I have the gas error light (yellow) always on, It turns off some time but it turns on again. I have some cleaning of the air system but it did not work. What could be the problem ?
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P0404 DTC is your EGR valve control circuits are incorrect. Your EGR valve may be staying open or staying closed. The EGR system needs to be checked and rectified. Leaving this repair is not a good idea. The next fault it will cause will be to destroy your DOC, (diesel oxidation catalyst). This will be followed by your DPF, (diesel particulate filter).
As it is at the moment your vehicle will not pass it's annual inspection inside the USA or the EU, (or most other places). Driving the vehicle as it is, it will be releasing excessive NOX, CO and HC. Your exhaust system will be running hot. Have it fixed ASAP, and you should get by with just the EGR work. Leave it and you are heading for a very very big repair bill.
Here is the explanation for the P0404 code, which is a generic code:
This means your EGR (Exhaust Gas Recirculation) valve is probably bad, or the vacuum line (or part which allows actuation which could be electrical) is in bad repair. The usual way to check the EGR is to manually articulate the diaphragm in the EGR valve. Not knowing what year your car is or what kind of engine does not allow me to show you a picture of where the EGR is located on your specific vehicle, but here is one on a different car to see what it looks like:
One side of the EGR is usually open to the atmosphere. If so, you can push on this gently while the engine is running. If the engine bogs a little while you do this, the EGR is usually good. If it doesn't, then it could be bad or there might be carbon blockage preventing it from working correctly. If it is vacuum operated, there should be a small actuator valve located between the EGR and the vacuum source. This could also be bad. If it is electrically operated, there should be a mechanical linkage which you can articulate manually to see if you get the engine bog. If this happens, the small motor which operates it is probably bad.
If the EGR is bad or malfunctioning, you probably will not notice a difference in how the engine runs. As the name implies, the EGR recirculates exhaust gases. The purpose of this is to actually cool and control combustion as it occurs in the combustion chamber. Without this, the combustion process becomes too hot and can form a gas called Nitrogen Oxide (NO or NO2) and is referred to as NOx. This is a nasty pollutant which forms acid rain and in larger quantities is very bad to human health. It is formed by the nitrogen getting oxidized during the combustion process at temps over 1700 deg F. I am not what anyone would consider a "tree hugger", but this is one of those gasses which is just nasty not only to the environment but directly to humans as well. The EGR process helps clean up this up, along with the catalytic converter to clean up most of what is left.
Needless to say, this is something which needs to get fixed.
You will need a system scan for DTC codes. They will tell you the area of your cars system that requires investigation.
Check out the net for budget scanning software such as Torque Pro or OBDII Doctor. Other then that you will need to put your vehicle into your favourite repair shop to determine the fault(s). Codes needed.