Re: Hello! CODE HELP 04 liberty 3.7
P0131 = O2 sensor 1 low voltage.
P0171 = system lean, bank 1.
P0172 = system too rich, bank 1.
The first code is telling that the O2 sensor voltage doesn´t go over 0.5 V for more then 2 minutes. This can be due to a lean mixture, or a faulty O2 sensor. A leak in the exhaust manifold or manifold gasket could draw in fresh air and fool the O2 sensor, causing the PCM (powertrain control module) to add more fuel and when this passes a certain limit, it will set the P0172 code.
So i would start checking for any exhaust leak first. Also check the O2 sensor wires if they are not touching a hot exhaust part and shorting out.
If that seems to be ok, then most probably the bank 1 upstream sensor (1-1) is faulty.
There could eventually be a problem with low fuel pressure (pump/regulator/filter) or plugged cat, but that should influence bank 2 as well.
If you have a scanner which shows live data, you can see the O2 sensor voltage. It should change constantly from 0.1 to 0.8 V on a good sensor, but any other problem causing the system be lean, will make the O2 voltage low.
But if you see the bank 2 upstream sensor (2-1) working properly, then most probably the 1-1 sensor is faulty.
If you have 100 K miles on it and the sensors where never changed before, then it would be a good idea to change both upstream sensors. They become laisy over time and cause the mpg to go down.