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Troublshooting egr code p0402. This OBD2 code reads "Excessive flow in the exhaust gas recirculation system". A little backround the service engine light comes and goes whenever it feels like it. It may stay on for a day or two and off for sevral starts and back on. In the past but not lately the engine would idle rough on occasion and smooth out as i accelerated. The idling rough would only last during sevral stops and then go away to reapear weeks later. Something that has been happening all along is the surging at slow and medium speeds. This is not eisily felt but being i drive it all the time i do notice it. I am not getting a clear answer from local mecanics on my diagnostic findings. This is what i have found: EGR valve appears to open and close propery creating a good seal. Vacuum solenoid is recieving 12v but its ohm reading is 33ohms. My repair book (Haynes repair manual) tells me it should be 20ohms. The auto mechanics i spoke with knew nothing about taking an ohm reading. The third item i tested was the backpressure sensor. With engine off ignition on referance voltage is 5 volts as it should be. I next checked for my .6 volts with engine running cold. This checked out ok. The next check is where the confusion lies. My book says " as the engine warms up, open the throttle to about 2000rpm and check voltage again. Voltage should be 1.5 - 2.5volts". Here is what i see. Cold .6 volts as engine warms and i rev at 200rpm voltage increases slowly to 3.5 volts.This takes about 20 minutes for the voltage to climb to 3.5volts. With engine hot idling at 900rpm voltage stays at 3.5 volts.Question: With engine hot at idle should the voltage drop down? Question: Is 3.5 volts too high? Quetion: Is 33 ohms too high for a vacuum soenoid? Question: Should i be looking somewhere else to find problem? Thamks in advance for any suggestions Mike
 

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Replace the DPFE Sensor (EGR pressure sensor). Very common failure for EGR codes.
 

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I 2nd that response. I had the same code on my 96. And found through investigation that replacing the DPFE may help clear this code. I've replaced the DPFE and haven't had anymore problems since. Its been almost 2 years.
Good Luck
 
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