There are several reasons your Toyota Tacoma could have failed the smog check for high CO (Carbon Monoxide). Your Toyota may be suffering from a Rich Fuel Mixture. CO is a by-product of incomplete combustion, more then often related to the presentation of too much fuel to the combustion chambers. In a "Rich Fuel" condition diagnosis would begin with inspecting all possible mechanical faults which could cause excessive fuel to enter the combustion chambers. This diagnosis would include inspecting your Toyota Tacoma for high fuel pressure, air restrictions and/or leaking fuel injectors. Something as simple as a a dirty air filter may restrict ample air from entering the combustion chambers. The lack of air will create a rich fuel condition and result in high CO.
The main emission components which determine the amount of fuel your Toyota Tacoma's engine control computer commands the fuel injectors to deliver include the Engine Coolant Temperature (ECT) sensor, the Mass Air Flow (MAF) sensor and the Oxygen Sensor.
If your Toyota Tacoma'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 engine is 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 Toyota'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. Read more about What Causes High CO here.
