A fluorescence-based temperature sensor system using a digital signal processing approach has been developed and evaluated in operation on a working automotive engine. The signal processing approach, using the least-squares method, makes the system relatively insensitive to intensity variations in the probe and thus provides more precise measurements when compared to a previous system designed using analogue phase-locked detection. Experiments carried out to determine the emission temperatures of a running car engine have demonstrated the effectiveness of the sensor system in monitoring exhaust temperatures up to 250 °C, and potentially higher. © 2006 IOP Publishing Ltd.