failing smog d15 civic?

check your compression, if it is too high, that can lead to elevated NOX. i had that issue 3 years ago. ran a steam clean treatment through it and it passes easily now.

HC is unburned fuel, NOx is the combustion temperature is too hot, if combustion temperature is hot you will get high HC and NOx. Does it miss fires? RPM too high, how is cooling system is it ok. Also check exhaust system specially cat efficiency test. Make sure you exhaust system tight and O2 sensor heater are working? Check DTC before you do anything sometime blow HO2S fuse can cause this problem.

you do no longer supply a year for this, that can exchange my answer. verify to make confident that the manifold atmospheric stress sensor is working and that it extremely is hooked as much as the intake manifold perfect. seek for loose connection on the intake edge of the engine. the reason I propose this is that the MAP sensor and cam sensor artwork collectively to make certain ignition strengthen. A vacuum leak or a defective MAP sensor will reason this issue. With the timing corrected, the O2 sensor might exchange the air gas mixture ensuing interior the engine working warmer. With the engine working warmer the NO issue might shield itself. even if it isn't the O2 sensors inflicting this issue!

also check your fuel cap