SCR catalyst diagnostics

A method, comprising:

• releasing NH3 from an SCR catalyst by initiating a thermal event;
• then
• oxidizing the SCR catalyst and a three-way catalyst positioned upstream of the SCR catalyst by ceasing all engine fuel injection;
• then
• after oxidizing the SCR and three-way catalysts, combusting a rich engine air-fuel ratio;
• and then
• indicating SCR catalyst degradation via a controller based on a difference in time for an intermediate oxygen sensor downstream of the three-way catalyst and for a downstream oxygen sensor downstream of the SCR catalyst to switch from a lean air-fuel ratio to a rich air-fuel ratio.

A system comprising:

• an engine coupled to an exhaust passage;
• a three-way catalyst arranged in the exhaust passage;
• an SCR catalyst arranged in the exhaust passage downstream of the three-way catalyst;
• an upstream exhaust sensor arranged upstream of the three-way catalyst, an intermediate exhaust sensor arranged downstream of the three-way catalyst, and a downstream exhaust sensor arranged downstream of the SCR catalyst;
• and
• a controller including instructions to
• release NH3 from the SCR catalyst by initiating a thermal event;
• then
• operate the engine with the upstream, intermediate, and downstream exhaust sensors each indicating a lean air-fuel ratio;
• then
• adjust engine operation to operate the engine with the upstream exhaust sensor reading a rich air-fuel ratio and the intermediate and downstream exhaust sensors each indicating lean air-fuel ratios;
• and then
• indicate degradation of the SCR catalyst based on when the intermediate and downstream exhaust sensors switch from a lean air-fuel ratio to a rich air-fuel ratio.