An exhaust gas treatment system for regulating NO x emissions in an internal combustion engine including diesel exhaust fluid, the system comprising: an exhaust gas pathway that is configured to receive exhaust gas from the internal combustion engine; a DEF injector configured to controllably inject DEF into the exhaust gas pathway; a selective catalytic reduction chamber positioned in line with the exhaust gas pathway such that the exhaust gas and ammonia (NH 3) both enter the SCR chamber at a first location, the exhaust gas and the NH 3 react within the SCR chamber to reduce NO x levels in the exhaust gas, and the NO x -reduced exhaust gas exits the SCR chamber at a second location; a pretreatment sensor positioned upstream of the SCR chamber and the DEF injector and configured to detect amounts of NO x; a tailpipe sensor positioned downstream of the SCR chamber to monitor the NO x -reduced exhaust gas leaving the SCR chamber, the tailpipe sensor being configured to provide an output indicative of an amount of NH 3 and NO x in the NO x -reduced exhaust without differentiating between amounts of NH 3 and amounts of NO x; and an electronic control unit configured to underdose an amount of the DEF injected into the exhaust gas pathway, receive the output from the tailpipe sensor, compare the output to a threshold, the threshold being greater than zero, and increase the amount of the DEF injected into the exhaust gas pathway if the output from the tailpipe sensor exceeds the threshold; wherein, the underdosed amount of DEF injected into the exhaust gas pathway is determined by the electronic control unit based on the output of the pretreatment sensor. an exhaust gas pathway that is configured to receive exhaust gas from the internal combustion engine; a DEF injector configured to controllably inject DEF into the exhaust gas pathway; a selective catalytic reduction chamber positioned in line with the exhaust gas pathway such that the exhaust gas and ammonia (NH 3) both enter the SCR chamber at a first location, the exhaust gas and the NH 3 react within the SCR chamber to reduce NO x levels in the exhaust gas, and the NO x -reduced exhaust gas exits the SCR chamber at a second location; a pretreatment sensor positioned upstream of the SCR chamber and the DEF injector and configured to detect amounts of NO x; a tailpipe sensor positioned downstream of the SCR chamber to monitor the NO x -reduced exhaust gas leaving the SCR chamber, the tailpipe sensor being configured to provide an output indicative of an amount of NH 3 and NO x in the NO x -reduced exhaust without differentiating between amounts of NH 3 and amounts of NO x; and an electronic control unit configured to underdose an amount of the DEF injected into the exhaust gas pathway, receive the output from the tailpipe sensor, compare the output to a threshold, the threshold being greater than zero, and increase the amount of the DEF injected into the exhaust gas pathway if the output from the tailpipe sensor exceeds the threshold; wherein, the underdosed amount of DEF injected into the exhaust gas pathway is determined by the electronic control unit based on the output of the pretreatment sensor. underdose an amount of the DEF injected into the exhaust gas pathway, receive the output from the tailpipe sensor, compare the output to a threshold, the threshold being greater than zero, and increase the amount of the DEF injected into the exhaust gas pathway if the output from the tailpipe sensor exceeds the threshold; wherein, the underdosed amount of DEF injected into the exhaust gas pathway is determined by the electronic control unit based on the output of the pretreatment sensor.