The engine management amber warning light on my daughter's Toyota Yaris (2007 model) comes on intermittently. A couple of years ago this was diagnosed as a faulty lambda sensor, which was replaced but the problem persisted.

At the last service they were again asked to investigate and then checked the emissions as I assume the diagnosis indicated a problem in that area but the emissions were fine. She has been told only to worry if the red light shows.

My daughter's main concern is that it may affect the sale of the car which is planned for the near future. It is going in for it's next service with a main dealer in a couple of weeks. Do you have any suggestions as to what might be causing this problem?