Hello.
The ecu Maps will take care of the Fuel to air ratios normally. The O2 sensor will only monitor the fuel and air if the ratio alters from the Stoichiometric / Lambda which is 14.7 parts air to 1 part fuel the 02 sensor will work on a closed loop system to over ride the ecu map setting to adjust the fuel to compensate and keep the mixture at the ldeal combustion rate of 14.7:1.
You can run rich by 20% of lamdba and still your Hydrocarbons will still be within the MOT requirements. Running lean will produce more oxegen/Nitrogen emissions which the MOT will not pick up.
But if you have a Naff O2 sensor get it changed asap, if your driving hard and the engine runs lean you can suffer from pre-detination wich aint good for your engine. If your running rich at high revs then your sooting your engine up with carbons, naffing your spark plugs and in both cases you will lose power.
Nick
