This is my first post, I just bought my 99 stepside ranger a few weeks ago, and unfortuneatly it came with a bright orange check engine light. Stopped by the local auto zone and got my free scan. It was code P0401 - Insufficient EGR Flow. Anyway I went through the whole process, took pictures, and figured i might be able to save someone a little time and money by posting the steps. If its already on here somewhere, I appologize. So here it goes...
First of all, what is the EGR System and what does it do?
Exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in most gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. Intermixing the incoming air with recirculated exhaust gas dilutes the mix with inert gas, lowering the adiabatic flame temperature and (in diesel engines) reducing the amount of excess oxygen. The exhaust gas also increases the specific heat capacity of the mix, lowering the peak combustion temperature. Because NOx formation progresses much faster at high temperatures, EGR serves to limit the generation of NOx. NOx is primarily formed when a mix of nitrogen and oxygen is subjected to high temperatures.
In Lehman's terms, the EGR system recycles exhaust gases to reduce emissions.
There are a few parts that you will need to be familiar with.
EGR Valve - Valve controlling the circulation of exhaust gases from the bottom of the manifold back to the top.
EGR sensor - This is basicly a thermostat for the EGR Valve. Once your engine temp reaches a certain point, the sensor will open the EGR valve allowing exhaust gases to pass through.
Vacuum Line - Comes out of the top of the EGR Valve (see picture below).
Exhaust Tube - There are two of these, they are attatched to the EGR valve, one is connected to the bottom of the manifold, and one to the top.
Sensor Line - There are two Sensor lines, one is the Res line and the other is the Hi line. They are connected to the EGR sensor and the Exhaust tube. They allow the sensor to monitor the temperature of the exhaust gases.
STEP 1
When you get the code P0401 EGR, there are a few different things that could be going wrong. I like to start with the ones that don't cost me anything, and work towards the more expensive ones.
So for step one, disconnect the Negative Battery Terminal.
STEP 2
Mark each sensor line with a sharpie or tape so that you can identify which one is Hi and Res. Disconnect the sensor lines from the EGR sensor.
With a flash light, check the lines for carbon build up. You can feel up and down the line for clogs, if its hard, it needs replaced, if you can squeeze it, it is still good. If they are clogged, install replacement lines, connect the negative battery terminal, and go for a ride until engine temperature gets to normal. If the check engine light comes back on, you will need to continue problem solving, if not, you are done. Congrats.
NOTE: If you can avoid disconnecting the bottom of the sensor lines, it will save you a lot of time as they are difficult to reconnect.
STEP 3
Before you begin, disconnect the negative battery terminal.
If you still have the check engine light on, your next step is to remove the EGR valve and check the exhaust lines for build up. PLEASE NOTE: The exhaust tubes kink very easily. I strongly suggest using pb blaster and waiting at least a half hour before trying to remove the bolts on the exhaust tubes. They are expensive and difficult to replace.
The Exhaust tube is going out of the bottom of the egr valve housing in this picture. As you can see mine was rusted and required two doses of pb blaster and a heated wrench. If you don't have the tools, a local mechanic should check this for you for less than $50.
If the Exhaust Tubes are free of build up, reconnect and move on to step 4. If they have carbon build up, you have thow options, remove them and clean them out, or replace them. I'm not sure of the cost as mine were clean, but I do know that they aren't cheap.
STEP 4
The next step is to test the Vacuum on the EGR valve. This will determine wether or not the EGR valve is working properly.
I had my mechanic do this step because i didn't have the tools, but he connected a vacuum testor to the top of the egr valve, zip tied the display to the windshield wiper, and took the truck for a ride. The valve was working properly most of the time, however did show zero vacuum one time, which is enough to throw a code. If the EGR valve is malfunctioning, it will either stay open or stay closed. If this is happening you will notice your engine being less responsive, and will need to replace the EGR valve. If your EGR valve is holding Vacuum, your problem is with the EGR sensor.
STEP 5
Before you start, disconnect the negative battery terminal.
Replacing the EGR sensor in a 99' is easy, takes about 5 minutes. There are two screws, one plug, and the two sensor lines that will need to be disconnected. PLEASE remember to mark your sensor lines, or do them one at a time.
Once you have your new EGR sensor in place, reconnect the negative battery terminal, and you should be good to go.
I am new to the forum world, so if I have made any mistakes, please let me know, will be glad to fix them. Hope this helps someone.
How To: P0401 EGR Fix 
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