P2420 experience - "Evaporative Emission Pressure Switching Valve Stuck Off"

[snowblower]

I just wanted to document my experience resolving a P2420 on my new-to-me Yaris. It's a 2014 with 293k km (183k miles).

First, the car drives fine even with the CEL on, and during the entire time that it is pending. It takes a few drives after clearing the codes for it to turn on the CEL. However you can see it pending on a code reader before that, and you can see the old permanent codes if they were cleared in the past. This was happening in conjunction with the evap monitor not completing.

As an aside, this car was a used car that I recently purchased. It had no permanent codes when I inspected it. So they must have been cleared with a dealer tool before it went to the used car lot (because a regular OBD scanner can't clear the permanent codes). Whatever the case, I knew that it wasn't completing its evap monitor, and I was expecting an evap problem when I bought it.

The vent valve is a solenoid operated valve that is located in the evap cannister that is in turn located under trunk of the Yaris. It's easy to get at.

The vent valve is normally open. Meaning that the gas tank vents to the evap (charcoal) canister, then the canister vents to atmosphere through the normally open valve. The ECU closes the valve to in order to test for evap system leaks.

The ECU runs the evap monitor test 5 hours after key off. (Although it could be 7.5 or as much as 9 depending on temperature). It commands the vent valve on (valve closed) then it monitors the evaporative system pressure. It should rise above atmosphere (due to fuel vapours), if it doesn't then the ECU assumes the valve is not working, or that there is a leak, and signals the code.

The root cause may be that the valve itself is not physically closing, or it could be the result of a leak. You have to check before assuming you need a new evap canister. The service manual provides a simple procedure for testing the evap canister. The evap canister has the vent solenoid built in. The service manual says to apply 12 volt accross the vent power and ground pins. There should be an audible click. If not then replace it. In my case the solenoid very obviously did not "click". There was no sign of it functioning. There was about 27 ohms of resistance across the solenoid power pins, and there was current flowing, but it would not "click". So I'm assuming that the solenoid was mechanically jammed. (I also checked that I had 12 volt power to the evap cannister connector on the car.)

Having concluded that the cannister (which contains the vent) was bad, I replaced it with a Toyota OEM canister. That fixed the problem.

Note, that it takes the ECU a couple of drive cycles, including the car sitting idle for 6 hours or longer after each drive, to conclude that all it is well, clear the permanent P2420, and finish the evap monitor. That happened as expected for me.

Also, note that the service manual (alldata in my case) documents a simple test where you pressurise the canister and it should hold pressue. My old evap canister didn't pass that. It leaked through the electrical connector. However the new canister did exactly the same thing, so I think that test description is either wrong or is incomplete. My guess is that you have to have a working vent valve and it has to be closed for that test to work. I didn't try that with my new cannister. Either way, it wasnt' relevant in my case because my vent solenoid was dead on my old canister.

Finally, the cannister is an expensive part. If you're not confident diagnosing the problem and declaring your own cannister dead then I suggest that it's worth paying a diagnostic fee to a dealer in this case.