I have a 96 ford Ranger 4 cyl. I got the check engine light, and read the code at the DEQ and it was a P0401 - Insufficient Exhaust Gas Recirculation.
I replaced the EGR valve (unnecessarily as the old one functioned with a vacuum)
I replaced the DPFE sensor (unnecessarily as the old one was functional according to the dealer diagnosis)
I checked the EGR solenoid with a 12v current and it supplies the EGR valve with vacuum.
I vacuum and pressure checked all the dpfe, egr, and solenoid hoses and EGR port tube. I checked the vacuum of the intake manifold at the EGR and it’s strong.
I reset the computer by disconnecting the battery after each part replacement.
What else is there?
The next thing to check would be the EGR exhaust gas supply tube.
Remove the two hoses for DPFE sensor from the supply tube and make sure those ports aren’t plugged with carbon. If that checks out then the problem might be with the orifice in the supply tube. Between where the two hoses for the DPFE sensor connect on the supply tube is an orifice. If this orifice also becomes plugged with carbon it will set this code.
Tester