Experimental tests have been carried out on a moving grate boiler at a 4 MW heating plant located at a sawing mill in south of Sweden. In order to investigate the performance of the combustion chamber, the local concentrations of O2, NO and CO are determined in the region above the grate using two water-cooled stainless steel probes of different lengths and diameters. The fuel used was sawmill waste consisting of bark, sawdust and shavings. The results of the study provided valuable insights into the combustion process.