This paper presents a start algorithm that is able to control the air/fuel ratio (AFR) during the cranking phase and immediately hereafter, where the ordinary λ-control is not yet enabled. The control is based on the ion sense principle, which means that a current through the spark plug is measured directly after the spark has disappeared. This current is a measure for the temperature and therefore of the combustion in the cylinder. This is an excellent way to start a CNG (Compressed Natural Gas) engine with unknown gas qualities. A typical example of application is when the vehicle is almost out of fuel and is refueled at a motel stop. The small amount of old fuel that is left in the system will mix with the new fuel resulting in an unknown fuel quality. The control system shall then be able to start the engine directly or after an accommodation over night.During the last condition, the oxygen sensor is still cold and thus not able to correct for fuel quality changes. By using the ion current signal, the start algorithm manages to start the engine with unknown fuel qualities. In addition, the algorithm also manages fuel changes in the period immediately after cranking.The key idea of the presented method is to supply the subsequent cylinders with a step wise changing AFR. A gas injection system that is able to dose cylinders individually is therefore indispensable. Based on the ion current signal, the “combustion status” is determined, and the changing AFR is continued or not. Immediately after start, when the engine is running, another algorithm takes over the control to maintain ignitable air/fuel ratios. Once the oxygen sensor is heated up, the normal λ-control can take over the task to correct for a changed gas quality.All experiments are performed on a 5-cylinder 2.0 ltr turbo charged Volvo production engine converted from gasoline to CNG operation. It is equipped with a newly developed sequential gas injection system, featuring injectors with electrical properties of gasoline injectors.