I just recently purchased a 2004 WRX Wagon
, I inherited a CEL
light with it. The code was PO457,
which means (Evaporative Emission Control System Leak Detected - Fuel Cap Loose / Off
After reading posts about the "T
" fitting and Vacuum Control Solenoid
located below the intake manifold, I checked these two things first and found both ok. The "T
" had all hoses attached and the Vacuum Control Solenoid
worked correctly when energized.
I then focused on the Drain Valve
, also called Canister Vent Valve
Drain Valve / Canister Vent Valve
I did a resistance check on the solenoid and found the coil ok, about 26 ohms
, I then energized the solenoid with 12v and it did not activate. (click)
(if the solenoid coil where bad it would of pulled a code for the vent valve, ECM only reads open or closed electrical circuit, not actuator position).
The Drain Valve
was designed to shut off air flow when energized, which it didn't do. When the ECM
does a evaporative emission test, it signals the valve to close so a vacuum can be pulled and then reads from the pressure sensor (located in the tank) a change in tank pressure, because the valve was frozen in the open position it just vented to atmosphere through the valve and prevented a vacuum being pulled in the fuel tank.
The canister vent valve was replaced and it solved my P0457
code, the ECM
passed all readiness tests and is ok for inspection.
The part sits below the canister, it has a disk that is spring loaded in the open position, when the coil is energized a "moving core
" presses against the disk to seal off flow, this is what seals the canister from atmosphere. I deduce that because it is the lowest part in the vapor train it collects moisture like a "P" trap under a sink and caused excessive oxidation of the moving parts.
(Update 5/5/2011, after disassembling the malfunctioning vent valve, I discovered that the moving core was frozen in the open position, this was most likely due to its inherent problematic location
The Vent Valve is #23 (click to enlarge)
I cleared the code and tried driving according to forum posts to get the ECM
to do a evaporative emissions readiness test
, but it did not work. It does appear that if you drive for the prescribed amount of time and then turn off the engine and remove the gas cap and then replace it and then restart, it forces or triggers the ECM
to do a evaporative emission readiness test
. I wasted a half tank of gas to trying to get the ECM to do a evaporative emission test until I stumbled on removing the gas cap. Another lesson that I learned from this experience is when looking for used autos, bring a code scanner that can read readiness test. If someone is selling a problem and resets the CEL, this light can stay off during your test drive, but the scanner will tell you without a doubt if the vehicle is waiting for conditions that will trigger the readiness test. If readiness tests are not complete, then beware. I was lucky that it was only one $86.00 part and I did not have to drop the gas tank.