Po446 toyota camry 1999 how to fix?

A P0446 doesn't point to a gas cap leak. A P0440 is a gas cap leak. A 446 is more probably the thre way vacuum switching valve located on the evaporative canister. The simplest way to check it's with a Toyota hand held scanner which can activate the valve remotely. If it does not click, it is not working. in spite of the problems some dealers have in diagnosing this problem, the dealer is still most likely the best place to take it.
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Toyota info System

check the egr valve, and the VSV. probably the problems.

A P0446 is a trouble code for an evaporative emission system problem.. Check the gas cap to make sure it is sealing properly, if it's, then it may be a bad evap charcoal canister or evap vsv.. it isn't an egr problem..

Well well well... I just want to tell you that I have had the same ALL TO FRUSTRATING PROBLEM with my 2002 camry SE, that I had just bought not more than two months ago.. 1st time light came on and it had one code.P0420 "Catalyst system below threshold" Then put new gas in and light came off. I thought interesting my problem has been fixed WRONG.. well the light came back on and it had a new code P0446 "Evap system malfuntion circuit" or some jazz like that. took it to a dealer and they estimated me about 1200 in repairs.. not to talk about the 108 to just hook up a comp. and diagnose it. MESSED UP.. well obviuosly I wanted to look more into this before spending that amount of money on repairs. I looked on many many diff forums such this one some running in lenghts of about 40 pgs. Everyone has this problem friend. I have seen soo many diff forums. NOOO FIX so far. I tried new gas cap. no big repairs still but I'll keep you posted email me at my email listed and we can keep in touch.. help me assist you then help me :

po440,441,442,446 are all evap emissions related codes. ranging from a loose gas cap to cracked canisters, flawed vsv's loose hoses, etc, etc. po420 430 are catalyst codes.one has nothing to do with another. The last poster has many issues with his car, your is more than probably a failed fuel cap. you must generally remember to tighten the cap till it clicks. forgetting one time will set the light.