Error code P0402

[Chaps]

So I got this code thrown back to me a week ago, and the guy at autozone said it was an O2 sensor problem. I replaced the O2 sensor he told me to replace, and the check engine light just popped up again with the same code. I need to pass emissions this month... Can anyone help me??

[Galant Evolution]

A p0402 is an EGR problem not an 02 sensor, you should do some research first before listening to the guys at auto zone, you prob just replaced a perfectly fine 02 sensor that didnt need replacing.


What does that mean?
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 (DPF). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0402 code means that OBD detected an excessive amount of EGR.


You may notice drivability problems such as the engine may surge off idle. There may also be other symptoms.

Causes:
A code P0402 most likely means one or more of the following has happened:

The DPFE (differential pressure) sensor is faulty and needs to be replaced
There is a blockage in the EGR (most likely carbon buildup)
The EGR valve is faulty
The EGR valve may not be opening due to a lack of vaccuum

Possible Solutions:
With a P0402, it is common for people to replace the EGR valve, or take it apart and clean out the carbon build up. another likely solution is to replace the DPFE sensor.

Check the voltage at the DPFE sensor both at idle and when the EGR is open.

However, if you mixed up p0402 and ment to write "p0420" this an a cat problem not 02 sensor.

Technical Description: Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?
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:
You will likely not notice any drivability problems, although there may be symptoms.

Causes:
A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

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 (HO2), replace if necessary
Replace the catalytic converter.