is above a certain level the exhaust flow rate sensor 32 is disposed within the exhaust system 12 and is in communication with the ecm 22 the exhaust glow rate sensor 32 generates a signal indicative of the mass flow rate of exhaust gas within the exhaust system 12 the exhaust oxygen sensor 34 is disposed within the exhaust system 12 upstream of the doc 14 the exhaust oxygen sensor 34 is also in communication with the ecm 22 the exhaust oxygen sensor generates a signal indicative of an amount of oxygen contained within the exhaust system 12 the amount of oxygen within the exhaust system 12 varies based on engine operating conditions and the amount of oxygen within the exhaust system will limit the amount of hydrocarbons that may be used for dpf regeneration turning now to fig2 a schematic diagram of a portion of an electronic control system of the engine 10 is schematically depicted the schematic diagram of fig2 would be executed within the ecm 22 of the engine 10 the control system receives a doc inlet temperature as shown at block 40 the doc inlet temperature is utilized to estimate a temperature change required for dpf regeneration as shown at block 42 block 44 shows that dpf temperature may be estimated if a doc outlet temperature sensor 16 is utilized a comparator 46 compares the output of the estimated temperature change for dpf regeneration from block 42 with the doc outlet temperature sensor output of block 44 the output of the comparator.