Hello Everyone,

Car is an '05 Cavalier, 5SPM, base model, close to 90KM.
I am getting the above error code, and it is the EVAP canister vent valve that is leaking.
I removed the valve, and it is working fine when exercised from an external 12V supply, no leak when the power is applied.
Checking at the connector at the back, the +12V is there, the ground is not. The ground to the solenoid valve is controlled by the ECU.
When I removed the short cable at the back, the wires was fine in there, measured with a DMM.
The manual says test the EVAP system (with smoke) with the ignition on and the engine not yet started. That is the condition when the EVAP went valve should be closed, when the ECU gives the ground, and that is when I measured.
I plan to measure the line from the ECU where it gives the ground to the back connector. If that line is continuous then the fault must be either the connector at the ECU, or the ECU itself.
I looked at the connector at the ECU, it looks pristine.
I read about leaking and corroded ECUs, but have not opened up mine yet.

Any comment on the above, is the test condition (engine on, not running) correct?

Anything else I should be checking?

Thanks, Peter

Have you inspected the hoses going to the charcoal canister?

Also a lot of the time the fuel cap seal goes bad and can cause this code.
If not that then the EVAP hoses.

Oh wait I misread that, you say when given an external 12v power supply the leak is no longer detected?

Hi William,

Thanks for the replies!
A bit of an update.

I do have the P0455 code as well, major leak. I think I had that too before, except P0442 showed before the P0455, that is the one I reported.

- As mentioned, I checked the purge valve, it held vacuum.
- I did check the vent valve when I took it out, it also held vacuum when exercised from an external 12V power supply.
- The hose from the vent valve to the canister seems to be OK, though I have not tested that.
- 12V is present at the vent valve connector.
- Ground was not present at vent valve connector when ignition on, engine not running. This condition, when the ECU is supposed to give ground, is questionable though. I do not have a "GM program" to exercise the vent valve from the ECU.
- The electrical line giving ground from the ECU connector to the vent valve connector is fine.
- I closed the vent valve with an external cable from the car'* 12V supply, the system did not hold vacuum, tested with a hand vacuum pump and gauge (similar to Mightyvac).
- Did replace gas cap, still error code for major leak.

I also built a smoke machine from a paint can, burning incense, feeding from an air tank. It only works for major leaks. If pressurized to 5psi - 10psi, the paint can pops open. At smaller pressure there is barely any air flow, the incense does not burn. This did work when the vent valve was open, though not when the vent valve was closed.

To the history of the car, the dealer replaced the steel lines (fuel and brake) under the car about two years ago, when one of the the brake lines rusted out.
It was replaced with a factory kit, including all the steel lines there.
The car came back with EVAP leak error code, had to take back the car three times. They thrashed the O-ring in the connector at the front in the plastic EVAP line when they replaced the steel lines.
They made me buy a new plastic line.
What a bunch of morons! I will never take my car to a GM dealer!

So it is possible that they also messed up the rear connector, and it showed up now.

Too bad that there is no access panel from under the rear seat to the gas tank!

I guess now I would need a "professional" smoke test to find the leak!
Any other suggestion?

Thanks for listening!
Peter