I have a 2000 toyota camry, 4 cycl. It has failed emissions because of two codes: PO401 & PO420. I have had the following replaced: EGR valve vacuum hose, 18TOM downstream O2 sensor, catalytic convertor, and a/f senor (I am not sure about the last). Each time, the check engine light goes off for about 500 miles, just enough to to pass emission but then it goes on again. Any idea what is wrong? It is time for emission test again. Thanks.
Did you check the EGR valve and the operation of its vacuum solenoid?
Are P0401 and P0420 the current codes or are those from a year ago?
They are from two years ago.
What are the codes from this instance? The codes from 2 years ago are not relevant.