Failed nct on emissions - Honda Civic

Son brought his 98 civic for nct 2/3 weeks ago and it failed on emissions, only has 43k miles on clock. Was told to put some additive in and use the V power petrol and that it would be ok. Murphy's law, it gave a worse reading on the re test. Examiner siad some sensors might have to be replaced ? Anyone have any prior experience on these ?


Thanks in advance

Secman

Failed on my emissons a few weeks back on a FIAT (enough said) and got the oxygen sensor replaced this has something to do with the lambda ratio (i havn't a notion to be honest) brought it back in and it passed by the skin of it teeth - don't care though its the last nct it will ever get.

my garage's emissons tester wasn't calibratd so he siad just bring it back and have it retested in he nct centre as it was cheaper than bringing it to another garage to hook it up to the tester.

Was lucky though i was convinced the cat was gone and they can be expensive - oxygen sensor was only 60-80 quid i think.

catalytic converter could be defictive. I have seen it in a Toyota Corolla before - the catalytic converter was changed and it passed the emissions test on retest.

how far over the limit was it? if not excessivley over try the cheapest options first.

is the air filter clean/new? cheapest fix/cause of high emissions, also try to run the car for a good while before test, drive it around for at least a half hour beforehand if possible. the hotter the engine the leaner the emissions (you'll know if the engine is at it's hottest if you hear a ticking (cooling down) noise from the engine when you switch the car off)

Thanks for the replies, think i know what the problem was, he put the additive to clean engine in on the morning of the test , so the tester was testing with the sh.t coming out ! Apparently he should have put the additive in at least a week beforehand. Anyone any thoughts on this explanation ?

Secman