A method and system for determining whether a desired amount of hydrocarbon is actually injected into an engine exhaust to react with NOx in such engine exhaust. The reaction is facilitated with a catalyst downstream of the hydrocarbon injection. The method and system measure a first temperature difference across the catalyst during the reaction. The first temperature difference is compared with an expected temperature difference across the catalyst during the reaction. The difference in exotherm is used to adjust the injected HC quantity, and diagnose injector failures.