P0405 is saying that the signal voltage coming back from the EGR pintle position sensor is too low (possibly 0).
The way it works, simplified, is that the power-train control module (computer) supplies a twelve volt lead voltage to the sensor. The voltage goes through the sensor, and depending on the position of the sensor, returns a signal voltage. When that signal voltage is consistently less than 0.14 volts, the code sets.
Most likely causes are going to be (1) a failed sensor, (2) no reference voltage from the PCM (basically the lead wire is broken/shorted/screwed up at the connector) or (3) The ground wire is weak.
Basically, you need to make sure that you have twelve volts coming from the PCM and that the ground wire is good. If those are good, then most likely your issue is the pintle position sensor (built in to the EGR), which would not be terribly surprising if it is an aftermarket EGR valve.
You can also check the signal voltage going back if you can get a lead from your multimeter into that wire.
Thursday, February 22nd, 2018 AT 7:15 PM