They think that because thats the most Common reason the light comes on if the car is still running fine. Without the proper codes it's complete guess work however experienced a mechanic they are...
Just trying to stop people making the same mistakes as me. Didn't get the codes, got told it was the lambda sensor as the car was still running fine, changed it and paid them, light still on. It was actually a leaky fuel injector dropping petrol into the cylinder after I turned off the engine causing a misfire so bringing on the engine light. Like I said it could be 1 of 100's! Have you seen how many fault codes there are connected to an IS200? Blooming loads!!! Lol... :)