FAILED EMISSIONS TEST P0401&P0420 CHECK ENGINE LIGHT RESETED MANY TIMES WHEN CUT OFF ENGINE LIGHT COMES BACK ON AFTER 5-6 MILES. I HAVE REPLACED EGR VALVE .STILL WA\ONT PASS INSPECTION. WHATS NEXT?

P0420 & P0430 You need new catalytic converter
The P0430 check engine light code, like P0420, means the catalyst system efficiency is below the required threshold. Simply put, this means your car is adding more pollution into the air than it's supposed to.

The error code p0420 does not rise only due to faulty catalytic converter but may also rise due to one of following reason:

1. Faulty rear O2 sensor

2. Damaged O2 sensor wiring

3. Exhaust leak

The error code p0401 indicates faulty Engine Gas Recirculation (EGR) valve. The remedy to the problem is:

a. Replace EGR valve

b. Repair damaged wiring of EGR valve

c. Clean the EGR valve tubing

Hi there:

DTC P0401 - Insufficient EGR Flow
EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emmissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor EGR (DPFE). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0401 code means that OBD detected an insufficient amount of EGR.

Symptoms
You may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. There may also be other symptoms.

A code P0401 most likely means one or more of the following has happened:
The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
There is a blockage in the EGR (tube), most likely carbon buildup
The EGR valve is faulty
The EGR valve may not be opening due to a lack of vaccuum

Possible Solutions:
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


DTC P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar.

This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms: The main symptom to the driver is the illumination of the MIL (malfunction indicator lamp). You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle.

A code P0420 may mean that one or more of the following has happened:
Leaded fuel was used where unleaded was called for
A damaged or failed oxygen sensor (HO2S)
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
******** spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination

Possible Solutions: Some suggested steps for troubleshooting a P0420 error code include:
Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2S), replace if necessary
Replace the catalytic converter

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.


Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using FixYa, have a nice day.

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.

what is the code that turned the light back on?????? if its a P0420, the cat is not good enough. you may need one good enough for California or a better quality converter.

the computer needs to be reset by performing a code erase then drive 500 miles

I would send the car for a smoke test for the emissions leak...Find out where the leak is comming from..Repair it..Reset the codes, then do a drive cycle and see what happens...

A p0420 code is set when the computer determines that the catalytic converter isn't doing it's job. In almost all instances that code can't set as a result of a bad o2 sensor so they probably didn't have to be replaced. The catalytic converter will need to be replaced, prefferably by an oem part. This can be pretty expensive and should be considered before purchasing the vehicle.

I have replaced a few bad converters on late model subarus in the past few years.

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

There is one 02 sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.

Possible Causes:
Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter

there is a section inthe manual that tells you how to prepare the car for emissions test which can take less than 2 hrs without driving 400 miles normally if there still a problem the check eng light will come back on shortly after the preperation. if the eng light comes on in acc position on igition switch and goes off the computer is fine.