Another P0401 code

[macdude67]

Sorry to beat a dead horse but this P0401 is being a pain. Bought this 1995 2wd 3.0 automatic a yr ago and was doing some basic tune up stuff. Noticed the check engine light did not light during key cycle, it had a burned out CEL bulb. Replaced bulb to fix no light situation. Used a scan tool and had several codes but none really effected drivability. One code for canister purge, corrected that with another known good canister purge solenoid. 2 other codes, one for bank one O2 sensor heating element and other for post cat O2 sensor heating element. Fixed those with new sensors. Cleared codes and only one to keep setting is P0401. Before P0401 would set within seconds of starting. so I replaced DPFE sensor and one small hose going from EGR pipe to sensor. Later replaced EGR valve and scrubbed intake EGR port to make sure it was clear and also replace EGR vacuum source solenoid. Now after 5 to 6 miles the P0401 code comes back, I can clear code during the drive cycle and code will not come back until 5 or 6 miles into next drive cycle? Gets pretty tiring playing wack a mole with these CEL codes. About to take the bulb back out.

 

[RonD]

What year is the 5.0l Computer?

P0401 Exhaust Gas Recirculation Flow Insufficient Detected

Yes, that code would be set by the DPFE sensor during a Test Cycle after driving a few miles
But could be a few things

The DPFE sensor has 2 hoses hooked to the EGR pipe that runs from exhaust manifold to EGR valve
When computer activates the EGR Modulator(solenoid) it will pass vacuum to the EGR valve causing it to open
When EGR Valve opens the DPFE hose closest to the valve will have a pressure drop, telling computer the EGR valve is working
No pressure drop = P0401

EGR modulator can be broken or no vacuum
It's a 2 wire solenoid valve, it gets 12volts with key on, the computer controls it's ground wire to open it, and computer uses PWM(pulse width modulation) to control how much ground to give the solenoid which controls how much vacuum is sent to the EGR Valve
There would be a different code if the solenoid part of unit was broken, but not if the valve part was not working, has no vacuum or is leaking vacuum, that would be P0401, no EGR valve movement

You can test EGR modulator with 9volt battery to see if its "clicking" open and closed(no polarity), but it probably is since just P0401 code, but you can then also test if the valve part is working by blowing thru it when it's activated and then can't blow thru when its not
Also test that it's Vacuum line has vacuum, engine on, and its EGR Valve hose is air tight, you can hold these 2 hoses together with engine idling which will cause EGR valve to open and engine should start to run rough and probably stall if you continue to hold EGR valve open, lol

EGR system is not used at idle so no static test using computer is possible
But........although rare it has been a computer issue when P0401 keeps coming up after all other repairs and checks have been done, just a bad circuit in the computer