The term "lean fuel mixture" is used to refer to an air/fuel ratio that has more air than fuel, which is often an indicator of issues such as an empty or almost empty fuel tank or malfunctioning fuel pump or fuel injectors. Vehicles that are experiencing a lean fuel mixture will often not start, and this problem can be solved through troubleshooting measures including adding more fuel to the tank even if the fuel gauge says the tank isn't empty and listening for the proper function of fuel pumps and injectors. Tools such as a funnel or even a medical stethoscope can be used as diagnostic tools to listen to the fuel pumps and injectors.
Lean mixtures are most commonly experienced when a vehicle does not have enough fuel. Those who are troubleshooting this issue should keep in mind that fuel gauges can malfunction and indicate that a vehicle's gas tank has more fuel than it actually does. Engine malfunction due to a lean fuel mixture can also be caused by problems unrelated to fuel, such as too much air getting into the engine. A proper air/fuel ratio is necessary for the proper functioning of all engine-driven vehicles, and rich mixtures, or those with too much fuel, are problematic as well.