Method of detecting failure in a secondary air supply system for internal combustion engines

A method of detecting failure in a secondary air supply system for an internal combustion engine including an exhaust passage in which said secondary air supply system is arranged for supplying secondary air thereinto, a first exhaust gas ingredient concentration sensor, and a second exhaust gas ingredient concentration sensor, said first exhaust gas ingredient concentration sensor and said second exhaust gas ingredient concentration sensor being arranged in said exhaust passage at locations upstream and downstream of said secondary air supply system, respectively, the method comprising the steps of:

• (1) causing said secondary air supply system to operate when said engine is in a predetermined decelerating condition;
• (2) detecting a first value of concentration of an exhaust gas ingredient, from an output from said first exhaust gas ingredient concentration sensor when said engine is in said predetermined decelerating condition;
• (3) detecting a second value of concentration of said exhaust gas ingredient, from an output from said second exhaust gas ingredient concentration sensor when said engine is in said predetermined decelerating condition;
• (4) determining whether or not said first value of concentration of said exhaust gas ingredient is lower than a first predetermined value;
• (5) determining whether or not said second value of concentration of said exhaust gas ingredient is lower than said first predetermined value;
• and
• (6) determining that there is failure in said secondary air supply system when it is determined that said first and second values of concentration of said exhaust gas ingredient are both lower than said first predetermined value.