Not on the scooby, but on my old 100,000 mile Primera.

According to the test station, my car was well within the limits for CO and HC emissions, but failed with a lambda of around 1.1. Thankfully a replacement lambda sensor (£12 from a scrappy, result!) has fixed the problem and allowed the car to pass.

However, I'm still puzzled as to why exactly the car failed. I understand that lambda is a measure of the fuel/air mix compared to the stoichiometric ratio, and that a value of one is therefore 'correct' and a value of 1.1 means that my car's mixture was too rich. But how come my HC and CO emissions didn't also show a problem? HC was just 9ppm (limit 200ppm!), so there obviously isn't excess unburned fuel coming out of my tailpipe.

Is it that the excess fuel is burned in the cat, which ultimately knackers it - ie. although nothing nasty is coming out of the exhaust yet, fixing the lambda value ensures that the cat will continue to work effectively?

Will I see an improvement in fuel economy now it's fixed?

 

Lambda 1.1 is lean. A 3 way cat only gets the CO, HC and NOx levels down at stoich, but at lambda 1.1 the CO and HC can be fine.