Find the small hose on, or very near, the EGR valve's tube to exhaust manifold, follow it back to DPFE.
These usually get water in them from exhaust humidity, you can try to dry them out but.............they usually need to be replaced.
And the DPFE is one of very few items on a Ranger that should NOT be replaced with another Motorcraft(Ford) part, use 3rd party, they seem to last twice as long.
DPFE sensor is not related to P0455 code
Lean code means computer is adding more fuel than the MAF sensor data would indicate is needed.
MAF sensor data is the volume of air coming in to the intake
computer then calculates the 14:1 air fuel ratio based on that air volume
computer adds that amount of fuel via the fuel injectors
computer then gets O2 sensor data about oxygen level in exhaust, if there is too much oxygen(lean) then computer adds more fuel, too little oxygen(rich) and it adds less fuel
This is called the Fuel Trim, 0 trim mean computer is adding the calculated 14:1 ratio of fuel and O2 is showing all is well, just the right amount of oxygen in the exhaust
-1 trim means computer is adding less fuel because there was not enough oxygen in exhaust, +1 means computer is adding more fuel because there was too much oxygen in the exhaust.
Normally fuel trim will run between -10 and +10, when a trim gets up to -20 or +20 computer will notify driver by turning on the CEL
P0171 mean +20 has been reached, engine is not actually running Lean, computer is adding the extra fuel, computer is just letting you know there is something wrong, either with the MAF data, the O2 data or the computer itself, the calculations are not adding up.
Dirty MAF sensor will under report the air volume, so computer's fuel calculation will be low and it will have to add more fuel.
Leak in exhaust manifold will suck in air, O2 will see that as more oxygen(lean) computer will add more fuel.
Vacuum leak of course, but this leak can be around the MAF sensor so while it is not technically a "vacuum" leak unreported air is being sucked in to intake so MAF data would be off.
O2 sensors are spec-ed to last 100,000 miles, they work by using a chemical reaction, like a car battery, so they do wear out when the chemicals get used up, just like a car battery.
Last edited by RonD; 06-13-2015 at 03:36 PM.