I use a Maxiscan MS309 to clear the warning light.
The Yellow light comes on often enough that I keep the thing in my car.
The cause is down to the ECU over fuelling the engine under a set of particular conditions... MMC is aware of the problem and is, I understand, working with Bosch to develop a revised ECU map for all 4.8s to cure the issue.
The warning is because the Lambda reading from the post car sensor drops out of permitted range for a second or so, so triggering the warning. It is harmless, a "technical" violation of the permitted emission levels.
I have found that it happens when the cats drop below optimal temperature, typically when driving at low revs at constant speed. One solution, on an automatic, seems to be to always keep the car in "Sport" mode... I've not had the light come on since September. Having said that no doubt it will light up this weekend....
Peter
