Emissions problem

[mo_ivy]

Hi fellow hexites. I need help and preferably before I’m broke! I bought my mk4 2.0 golf gti about 6 months ago, its on a 02 plate, had no problems for the first few months but the engine management light came on. I took it in for an MOT at VW but it failed on the emissions test so had a diagnostics test done by a garage and the lambda sensor came up faulty, had this changed (bought from GSF) straight away and hooked it up back up the computer, it still came up as the lambda sensor, the mechanic hooked some machine to my exhaust and took some readings, he told me my emissions are still high but the lambda sensor is working fine, best option is to change the catalyst converter. I had the CAT (bought from cats 4 less) fitted on last weekend and had an emissions test done, still high and still comes up as the lambda sensor. As soon as I had the CAT fitted another problem arose, the exhaust is blowing slightly which doesn’t sound healthy at all. Can someone please help me, my mechanic has now suggested an oxygen sensor but all this seems to be trial an error. I don’t really want to take it to VW as they will charge £80 an hour so please any help or suggestions will be appreciated. Many thanks and sorry for the long thread.

[5lab]

could possibly be the 2nd sensor is also faulty, or the fault is in the loom somewhere?

[GoNz0]

faulty air flow meter in the air intake can throw a 02 code in its early stages. i doubt it has a post cat sensor as there not part of UK emmision standards, so its either Aiflow meter or something as stupid as a clogged air filter ?

[mo_ivy]

I doubt its the air filter as ive just had a major 40,000 service done at VW, that was changed. I dont understand why nothing other than the lambda sensor comes up on the diagnostics test. How much would an air flow meter cost?

[SilentDeath]

not sure if its pre-cat or post-cat but my 11 year fiesta has a sensor near the front of the exhaust, cant rememeber where exactly though.

I would check the airflow meter is connected properly, might have been left lose when they changed the filter.

Also oxygen sensor = mass airflow sensor. It just tells the ecu how much air is going in and at what temperature so it can adjust the amount fuel.
I dont think they will be THAT expensive, IIRC it works by having several wires with a voltage through them, air will change the resistance slightly which the ecu measures by changes in current.. so just a small box with a few wires..

Also if you check your sparkplugs, they will probably have a nice coat of carbon deposits soon, which wont help with your emissions and might help tell you whats wrong.

Is it possible the engine management light came on becuase the sensor was reading high and not becuase it thought it was broken? It seems garages are not particually good at interpreting the codes.

[kickstart 1]

Really dont know if this will be of any use .but costs nothing ! ..I had similar problem at one of my M.O.T's ,the lambda sensor had gone ,was fitted with new one and still failed my emissions test,my garage solved the problem very simply ,dont know if it will work for you ? All they did was took my car for a good blast down the road for about 15 mins , came back linked it straight up to emission machine ,et voila problem solved ,they commented that sometimes its all the car needs to pass.Like i said may help or not ,but wont cost anything to try !

[Butcher]

Giving it a good blast will often shift some of the deposits in the engine which makes it run a bit cleaner.
You could also try running it on shell v-power for a few weeks as it has a high level of detegents which will clean out the engine (though you'd have to somehow pass the MOT first ).

[5lab]

could also be the cold-start-sensor playing up - if it always thinks it's cold it will be running the car rich.

fully agree with the italian tuneup (blast down the road). you want to sit on the motorway in third. altinating between foot flat on the floor and nothing.. if you've got a revgauge, try and get it to cruise within 1,000 rpm of redline