We'd suggest checking cylinder compression to ensure your engine doesn't have heavy carbon build-up which could cause the engine to run hot; causing high NOx. Also make sure the engine is not running lean. If your smog test results indicated 0.00 CO levels, chances are the engine is running lean. Look at the oxygen sensor first for proper operation, and than the remaining emission sensors... mainly the MAP and TPS. This Honda Civic more than likely doesn't have an EGR valve, so that's one component you need not worry about. If it did however, it would be our first selection of emission components to test. On EGR valve equipped vehicles, the EGR valve is primarily used to control NOx. Non-EGR equipped vehicles rely on proper fuel control, engine temperature, and catalytic converter function.
