most likly have verious codes atrached with this. Like p017p and 2 or 3 others. Multiple sensors would pick this up.

If only thr bank 1 is showing up. Its a bad lamda generally. To be accurate use a diagnostic tool you can buy for your car to read the sensors. If the lamdba is not reaching high readings above say 700.. its a very good clue to a fauly sensor.

Bank 2 means there is a problem between the catalyic converter and that second sensor. Leaks/bad cat or fauly sensor. Checking the pipe will again with paper like mentioned above will rule this out. Reading the sensor if it working will rule that out.

Again other things can be impacting this. But you can rule out some of the possible issues by doing the above.

JaymanD12

Theres a lot here to look into and seems i can do a lot by myself but i dont often know how to do these things.I was told to give the car a good service. Spent 100 euro on parts and another 100 on labour. I failed the nct retest. Im going to have a look at the pipe this evening for a leak like you suggested. If i find one how is it repaired. Do you know how to read engine light for bww? Thanks

JaymanD12

Hi i got a friend who could check my error code and it showed lamba. It was the one after my cat. I check and sure enough there was a hole in my pipe. I have it booked to get fixed on wednesday. Thanks for the help