I have a T-registered Vauxhall Corsa three-cylinder, which has high emissions. It's done 56,000 miles, so still quite young in terms of mileage, with two owners from new. It was owned by a pensioner who used it for short trips, and had been run low on oil for a while some time in the past. It's since had a service with oil/filters and plugs changed - but there's been no effect.

Could it be the lambda sensor that has failed or something worse? I had someone suggest to me that the piston rings had failed.