found this on a lambda website quite interesting
ECU's are progressively becoming able to accurately identify a failing lambda sensor, but for older engine management systems, this is not the case. Fault codes do exist for lambda failure but most older ECU's only detect an absence of signal, for example if you had cut the sensor wires, or average 'too rich' and average 'too lean' mixtures over a long period of time. These fault codes do not always work but the increasing number of Lambda-code related MOT failures we encounter would support their usefulness. It still applies that the absence of a lambda-related fault code cannot be taken as a guarantee the Lambda is functioning correctly. This is especially true of some ECU systems with sophisticated LOS strategies, such as Toyota or Lexus, which will cope with a failing sensor but fuel consumption will be markedly up as a result of more fuel being used to maintain driveability