There is also a vac. switching valve for the egr that also goes bad.to find the valve follow the vac. line that is hooked to the top of the egr to the valve.it is located on the back of the motor up under the intake and is a pain to replace. if you have a hand held vac. pump hook it to the egr valve and pump it up and see if the engine dies if it does i would replace the vac. switching valve.if it does not die then i would start looking for carbon on the inside of the intake on the back side of the throttle body plate.and also carbon will build up at the intake where the egr bolts to it...before i messed with the vac. switch i would look inside of the throttle body behind the throttle plate and see if the passage that goes to the egr valve is stopped up with carbon and you will need a small round mirrow and a flashlight....if the car does not die when you do the vac. pump test it is easier to take the throttle body off to clean and to check....been working on toyota and lexus for 25 yrs and have seen it all so let me know if i can help in any other way..........good luck and if i did my job let me and fixya know and a thumbs up............thanks.......
It was a bad vacuum hose in my case:
http://www.fixya.com/cars/r25932963-p0401_error_insufficient_egr_flow
You could have a resticted EGR passage, a bad modulater valve or a bad vacuum switching valve.
SOURCE: 1999 Toyota Camry, 4 cylinder. engine
I had the same code. Apparently this can be caused by a loose gas cap. Mine was tight but there was minor rust around the top of the fuel pipe where the cap goes on.
I took some sandpaper and did a light sanding around the opening to get a better seal. Problem has been gone for a year now. Same gas cap!
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Diagnosis of this code is not just throwing a part on, you need a dealer to look at this.
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