My car failed its MOT on the emissions does that mean the Lambda sensor should be replaced?
Not necessarily, further tests would need to be made to see if the problem was somewhere else in the system. To change the Lambda sensor immediately would be like a Doctor replacing his thermometer because it said the patient had a temperature of 102 degrees! Frequently there is another fault in the system and although the sensor is reporting this correctly the ECU can only make adjustments over a certain limited range. If this was not the case the engine would stop completely when a fault occurred rather than just run poorly.
