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Check engine light comes on after I fill up my gas tank

I have a 2013 Honda Accord Sport with 115k miles. Since September, my check engine light comes on 5 minutes after I fill up or when the tank gets down to 1/4. It goes off in a couple of days and stays off until I fill up again. My fuel mileage has also gotten terrible.

I got it tested and the code P0420 came up. The guy at Auto Zone (seemingly inexperienced) told me it was the catalytic converter and it would be a million dollars (exaggeration). I went to another place and the gentleman (seemingly very experienced) told me to change the gas cap, which I did. He also said it was not likely to be the catalytic converter because they are made to last longer on relatively newer cars. I thought that worked until the light came on again.

Then I went to my mechanic. He put that reader thing (idk what it’s called; please inform me) and got the same code. He said O2 sensors and/or catalytic converter. While I was there, I got new spark plugs. That was Monday; the engine light came on again this morning (Wed.) when I filled up.

Can anyone provide some insight? I am going to order the sensors, but I do not want to waste the money if there may be a less expensive option. I have experienced no other issues with this car.

Of note: This issue started after I put supreme gas in my car, listening to the ads that said it wo0uld help clean my engine. Ever since then, the light keeps coming on.

Thanks, in advance, for any help provided.

First, premium gas has no more detergents than regular. If Honda doesn’t say premium is required, it isn’t.

The O2 sensors are relatively cheap and don’t last forever. They are cheaper than a catlytic converter. Make sure the mechanic checks that there are no exhaust leaks between the engine and cat. That can throw the P0420 as well. If there are no leaks, change the O2 sensors. That should take care of the code.


the fine folks at Autozone are not mechanics. They can sell you parts and tell you what codes your vehicle has, but do not listen to them for repair advice. You did good getting the code form them and going elsewhere.

Supreme gas has no relevance on this issue.

Check your owners manual for warranty on the emissions system of this car. Many are 8 years and so many miles. You may be past the miles, but may not be. Check it out- especially if someone wants to put a Catalytic Convertor on for you.


Thanks! I will have him check for an exhaust leak before I buy the sensors.

Hi again!
So I got the sensors replaced, but the engine light is still on. It was actually off before he replaced the sensors, then it came on again about an hour or two after I left the shop. Also, my battery died twice
Any ideas/suggestions?
This is really bugging me, as I am planing to drive from Houston to Jacksonville, FL for the holidays.

Assuming the light is on for the same reason as before . . . P0420 . . . that would mean the conditions were met for once again illuminating the light. That means the mechanic cleared the code, which automatically reset all the readiness monitors to incomplete. You received the car after he replaced the parts and cleared the code, but BEFORE the catalytic converter monitor was allowed to complete. It completed shortly after you took possession of the car, the conditions were met for illuminating the code.

Translation . . . the mechanic in all likelihood did NOT “verify the repair” . . . it’s possible he doesn’t even understand the concept of “verify the repair”

Supposing there are in fact no exhaust leaks, as mentioned by @Mustangman, there is a fairly good chance the catalytic converter is in fact bad. That means it’s not efficient enough to keep the code from being generated.

There is another possibility . . . but it’s remote, in my opinion . . . the engine control module may need updated software to address this issue. But I think it’s unlikely, because if it were the case, you’d think you would have had the P0420 a lot sooner than now. The car is 6yrs old and has 115K, after all. If the software were truly the problem, I think you’d have had problems 4 or 5 years ago


Have you switched back to regular gas? If not, do that and see if the light comes on after two tankfuls. BTW, be careful not to overfill the gas tank, always stop at the first click, never add any more. But that is not what is causing your current issue.

Now that the O2 sensors have been replaced, either pull the radio fuse or disconnect the negative battery cable for 10 minutes, then reconnect or reinsert the fuse. This will erase the computers memory and the computer will have to relearn all the engine parameters. That might help, costs nothing to try.

BTW, Sometimes regular grade gas does not have as many detergents as mid and premium. That is even true with some major brands, but it is by station, not by brand and they are supposed to have a notice when the regular doesn’t have the same level of detergents. There is a Mobil/Exxon near me that does this but they do warn you. Look for a station that says they use Top Tier gas, it will be the best for cleaning your engine. Costco gas is top tier.

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You can’t overfill your gas tank.

The Onboard Refueling Vapor Recovery system has a component called an overfill check valve that prevents that from happening.


The overfill check valve does not stop someone from overfilling their gas tank. It is supposed to stop the excess fuel from getting into the vapor recovery system, but when you over fill and then go bumping down the road, the valve can bounce open and closed allowing some raw fuel to get into the canister. I do agree a lot less gets in than would get in without it though.

For the OP, before you spend a ton of money on a new cat, and after you have gone through two tank fulls of regular gas and reset the computer by removing power, if you still get the light, you need to find a mechanic that has a dual trace oscilloscope. This is used to trace the output voltage from each O2 sensor at the same time. This will confirm if the cat is actually bad or if something else is going on.

One more thing, a P0420 code will not cause damage to your engine. You don’t want to drive with it on forever though because if another code should appear, you won’t know that unless you hook up a code reader. If the check engine light starts blinking, then you have a code that can cause damage, at least to the cat anyway. Also with the code on, you are probably polluting the air a little more.

You might check on the cost of a rental . It might be expensive but so would a break down during the holidays.

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Thanks for such a clear explanation. I read up on an ECU reset; don’t know if I want to try that. It is all a bit upsetting, though. I’m thinking of just going to trade it in so I don’t have to deal with this anymore. :woman_facepalming:t5:

Vent/Rollover Valve

It provides a method of controlled escape for gasoline vapors during the refueling process. It has a mechanism which closes the vent in the event the vehicle rolls over, to prevent spilling of VOCs or fuel in general. It also acts as a fill limiter.


Thanks, Keith!
I have been using regular gas; I only tried the premium that once. And I always stop at the first click.
I will disconnect the battery and see how that goes.

before you dump the car . . .

Have somebody scan for codes again

Please tell us which particular code . . . if there is more than one . . . was the one that commanded the check engine light on again

You obviously had/have 2 separate problems. Catalytic converter and evaporative emissions

While I’ve been talking exclusively about the catalytic converter problem, several others have been concentrating on evaporative emissions

I urge you to NOT clear any codes or disconnect the battery or anything along those lines, at least not just yet

By doing so, you are erasing all freeze frame data, which is valuable information for a mechanic

Kind of comforting to know it won’t damage the engine. It just randomly comes on then randomly goes off. No other issues with the car.
My plan was to hold on to it (I pay off the loan in 8 more months; bought the car used, a certified pre-owned) for my kid, but I’m ready to give up. If it’s the CAT, I’m throwing in the towel! lol

I will respectfully disagree with @db4690 about erasing the codes by disconnecting the battery. You should verify that the only code is the P0420. I only brought up the part about overfilling because of it happening right after filling up the gas tank, but overfilling the gas tank by itself does not cause any codes. Overfilling over time can cause some saturation of the canister and that leads to a different code. Since you are not overfilling, then just ignore all previous arguments on the subject as they are not germane.

The freeze frame data is of little use to a mechanic who is only guessing what the problem is. It could be of use if the freeze frame date were written down each time the code appears and is cleared to see if the code appears under a certain condition, but even then I don’t see it being of much value. The dual trace (aka dual channel) O’scope is really the best tool for troubleshooting a P0420/P0430 code.

Do you have a V-6?

I fundamentally disagree with you about the value of the freeze frame data

Well I fundamentally disagree with me on the value of freeze frame data in general, I just don’t think it has much value in this case.

I expect the problem is that you need a replacement cat. That’s a little uncommon at 115K, but not unheard of. There may be more than one cat on your car btw.

Before replacing the cat, if this were my car I’d probably do a fuel trim test. That would show if there were any air/fuel mixture problems caused by exhaust leaks, vacuum leaks, MAF sensor inaccuracies etc. Give yourself a chance to get lucky in other words.

I’d also want to solve the dead battery problem before replacing the cat. The computer that interprets the sensor readings that flag the check engine light just isn’t able to do its job properly unless the battery voltage is correct. Best of luck.

Note: The cooling system must maintain the correct engine operating temperature for the cat monitoring to work correctly. Ask you shop to verify.

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Then we’re in complete agreement

I disagree with you . . . and you apparently disagree with yourself

You might want to carefully reread your own comment from a few hours ago :wink: