Code P0401= Insufficient EGR Flow Detected
Code P0171= System Running Lean.
If you do have a dirty air filter, some dirt may have made its way past the filter and contaminated the Mass Air Flow sensor. Now that the MAF is contaminated, it calculates less than actual air flow which makes the car computer think it has a light load. Therefore, the injectors pump in less fuel to the combustion chambers and the O2 sensors feed information back to the PCM (car computer) saying "Hey: System Lean!!" The computer determines this as a fault and reports P0171.
Remove the MAF, spray the elements down with brake cleaner, blow dry with shop air, and reinstall. If the cleaner emulsified and removed the contaminants, this should remedy the P0171.
Now the harder part: The EGR.
Find the vacuum tube which activates the EGR. Apply vacuum to it during engine idle. If the engine runs rough and/or stalls, the EGR is likely doing its job correctly and the problem may rest with the DPFEGR Sensor.
If there is no change to engine idling with/without vacuum to the EGR, the EGR and/or passageways are clogged.
I purchased a '98 Windstar for my wife and wanted it in 'perfect' running condition. So I rebuilt the engine to make sure everything was in proper running order and was suprised to see the quantity of carbon deposits in the intake manifold and associated passageways. It had about 130,000 miles at the time. My point is: the intake passages (esp. EGR related) may well be clogged, resulting in your P0401.
Thursday, February 19th, 2009 AT 7:55 PM