MERCURY 2000 ODB2 CODES P0400, P00A0, P0401

With the code coming up for an EGR valve I would consider it unlikely that it left the factory without one as it would have had the code from day one and it would have been fixed at a service under warranty.. If you got the car second hand then it is possible that the EGR valve failed and the previous owner found it easier to blank it off to sell it . I would consider fitting a new valve and getting it working properly as it is critical to passing the road worthy certificate., performance and economy.

P0401 is EGR problem
P0300 is a cylinder 3 misfire.
Replace EGR valve and do a full tune-up.

Google search the codes. Could be a hose.water pump.radiator. heater core leaking.

Google the codes

Lots to read & study

OBD2 PO401

OBD2 PO455

Think you will a repair shop to smoke the
Evap for the 455

Stop, DO NOT replace expensive parts or O2 sensor! Ford Explorer codes p0174 , p0401 , rough idle and check engine light on. First thoroughly check all of the hoses and elbows connected to the intake and EGR (vacuum system) for splits, cracks, holes or tears, this could quickly cause these codes and fixes for less than $10. Check all connectors and the EGR valve sensor it could have a crack as well, inspect thoroughly because a split sensor could appear attached to the vacuum hoses but hoses could just be sitting there not actually attached due to a split or crack. This was my problem replaced the EGR valve sensor and the two little vacuum hoses attached to them for $50-60 with in stock parts at the local parts store. Took less than 10 minutes to replace and is easily accessible, the check engine light turned off immediately without having to reset the codes by disconnecting the battery, idle became normal and fuel efficiency better. If you have THOROUGHLY inspected every hose, connector and elbow and found no issues, clean the MAF with electrical contact cleaner ( let dry) and clean the EGR with carbon cleaner. A dirty MAF or EGR could also cause these codes to pop up. If this still doesn't work, then consider replacing the EGR, if that isn't it then try the O2 sensor. Good luck!

P0401 is EGR valve, low circulation detected.
P0420 Catalyst system.
P0133 is O2 sensor bank 1 sensor 1.
All these codes are emissions related, so i would start with the EGR valve, it's probably clogged with carbon. The catalytic converter could be also, check that for excessive backpressure. The oxygen sensor can be tested, replace if needed. Good luck, i hope this helps.

for p0401:
In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.
Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open.
Clean out the EGR valve and/or tubing to remove deposits.
Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model).
Replace the DPFE sensor (with a good quality / OEM one).
Replace the EGR valve.

Barring any vacuum leaks, P0401 is indicative of a bad EGR valve. Hope this answers your question. Best of luck.
Greg

Late suggestion; it may be that the code needs to be reset via a code reader and that the problem will remain in memory until forcibly removed. That is fairly common.

Jason,
-after you change the part you have to reset/clear the code

-this can be done by either:
(1) using the OBD2 code reader that Autozone used OR since option 2 (below) is very quick
(2) disconnecting the battery for about 30 seconds (this will delete all codes/radio presets/etc) but it will also clear the 'check engine' code
-this is what I do AFTER i write down my radio station presets :)

-hopefully the new EGR valve resolves your issue after you CLEAR the code
...but that code just means a an insufficient amount of Exhaust Gas recirculation (pressure difference) used to keep the emissions in check
-if it comes back, then there are a few other fixes to try also

-let me know if you need further assistance
-please remember to rate me `fix-ya`if it works
thanks