Ford Duratec/ZeTec Trouble Codes P1151, P1131, P0174, or P0171
All four of these codes indicate a problem with a lean condition. This engine is known for vacuum leaks, so before you start throwing oxygen sensors at the problem, it pays to perform some diagnostics.
Start by cleaning the MAF sensor. Spray the sensor with CRC MAF sensor cleaner (even though Ford tells you to replace it because it can’t be cleaned). Next, check the plastic vacuum lines, especially where they meet with rubber boots. If they are cracked or the rubber is swollen, replace the line. Then check the rubber elbow that holds the PCV valve. That elbow is a know failure point. In fact, the entire PCV tubing is a trouble spot on these engines. (thanks Ford).
After you’ve checked for vacuum leaks, check the operation of the oxygen sensors. The easiest way to do that is to disconnect the sensor from the harness. Connect one lead of your digital voltmeter to the black wire on the sensor and the other lead to battery negative. Then start the engine and add fuel (either propane, starting fluid, or carb cleaner) into the throttle body to force the mixture to rich. You should see a corresponding increase in voltage as you add fuel (from 0 volts to slightly less than 1 volt). Start and stop the fuel enrichment and watch the meter to see if the volts drop as you stop adding fuel. If you see no change, the sensor is dead. Replace it.
You can also use the fuel enrichment procedure to locate intake gasket leaks.
Code errors
P1131 Lack of Upstream Heated Oxygen Sensor Switch – Sensor Indicates Lean – Bank No. 1
P1151P1151 Lack of Upstream Heated Oxygen Sensor Switch – Sensor Indicates Lean – Bank No. 2
P0174 system lean bank 2
P0171 system lean bank 1
