Verifying My Fix for Very Small Vacuum Leak

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CharlieS

CharlieS

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Edit: what I am trying to figure out is how to verify that my fixes to the P0456 worked.

Hi all,

Is anyone familiar with how and when the emission system self checks?

I've been getting a periodic P0456 (very small vacuum leak). It triggers a check engine light.

I can clear active and pending codes, but there is a corresponding permanent error code I can't erase with the basic code reader I own.

I put on a new Toyota OEM gas cap from the dealer, and replaced the Vacuum Switching Valve Assembly (bolted to the intake manifold).

I have a 12.5 gal LRA (it has been on there for years) and I checked all the hose clamps and visually inspected hoses. Everything looks okay.

I also cleaned some crusty gunk from the (LRA) gas filler neck rim thinking it might have been enough to make an imperfect seal, even with a new cap.

I made sure the (new) gas cap threads and gasket was very clean.

I haven't got a code now after a handful of long drives, but am not clear what conditions, drive cycles, etc are required to retest and trigger the error if it still exists.

I should probably also smoke test, but I got lazy. I'll do that tonight.

I'm hoping it isn't the charcoal canister, mostly because I don't want to have to drop the LRA tank to get to it, but also because the darn charcoal canister is expensive.

I live in a state with annual safety and emissions inspections, and my appointment is for tomorrow. I'd prefer to see if I am going to get a code before I pay for the inspection, or get it failed for this emissions code/CEL.

Any insight on the conditions for triggering a test?

Thank you.
 
Last edited:
I understand that. Thank you.

But if I know what the readiness conditions are, I can drive the required drive cycles and conditions, etc.

I'm basically looking for a way to verify that my fixes worked.

I've driven 160 miles and 5 drive cycles since the code clearing.

I updated the thread title and added a clarification at the top if my post.
 
Last edited:
In California, it takes 10 cold starts and 200 miles for the emission inspection system to ignore permanent codes.

Most common evap leaks comes from the canister purge valve, vent valve, and least likely is the charcoal canister itself
 
CharlieS said:
I understand that. Thank you.

But if I know what the readiness conditions are, I can drive the required drive cycles and conditions, etc.

I'm basically looking for a way to verify that my fixes worked.

I've driven 160 miles and 5 drive cycles since the code clearing.

I updated the thread title and added a clarification at the top if my post.
Click to expand...
It would show a pending code after 1 drive cycle+test fail, and a current/perminant code with a CEL if it fails again. It looks like your repair is good.
 
Don’t most code readers have a simulated emissions test/monitor check? Even my $40 harbor freight Bluetooth dongle had an emissions check simulator, helped a lot with an old v8 mountaineer I had.
 
dudemancool1904 said:
Don’t most code readers have a simulated emissions test/monitor check? Even my $40 harbor freight Bluetooth dongle had an emissions check simulator, helped a lot with an old v8 mountaineer I had.
Click to expand...
Using Techstream/GTS+ there’s an EVAP test in the Utilities menu of the ECM/ECT, but I’m not familiar with what the aftermarket scanners offer.
 
They weren't able to inspect it. Back to troubleshooting. Guess I might need to drop the LRA so I can access the charcoal canister.
 
CharlieS said:
Vermont
Click to expand...
So you need to do 15/200. In CA it's 10/200.

Have them test it anyways if you have completed 15 warm-ups. If they refuse, take it elsewhere.
Screenshot 2026-04-24 145753.webp
 
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