Hello guys hopefully someone can help me,
i m in aberdeen,
Have 3 fails on Mot for emissions,

first was for high lambda with co2 and hc fine, got new exaust box fitted,

next fail was for high co2 with lambda and hc fine, two bottles of cataclean sorted that out,

went back and then lambda failed again,

first test was in kwick fit and they could nt fix the lambda problem,

next test was at council and they could nt fix the high co2, said get a new cat, but cataclean seems to have sorted high co2,

then went to peter vardy today and the car failed on lambda again,

I also got a new lambda sesnor fitted two days ago, I drove around like mad before the test today and it still failed on lambda after passing in a previous test, like 2 weeks ago,

is this a common problem? fail co2 and pass lambda in one place and pass co2 but fail lambda in another place,

car is a 1998 audi a3, brought it to local mechanic as i trust him, he said change spark plugs might do it,

only problem is I have to keep paying for test after test and re test, i ve got a diagnostic in kwick fit they said was fine, and another diagnostic in nivlem, they could nt identify any problems either, both these cost £100 and £35 respectivly, i dont want to go to Audi and have them do a £90 diagnostic to tell me I need a new lambda sensor after I ve just fitted one, got one from ebay for £17, audi charge £100 for a new sensor, excluding labour,

I m only a poor student so cant afford these prices so I m forced to seek other methods which thus far have proved futile,

Thanks for any responces