1993 Isuzu Trooper has high CO, check engine light not on, and it failed the smog test.

Your Isuzu Trooper is going to need a Fuel Feedback Test. A smog check repair station will have to find out why your Trooper is producing high CO. The diagnosis for this fault should begin with inspection of the fuel injection "feedback" system. This includes ensuring your Isuzu's oxygen sensor is working properly, the missions computer is receiving the signals from the oxygen sensor, and the OBD 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. To perform a feed back test, a 5-gas analyzer (or smog check machine) and propane will be needed. Unmetered propane should be introduce to the intake system to ensure the oxygen sensor is reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery. If no change in fuel delivery is seen we know we have a feedback problem. Then the diagnose should turn to which part of the feedback system is defective. Using a voltmeter you must observe the voltage output of the oxygen sensor as propane is added. Voltage should increase. If there is no increase we have a defective O2 sensor. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output the technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the computer output terminal to the fuel injectors are lowering "injector pulse rates". At the end of the feedback test it will be know whether the high CO fault is electrical or mechanical.