Q: idling problem with peugeot 406

Hello and thank you for contacting YourMechanic. When you start the engine up when it is cold and the engine idles rough or does not want to stay running, then this is an indication that the engine temperature sensor has failed. When the engine is started cold, the engine temperature sensor sends a signal to the computer to provide a richer mixture. As the engine temperature sensor begins to fail, it sends a higher than actual temperature signal to the computer. The computer leans the fuel mixture thinking the engine is at normal operating temperature. This will cause the engine to run rough. As the engine warms up, it not longer needs the richer cold idle mixture and the engine begins to run smoother. I recommend replacing the engine coolant temperature sensor. If you need further assistance in diagnosing your vehicle, then seek out a professional, preferably a YourMechanic professional to help you with your vehicle.