A light emitting diode (LED) based illumination module performs on-board diagnostics. For example, diagnostics may include estimating elapsed lifetime, degradation of phosphor, thermal failure, failure of LEDs, or LED current adjustment based on measured flux or temperature. The elapsed lifetime may be estimated by scaling accumulated elapsed time of operation by an acceleration factor derived from actual operating conditions, such as temperature, current and relative humidity. The degradation of phosphor may be estimated based on a measured response of the phosphor to pulsed light from the LEDs. A thermal failure may be diagnosed using a transient response of the module from a start up condition. The failure of LEDs may be diagnosed based on measured forward voltage. The current for LEDs may adjusted using measured flux values and current values and a desired ratio of flux values. Additionally, the LED current may be scaled based on a measured temperature.