Replacing Evap Canister - Did it cause all these problems?

2002 Hyundai Sonata

Hi all! Newb at diy car repair and cars in general (beyond the fact that they go vroom), so please excuse the lack of knowledge of processes, prices, and names of parts.

So I’ve had some problems with my car. Among other things I was having trouble pumping gas. The pump would shut off automatically every dollar or so. I’ve also had some hard starts and twice I had trouble starting the car. It would turn over and ALMOST start but just wouldn’t catch. It only happened twice, and I was able to start it after waiting a minute and trying twice more. In addition to this, my CEL turned on yesterday with a P0171 code - it’s a too lean code. I took the car to a mechanic right away, and he said that all of the above problems, including the code could be solved by replacing the evap canister. I had already suspected I would need a new canister because I had a small evap leak code for a while that wasn’t solved by replacing the gas cap (went away on its own eventually).

Could the evap canister really cause/solve all those problems? When replacing the evap canister should I also replace the vent valve and hoses attached to the canister? I can’t seem to find a full assembly for my car - all I see for my car is just the canister itself - just a plain box with nothing attached, but I see them for other cars, so it looks like maybe it’s all supposed to be replaced at once?

I see the canister for my car in various places for about $180. The mechanic I went to says the part is about $350, but I’m not sure if that’s for the full assembly and if the other pieces are worth almost $200. Should I replace more than just the canister? Can you offer a list of pieces that typically come in a full canister assembly? Since I can’t seem to find more than just the canister it looks like I’ll have to buy everything separately if I do need to replace all of it.

I’ve watched some videos about replacing the canister, and it doesn’t look super complicated. All videos seem to show taking the wheel off first though. Wondering if it’s too difficult to access without doing that. Not difficult to take it off, just a bit of a pain if I don’t have to.

Any opinions/suggestions/advice on prices, parts, mechanic’s analysis, repair process and difficulty level would be greatly appreciated!

I can tell you’re a newbie.

You know how?

You didn’t provide the year of your vehicle.



First you ask what you are getting for your money. Second , do you expect the mechanic to work for nothing? Also the mechanic will provide some warranty for the part he furnishes and the labor.

What? Of course not. Where did I say I wanted free labor? I certainly want to save money which is why I would consider doing it myself and am trying to find cheaper parts. The $350 I stated was JUST for the part. The labor was quoted as an additional $91 and the “Limited Lifetime Warranty” is free. If the process is more difficult than it looks then the $91 may be worth it, but I’d still like to see if I can find the parts cheaper elsewhere. I know there are mechanics who tend to mark up prices.

Whoops! Sorry! It’s a 2002 - editing post now. Thanks for reminding me!

The vent valve is a separate component from the carbon canister.

The vent valve is what allows the gas tank to vent when refueling.

If the vent valve gets stuck, the gas tank can’t vent, and gas pump keeps shutting off.


If you can fill it on slow speed gas delivery try that, I am not convinced the code has anything to do with your cannister. Get a second opinion.

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I probably mean something else lol. When I search for canister assemblies I see ones that don’t fit my car but generally all look like this:

There are lots of additional pieces on them. The ones I see for my car mostly just look like this:

Maybe my car’s just different and doesn’t have all the extra pieces and hoses I’m seeing on canisters for other cars?

The gas pumping thing is frustrating, but I can live with it. I’m only concerned about the code because I have to have an emissions test done soon. I’ve been trying to read up on the code a bit and don’t really understand how the canister’s connected, so I’m not fully convinced either. Looks like getting a second opinion may be the best thing to do :-/

Edit: Also concerned about hard starts and the couple times I had trouble starting at all. I think I can understand how the canister could be responsible for that though it seems more likely a problem with purge valve.

That is the nature of the business

Shops make a fair profit on the parts they sell. In other words, they’re marked up. If they didn’t, they would soon be out of business

No shop is going to sell their customers parts at rockauto prices

Were I having these symptoms replacing the canister isn’t where I’d start. The canister is part of the evap system, who’s function is to prevent fuel vapors from exiting the fuel system and polluting the air. Instead it routes those vapors to the canister, which are later pulled out of the canister and burned in the engine. One of its main jobs is to prevent refueling from pushing fuel vapors into the air, and there are various valves (some passive, some electrically operated) involved. If one of those valves fails, that could cause the difficult to fuel symptom. Replacing the canister wouldn’t solve that problem.

The too lean problem could be (unlikely, but possible) caused by something in the evap system preventing fuel from being pumped from the tank to the fuel rail, but if that’s happening it would show up in a fuel pressure test.

If I suspected a canister problem on my Corolla I wouldn’t replace it as the first recourse, but instead I’d remove, inspect, and test it first. Ask your shop if there’s a cleaning, inspection, and test procedure for the canister. On my Corolla as I recall you use low pressure air to blow into certain ports and verify the air comes out the correct port on the other side, and not the wrong port. There’s an air filter on one of the ports that can clog up too.

Suggest also to google that code you got. There’s a website called obd codes or something like that, will give you a complete list of what can cause it.

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Also, you might be causing these problems if you squeez more gas in after the automatic pump shuts off. Doing that will saturate the canister.

So they don’t make their money from the $130/hour labor prices, “shop fees”, and “Limited Extended Labor Warranties” (Pepguard Limited Extended Labor Warranty = $157.38)? O_O
I can understand a smallish mark up - say they buy at wholesale price or less, sell at retail and add a fee for ordering and supplying the part, but if I’m seeing the part at an average of $180 everywhere I look then charging $350 seems unreasonable.

I’ve googled the code a bunch since I got it which is why I was so confused as to how the canister could be the problem. I keep seeing that the purge valve or even the fuel pump could be the issue, but not the canister. I may just replace the purge valve and vacuum hose and see what happens. The parts are inexpensive enough and the replacement process doesn’t look too bad. If that doesn’t work then I’ll move on to something else :-/

For the P0171 code the freeze frame data will give some indication to the cause of the problem but by just guessing I would start by cleaning or replacing the mass air flow sensor.


Definitely not. Since I got the car I’ve been an undergrad student and am now a grad student. I never had enough money to fill the tank lol. I’d buy $10 in gas at a time and go until the empty light came on. I only started filling the tank when the pumping problem started. It takes so long to fill the tank that sometimes I’ll just spend a bit more time at the gas station when I have the time to spend rather than curse the slow pumping when I’m in a rush. When I do fill the tank I turn the car (not engine) on so I can check the fuel needle. Otherwise I can’t tell the difference between the bazillion auto shut offs I get pumping gas and the shut off that indicates a full tank.

I wondered what the freeze frame data was for…but didn’t bother to check it out :-/ Anyway, thanks! I’ll check it in the morning. Even if I have to replace that sensor, it looks easier than replacing the canister, so I’ll add it to my list of hopefuls :slight_smile:

Edit: That sensor is in that big hose connected to the box the air filter goes in…Is it just a coincidence that this code popped up the day after I replaced the air filter?

Inspect the induction hose from the mass air flow sensor to the throttle body for cracks or leaks, un-metered air will cause a lean fault.


I agree with @Nevada_545 that the P0171 code has nothing to do with the canister and more to do with the Mass Air Flow (MAF) sensor. Either the sensor is dirty or there is an air leak after the MAF. The MAF is usually located in the duct between the air filter and the throttle body and close to the air filter.

It is possible that any leak in the canister system would also allow unmetered air into the engine also giving a P0171 but usually a leak in that area will give a code for the canister system before giving a P0171 code.

The P0171 code does not mean the engine is running lean. It means that the computer is adding more gas to the engine than what it predicts it should have to add. In other words, the O2 sensor is calling for more fuel to keep the air/fuel (A/F) ratio in the correct range.

As engines get older, this is common. The computer allows for up to 20% more fuel due to engine age or other differences. When it exceeds the 20%, the CEL lights up and you get a P0171.

I concur w/advice above, the lean code is most likely something other than the evap system, and cleaning the MAF is a good guess as to where to start absent other diagnostics, like fuel trim data.

The parts cost for the oem canister itself is about $250 from what I see. About 1/2 hour in labor to replace it. The $180 canisters you are seeing are probably aftermarket versions, and may not be as good as the oem version.

The parts of the refueling system that could be causing the fuel pump shutoff problem are

  • fill vent valve (top of tank)
  • canister close valve
  • canister air filter
  • purge control solenoid valve
  • canister
  • fuel shutoff valve (inside tank)
  • fuel tank pressure sensor
  • 2-way valve in the pressure sensor line
  • restrictive 4-way valve (between tank and filler tube)
  • roll-over valve (top of tank)