Gas Cap Warning Light

[VQ Dawg]

I have been getting a P0455 CEL code (gross EVAP leak). I checked that the cap is on correctly and reset the light a few times, but it comes back on in just a few drive cycles. I am about to start chasing down the source, but am a bit curious about the gas cap warning light. Does anyone know what triggers this light vs. the CEL? I looked at the cap and the mounting area and I can not see anything that is specifically reading anything in the cap area. I was thinking maybe the PCM uses the tank pressure sensor to trigger the gas cap light only and if a EVAP leak is detected somewhere else is triggers the CEL?

I ask because it is cheaper just to buy a new cap than to test my current one, but if there is something monitoring the cap directly for the gas cap warning light then I would be wasting my money. My guess is the gas cap light is not really very reliable and the problem could still be the cap even if that light does not come one, but I wanted to see if anyone had any insight before I waste too much time trying to chase the source.

 

[VQ Dawg]

Never mind. I just found some info from a 2008 service manual. It states that the gas cap light is illuminated if the initial vacuum after fueling can not be reached and the purge flow is excessive. Seems to me that a bad cap could still throw the P0455 code if it is not functioning properly so I can not rule the cap out yet.

 

[drchuck]

Don't get me started on this. Please refer to the previous thread - which began about 3 years ago - and is still not resolved. I did learn that one can reset the codes by simply disconnecting the battery terminal for about 10-12 minutes. If the cap is the problem, the light will stay out. Driving cycles take too long to diagnose. My light comes on about every 712 miles. It lived at my dealership for over a year while they dinked around with it. One thing in reading your second post - be careful to stop filling as soon as the pump pops off. Trying to squeeze that last cup in can fill the evap hose, and as long as it's wet (and its filter) the values are going to be other than what the computer is looking for. Also, try to fill up on level ground. Yesterday I needed fuel, and the station had a steep drive. The CX was nose down about 15 degrees and afterward I realized that the pump probably overfilled at that angle. Sure enough today the lamp came on again. Again, the bottom line is to check the cap only, just reset the codes with the battery connection. I will say that to my knowledge there is absolutely no difference in performance or mileage with the light on. Unfortunately I made a 500 mile trip home with it lit up, about 2 years ago, and the car ran fine.

 

[VQ Dawg]

Thanks for the info. I saw your posts and am hoping I have better luck than you did. The light on my car comes quickly every second trip after reset. It is very consistent. I have one of those Bluetooth readers so I can reset the light easily. Coincidentally, the very next drive after my previous post, the car threw another p0455 code and the gas cap light. I went ahead and ordered a new cap so hopefully it solves the problem. If not I will be under the car testing the entire EVAP system.

 

[drchuck]

I hope that works for you. Probably will. Just FYI, be advised that the entire emission system back there is covered under an extended warranty, both time and years. You can check with your dealer, or go on the EPA website, in addition to the information regarding warranty in the vehicle manual, to verify this post.

 

[VQ Dawg]

Thanks for the tip. I checked to see if this falls under the emissions warranty and it does not. Not in my state at least since there is no emissions testing required.

 

[VQ Dawg]

So this problem continues. I installed a new gas OEM gas cap and no dice, the light comes back on after resetting. I took it to the dealer to run a diagnostic and they said that they could not draw a vacuum to run the test and recommend replacing the canister purge valve. I replaced it this past weekend and reset the light. Exactly two drive cycles later the P0455 code comes back. This is getting frustrating now. I hate throwing money at a problem to result in no change. I inspected every line on the car and everything looks like new.

The only thing I find strange is that when a run a live monitor with my OBD2 reader, I get no response from the total system pressure and EVAP system pressure PIDs. They always read 0 and -1.18psi respectively. this tells me that either the sensors are bad or the car does not support this function and the reader is just showing defaults. I can see the purge valve command change (the valve that goes to the intake) and I get no change in short term fuel trims. This tells me that the O2 sensor are not detecting a rich condition when the valves open because it is not drawing in vapors from the EVAP canister.

I plan on going back to the dealer since they gave me an incorrect diagnosis, but if anyone here has any input I am listening intently. My next guess is the engine side purge valve and a smoke test.