The wavelength switching probe laser as described in this paper is the key component in a "J-line scan" diagnostic system, that was developed during the past year at Rocketdyne. Typically, the J-line scan experiment consists of performing small signal gain measurements at a set of wavelengths, that correspond to different rotational transitions of the CO2 molecule. From these gain measurements the rotational temperature of the CO2 (001) state may be computed, which in turn yields the amount of specific optical energy available in the active medium. The significance of being able to measure this quantity is substantial. A low power wavelength switching CO2 laser was designed and constructed, that allows scanning of up to 8 individual, selected wavelengths in rapid sequence. This probe laser incorporates several novel features. The wavelength scan of the probe laser is accomplished by a galvo controlled mirror, switching 8 prealigned gratings in sequence into the optical cavity of the probe laser. Accurate and rapid angular registration of the switch-mirror is achieved by using control loops. After the laser is switched to the next wavelength (in less then 10 milliseconds) the laser cavity length is then rapidly readjusted to lock cavity resonance to line center. An electronic control center was also developed, that provides fully automatic operation. This system is now fully operational. The present paper describes the system together with experience gained by using it on a medium scale Gas Dynamic Laser (GDL) device.