Method for determining malfunction of nitrogen oxide sensor and selective catalytic reduction system operating the same

A selective catalytic reduction system for nitrogen oxide of exhaust gas, comprising:

• an exhaust pipe;
• a first nitrogen oxide sensor disposed in the exhaust pipe;
• a selective catalytic reduction apparatus for eliminating the nitrogen oxide, disposed in the exhaust pipe behind the first sensor;
• a second nitrogen oxide sensor disposed in the exhaust pipe behind the selective catalytic reduction apparatus;
• and
• a control portion for determining whether the first sensor is malfunctioning, wherein the control portion performs:
• comparing a nitrogen oxide amount detected by the first sensor with a predetermined value;
• if a difference between the predetermined value and the amount detected by the first sensor is in a predetermined range, determining whether an engine is in a steady state;
• if the engine is in a steady state, changing at least one engine control parameter, thereby changing an actual nitrogen oxide amount;
• after changing the parameter, determining whether the amount detected by the first sensor changes;
• and
• determining a malfunction of the first sensor in a case if the amount does not change.