Got high emissions on my x reg 1.8 sel passat first fast idle test was0.621% got to be 0.300% and on the second fast idle test was 0.606% also on the natural idle test was 3.494% got to be 0.500% can anyone tell me what it could be before i spend money on something i dont need.cheers.

Dieseling or running-on is a fairly natural consequence of an engine that is idling too fast or being switched off before the idle has been restored after being revved.

Changes to engine design to improve acceleration and economy and meet new emission regulations required higher idle speeds and dieseling became a common problem which required the introduction of a fuel cut-off solenoid on many carburettors.

Dieseling is sometimes the result of overheating or too much carbon in the combustion chamber.

The design changes mentioned above required changes to carburation, including the choke design and operation. Complying with new emission regs required a quick engine warm-up and for the automatic choke to disengage in about two minutes. A higher fast idle was also required and mostly at least one vacuum servo was used to ensure smooth running during the warm-up period.

In order to disengage the choke quickly, it was often arranged to be heated, either by electricity alone or by electricity and hot water.

If the fuse has been removed the choke will not work correctly. Why the previous owner removed the fuse is a mystery, perhaps there is a fault.
The choke must be put back into good operation in order to achieve proper fuel economy and smooth running, or alternatively the carburettor could be converted to manual choke.

change the O2 sensors and if that doesnt work then its a new cat ,you can test the sensors first if you have a automotive test meter though its easy with the right gear ,the two wires with the same colour should read 12v heater circuit and the other two should read a variation from 8v to 12v depending on engine speed

hi.
if there are no faults showing, then the most obvious thing to do would be to change the air filter, this is a relatively cheap part, but it can make a huge difference to emissions, even if it looks perfectly clean.
also make sure you take the car on a good run before taking it to the test centre to ensure that the engine and catalytic converter are up to operating temperature.

it is possible that the catalytic converter is clogged up and not working properly. the only cure for this is to replace it. the mot tester can usually tell you if he thinks it is the cat. my mot tester is always good for advice, just ask what he thinks could be causing it if the air filter does not make enough of a difference

the O2 sensor works on one volt .5 is around the running voltage, i would refit the air hose as it will run the converter hotter at idle.

Did you install new spark plugs? Definitely replace the O2 sensor.

Several things.....

EGR Valve
Bad Spark Plugs
Bad EVAP Canister.....

All related items to fuel and spark.

Do you have bad fuel mileage? How does the engine run? Smooth? Chugs? Idles rough?

check both lambda sensors for breaks in wires and for open circuit on heater side of lambda sensor on the one under the car on the second cat,failed heater element will not heat gas engough for sensor to read causing false readings , also check for slow i.a.c valve on inlet manifold.

High CO usualy means the O2 sensors are not reading properly. Take it to a parts store that can read codes, I know Auto Zone does in my area. If it is the O2 sensor it is an easy fix but the sensors run about $50.00 each.

Clean the MAF (mass air flow sensor.)Follow the hose from the air filter to the air flow sensor.Use a proper cleaner rated safe for this purpose.

The advice that the garage gave you seems right. If any bushes were worn they would show up on the MOT Test. Try taking it to another garage to check out the Cat for your peace of mind.