I am hobby mechanic taking care of my own equipment only (2003 corvette z06, 2012 Yukon xl slt 5.3, 2018 sierra Denali 6.2)
Right now I get a code P0449 in the sierra. when the circuit is closed I get 12v between the two ports and when it supposed to be open I get 9v that still operates the solenoid. checked each port to ground:
red wire reads 12v constant (venting and not venting)
white reads 3v when venting and 0v when not venting
Hope you can help.
Thank you.
That solenoid gets Power (usually Key On but sometimes always) and is Ground Side Controlled (through the White wire) by a transistor in the computer.
The computer monitors the voltage on the Ground (White) wire.
When the computer isn't providing a ground on the White wire (Solenoid OFF) it expects to see battery voltage there (12 volts +-) ((because the 12 volts from the Power (Red) wire to the solenoid will flow through the UNGROUNDED coil in the solenoid and into the (White) wire entering the computer)).
When the computer switches on the Ground that voltage gets pulled down to 0 and the computer expects to see that too.
When the computer doesn't see these voltages at the right times, it throws that P0449 circuit code.
You've already established that the Power side of the circuit (Red wire) is good.
The question is if the computer controlled Ground side (White wire) of the circuit is OK.
The easiest way to test the circuit is with a test light.
Disconnect the wiring connector to the solenoid.
Touch the probe of the test light to one terminal on the connector and touch the Ground wire of the test light to the other terminal.
When the solenoid ISN'T being commanded ON by the computer the test light SHOULDN'T light.
When the solenoid IS being commanded ON by the computer the test light SHOULD light.
If that's happening then you're done testing. You need a new solenoid valve.
However, if the test light stays lit (even a little) when the solenoid isn't being commanded on by the computer, then there's either a short to ground on that White wire, or the driver (transistor) is burned out in the computer.
Wow..... thanks for replying. that means my computer is toast.. since I get light on both situations , weak on venting (9v) and strong on not venting (12v) but still there is a light.
is there a way to test a short to ground on the white wire before i replace the computer?
You can just disconnect the connector on the PCM and test again for a short to ground on that solenoid connector
.
If the test light still lights, there's a short to Ground in the wiring
.
You can even go one step further.
.
If you don't see a ground on the wire with the connector disconnected, you can reconnect the connector and backpin the connector pin at the computer connector to see if it's providing a Ground when the computer isn't commanding the solenoid ON
.
That would be conclusive
Thanks a lot! appreciate the help.
Problem fixed! problem was a corroded connector.
thanks everybody
Check all wiring and hose connections to the EVAP solenoid and replace the solenoid if necessary. Also, get a new gas cap just for quality.
