Confused.
You stated p0420 in your title which is catalyst below efficiency threshold.
Then you stated p0405 which is
Exhaust Gas Recirculation Position Sensor Circuit Low Voltage
The Engine Control Module (ECM) uses the Exhaust Gas Recirculation (EGR) position sensor to determine the position of the EGR valve.
The ECM sends a reference voltage through the 5 V reference circuit to the EGR position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the EGR valve position, is sent from the sensor to the ECM through the EGR position sensor signal circuit.
P0405 (DTC) will be set when the ECM detects that the EGR position sensor voltage is less than 0.25 V for greater than 5 s.
If your catalyst test passed, I'm assuming the 0420 is a mistake so I will ignore that.
0405 means either the five volt reference signal is not getting to the actual EGR valve or the response signal from the position sensor is not getting back to the PCM.
Check the continuity between the actual pins on the pigtail and back probe for each wire to make sure the pigtail does not have a discontinuity in one of the wires.
Check the voltage at each pin and make sure you are getting 5 volts at one of them. You'll need a wiring diagram if you want to do this properly and only test the 5 volt line once you figure out which color wire it is.
If you are getting the 5 volt reference signal but no response signal from the position sensor, either the EGR valve is clogged with carbon deposits and is more or less Frozen closed, or the position sensor detector circuit is done.
Given the cost of a new EGR, you owe it to yourself to remove the one in place and see if it is in fact completely clogged and unable to move.
Before you do that, Ensure you can get a hold of a new EGR gasket.
You can soak the internals of these egrs in carb cleaner repeatedly to get most of the carbon out. Do not bend the center pintle or it is ruined. It usually spins independent of moving up and down. Usually a torx bit will fit. Light scraping is okay, scraping off the pintle is not okay
If you clean the carbon out your DTC may change. Please report back if any of this actually works for you.
