Microbial population or biomass ('bugs') to reduce the influent load and reach required effluent permit limits. The ability of the biomass to adapt to its ever-changing environment and continue to perform under various conditions is the key component in successful biological wastewater treatment. Operators need quick assessment tools to determine if the treatment process is operating well just as a doctor listens to a patient's heart through a stethoscope and takes temperature and blood pressure. The parameter most indicative of biomass health and stability is biological oxygen uptake, or respiration rate. By measuring the oxygen uptake response of the biomass, they can 'listen' for symptoms and diagnose potential operating problems. Timely response to these potential problems can mitigate or eliminate their impact on the treatment plant. In this paper, a method of monitoring the plant operation using a combination of chemical oxygen demand (COD), DO and SOUR is proposed. A troubleshooting guide is presented and additional uses of oxygen consumption testing are discussed to improve plant operation.