Getting to the bottom of what is causing your Mitsubishi Montero to trigger trouble code P0174 will require some diagnostic time. You'll need to find out if its a defective sensor that's tricking the ECU into thinking there is a lean fuel condition, or in fact there is a lean fuel condition due to a defective mechanical component.
P0174 indicates the oxygen sensor in bank 2 has detected a lean fuel condition. Bank 2 is usually the opposite side of the #1 cylinder. Lean fuel condition means the fuel mixture contains too much oxygen and not enough fuel to promote efficient combustion; the ratio of 14.7:1 (air/fuel) is not present.
Out first recommendation would be to inspect the MAF (Mass Air Flow) sensor for proper operation. The MAF sensor will collect dirt and dust over time and should be cleaned, if not replaced. If you have the proper diagnostic equipment you may be able to diagnose your Ford Thunderbird MAF sensor at home before buying a replacement. Or else, try cleaning the component (only with electronic circuit cleaner).
While working on the MAF sensor also ensure there aren't any vacuum leaks downstream the MAF sensor. Downstream means AFTER the MAF sensor; once the MAF sensor has detected a certain amount of air entering the intake manifold and it sends this data to the ECU, no additional air must enter the intake manifold or else a lean fuel condition will be created.
Components which may cause inadequate fuel delivery should be inspected, such as the fuel filter, fuel pump, fuel pressure, and possible fuel injectors.
1. The MAF (Mass Air Flow) Sensor is dirty or faulty. Cleaning the MAF sensor with electronics cleaner (comes in a spray can) may resolve this issue.
2. Possible vacuum leak downstream of the MAF sensor. You should inspect all vacuum and PCV hoses.
3. Possible fuel pressure related issues (check for dirty fuel filter and correct fuel pressure).
If it is found that the MAF is working correctly, you may than turn your focus to the oxygen sensor and measure its voltage output based on the fuel content in the exhaust. While introducing propane to the intake system, the oxygen sensor reading should show an increase. Using a voltmeter oxygen sensor voltage should observe as propane is added. The ECU should decrease fuel delivery at the instant propane is added to the system. If there is no change in fuel delivery there is emission system feedback problem. If there is no increase in voltage, more than likely the O2 is defective. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output, voltage increase should also be present at the computer input terminal. If voltage is present at the terminal as well, next you must ensure the computer output terminal to the fuel injectors are lowering “injector pulse rates”.
As you can see, there is some technical know-how involved in diagnosing smog check failure. If you go about replacing every part that someone suggests might be the problem it'll cost you a fortune. We highly recommend having a smog check repair station conduct a diagnosis to get to the bottom of your vehicle P0174 trouble code. A smog station will typically charge 2 hours of labor to diagnose a smog check failure.