Failure diagnosis apparatus for exhaust gas control apparatus (as amended)

A failure diagnosis apparatus for an exhaust gas control apparatus having both a selective reduction function and a filter function, the failure diagnosis apparatus comprising:

• a NOx sensor arranged on a downstream side from the exhaust gas control apparatus, the NOx sensor configured to detect an amount of NOx contained in exhaust gas;
• and
• an electronic control unit configured to
• a) estimate a PM accumulation amount as the amount of particulate matter accumulated in the exhaust gas control apparatus,
• b) estimate a NO2 ratio as the ratio of nitrogen dioxide in NOx flowing into the exhaust gas control apparatus,
• c) compute a reference NOx purification rate as a NOx purification rate pertaining to a case where the exhaust gas control apparatus is normal by using, as parameters, an estimated PM accumulation amount as the PM accumulation amount estimated and an estimated NO2 ratio as the NO2 ratio estimated,
• d) compute an actual NOx purification rate as a real NOx purification rate of the exhaust gas control apparatus by using the amount of the NOx detected by the NOx sensor as a parameter, and
• e) determine that the exhaust gas control apparatus fails on condition of the difference between the reference NOx purification rate and the actual NOx purification rate exceeding a threshold.