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My 1998 Honda Civic DX failed the smog check for high CO

I have a 1998 Honda Civic DX. It only failed the 15mph CO (%) portion of the smog test. The Max is .55 and my Honda Civic registered at .99%. The 25 mph portion passed with flying colors. What do I have to repair or replace to pass? I properly tuned up my car beforehand and drove for about 25 mintues before the test. I changed the oil, replaced the spark plugs and topped up the fluids a week before the test. I also put a full tank the night before. The air filter, distributor cap and wires were changed a few months ago. The O2 sensor is about 2.5 years old and so is the fuel filter. I also put in a fuel injector cleaner a week before the test as well. I dont know what to change anymore. Please help?
Answer:

There are several reasons your Honda Civic could have failed the smog check for. We can only recommend an experienced smog check repair station conduct a diagnosis regarding your Honda Civic's smog check failure. Along with the oxygen sensor there are several additional emission sensors which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio). The main emission parts include the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If your Civic's 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. When your Toyota's engineis cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Insuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). Your Honda's MAF 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. It is very important to diagnose the entire emission control system and ensure its proper operation. posted by SmogTips Support

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