Got a Megane 1.6e Sport and it's just failed it's MOT on the emissions. The fuel mixture goes up and down like a yo-yo according to the analyser and I think it's related to the temperature because when it's cold it runs fairly well, but as it warms up it gets lumpy - probably over rich mixture. Once warm, the dashboard temperature gauge goes up and down from 'normal' up to hot and back down again in about 8 - 10 seconds and repeats it several times.
At the right hand end of the head there are two sensors living presumably close to the thermostat. Disconnect the top black one and the dashboard gauge drops to zero, so presumably the lower white one is the engine/water sensor for the ECU?
I have cleaned and lubricated the connectors and the connectors to the Lambda sensor but it has made no difference.