P0420 on a Lotus... aka Cat Efficiency Below threshold... The factory computer monitors the differences between the front and rear O2 sensors and if they don't follow a pre-defined pattern the factory computer decides that the cat is not working well enough to satisfy the emissions (smog) limits. The check engine light will be illuminated and the vehicle definitely won't pass any smog tests.
Ultimately to be an environmentally responsible individual it may be best to replace the cat with a high quality unit. Personally I prefer the stainless core cats manufactured by Random Technologies as they do not offer a significant amount of backpressure or performance loss and are still very successful at passing tailpipe smog tests.
Some Lotus owners do not drive their vehicles on the street and these emissions checks are merely an annoyance.
These are the stock 2006 Exige settings.
Here are the recommended settings to disable the code.
This will leave the emissions monitors as incomplete.
Support for Lotus Elise, Exige and Exige S
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