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The fuel injectors in a heavy-duty diesel engine with a selective catalytic reduction (SCR) system operate in harsh conditions that cause them to deteriorate, resulting in changes in the fuel injection quantity. In this study, a method for determining the injection quantity of fuel injected using the output of a NOx sensor in a heavy-duty diesel engine equipped with a SCR system is developed. A mathematical model was derived to calculate the fuel injection quantity based on the oxygen concentration in the exhaust gases. Bench tests were conducted at various engine operating conditions. The results indicate that at high engine loads, the absolute error between the oxygen concentration measured by the NOx sensor and that measured by a gas analyser was less than 2%, which is acceptable, and the reactions in the SCR reactor consumed no more than 1% of the oxygen in the exhaust gases, which is negligible. An analysis of the model demonstrates that the calculated value of the injection quantity based on the oxygen concentration in the exhaust is more accurate for lower oxygen concentrations (i.e., at greater engine loads). The test data reveal that the error in the fuel injection quantity obtained from the model was less than 2% at high engine loads. This research demonstrates that the on-board calculation of the fuel injection quantity based on the oxygen concentration signals from the NOx sensor is possible at high engine loads. [ABSTRACT FROM AUTHOR]