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One of the more common codes that I typically see on a 1997-2001 Toyota 2.2L 4-cylinder Camry is a P0401. It is a code that indicates that there is a problem with the EGR System, not necessarily a problem the EGR valve itself. I hear stories of people pulling getting this code and replacing the EGR Valve without doing any testing because they don't know what to test. They spend needless money and still haven't fixed the problem.
There are three parts that will generally make this code appear on your car. Yes the EGR Valve is one of them, but you also have the EGR VSV, and the EGR Modulator. The first parts of inspection will be to check the function of the EGR Valve by pulling a vacuum on it with a vacuum pump. You can get a vacuum pump at and auto parts store for around $20 and even rent one at some stores.
Hook up the pump to the only vacuum port on your EGR valve. There will be a small hose attached, that you will need to disconnect and install your pump onto the same port. With the engine running at an idle you should be able to pump the vacuum pump to create vacuum on the EGR valve. The diaphragm inside the EGR Valve should raise and cause the car to die. If your car does die then your EGR Valve and it's ports are fine. If your car stumbles or doesn't have any change in the way it runs when you're pulling vacuum then you probably have a bad EGR Valve or a clogged port in your intake.
If you think you have a bad valve then you should take it off the engine and test it with the same vacuum pump. You should be able to see the diaphragm inside the valve move as you are applying vacuum. If the diaphragm does move then you may have clogged ports in your intake and may need to have the intake cleaned. If it doesn't move then you should replace the EGR Valve.
However, if your Camry stopped running when you applied that first vacuum then you should start looking to the EGR VSV. I know VS what? It's a small blue solenoid located on the back of the engine block. It will have a couple of vacuum hoses, and a small plug attached all be held up there with one 10-mm bolt. Remove the VSV and test it by applying power and ground from your cars battery (VSV should not be plugged into harness when testing.) If you hear the VSV click when you apply power then you should replace the EGR Modulator.
The EGR Modulator is located to the right of the EGR Valve above your intake. It will have several vacuum hoses coming off of it and slides into a holder. The EGR Modulator can be purchased relatively cheap at your dealer. There is no easy testing for the modulator. If you replace the modulator and still have a problem and the light comes back on, you should then replace the EGR VSV also known as the Vacuum Switching Valve. Sometimes the VSV can have an internal break and still click when you put power and ground to the terminals, but still be bad.
So you could be wasting your money by just throwing on a valve. Remember the only way to fix something right is to know the proper testing procedure so you can come to a correct fix.