We'll have to figure out what is causing your Mazda's engine to run lean. Is there a problem with the oxygen sensor not reporting accurately to the engine control computer, or is there a mechanical fault which is actually sending the engine less fuel then needed to combust efficiently?
The first step is to conduct a fuel feedback test (utilizing a 5-gas analyzer, typically a BAR-97 smog machine). This test will ensure the oxygen sensors are working properly, your Mazda Tribute's computer is receiving the voltage signal from the oxygen sensors, 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.
Additional sensors which should be inspected for proper operation include the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. Both these sensors also play a significant role in determining fuel delivery. The ECT (when cold) tells the computer to run rich, and visa-versa. The MAF measures air flow. If the MAF is measuring low air intake to the manifold, it will tell the computer to deliver less fuel. It is imperative that the MAF be measuring air intake accurately in order for the computer to command the fuel injectors on-time correctly. Many lean-fuel conditions are caused due to dirty MAF sensors.
At the end of the feedback test it will be know whether the high CO fault is electronic or mechanical, and the technician can further diagnose the fault. If the fault is determined to be mechanical, the locations which need to be inspected are any and all points of "fuel entry"... such as and most predominantly the Fuel Injectors. It is not uncommon to a stuck "closed" or "sticking" fuel injector to cause a lean fuel condition.
To answer your question in short... you'll more than likely need to obtain a hands-on diagnosis from a reputable smog check repair station. Typically smog repair stations charge a flat two hours of diagnosis time to uncover emission failures. The average hourly labor rate at a smog check repair station is $65.00 - $85.00.
