A high NO failure is usually caused by a defective EGR valve and/or systems (if your Honda Civic is equipped with an EGR system). High compression, high engine temperatures, and/or vacuum leaks which cause lean fuel mixtures (low CO) will also cause high NO failures. If after all the emissions systems on your Honda Civic are inspected and no faults found, consideration should be given to a defective Catalytic Converter.
If your Honda Civic is equipped with an EGR valve we recommend inspecting this component first. The EGR valve is responsible for re-circulating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture, keeping temperatures under 2500 F, and lowering NO.
If your Honda Civic is not equipped with an EGR system, or after the EGR system has been inspected and no faults found, or faults found and repaired, you should inspect the "fuel feedback" system. This includes ensuring the oxygen sensor is working properly, the computer system is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. As mentioned, a lean fuel condition will cause high NO as well.
Additional Honda Civic sensors which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio) are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If your Honda Civic's engine coolant temperature sensor is not operating properly (not sending the correct voltage signal to the Engine Control Module) the ECM will not be able to properly calculate how much fuel to deliver to the combustion chambers. The ECM may lean out the fuel mixture, causing the high NO. The MAF (mass air flow)sensor measures the amount of air entering the intake manifold. This sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers.
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