Do I keep spending money to diagnose my Sonic?

I have a 2014 Chevy Sonic with 77,400 miles on it. It was Hertz Rent A Car that I purchased in 2016. Before you groan, here is the story of why I bought it. A drunk blowing .12 totaled my Chrysler Sebring the second week of January, 2016. It was 24 degrees when he hit me, 18 degrees when the tow truck finally arrived, and -10 when I started car shopping two days later. There was a snowstorm the next day, wrecking a lot of cars, so a ton of us were trying to buy a replacement car when inventories are low. I bought whatever was available for under $10,000, because State Farm did not compensate me fairly for the Sebring, of course. Anyway, the car’s been pretty good, but the check engine light went on in July and did not go off with a new tank of gas, several starts and a new gas cap, as it had once before. However, it ran absolutely fine, with no symptoms of anything wrong. After about two weeks (I am now retired and don’t drive that much), I needed to fill up with gas. I tried to start the car to leave the gas station, and it barely started. It ran very rough and knocked after filling the tank. I did get it to run and got home. It started and ran well for the rest of the tank of gas. I took it into the local shop who scanned the code for me for free. It was P0171, which means fuel trim system lean. The shop owner and I looked at the technical documentation for that code together, and my car had none of the symptoms described. So he reset the code and had me go fill the tank with gas. It did the same on the next fill–starting very rough (flooding). The check engine light came on immediately. The local shop ran the code again. Again P0171, with P0324 (knocking, no surprise) and P0496 (evap system flow during non purge) also. He said he could replace a sensor in the evap system for the starting issue, but felt something else was wrong, and the check engine light would come back on. He told me to drive it as is and try different amounts of gas when filling it and let him know if there are any other symptoms. After 3 months, there is not. The car runs and performs just fine except for a very tough start immediately after filling fully or partially with gas, when the car is mostly empty or only half or one-third empty, and all combinations thereof. I don’t know whether to proceed to spend money trying to figure out the problem. The independent shop mechanic said not to bother (honest guy! Or just afraid to dig into it). I haven’t found a good GMC dealer in my area. I would appreciate any help you can give me. Lisa Mc

For starters I have bought x-rental cars without problems.

Now for your problem, I think there is a fault in your evap system. Not an expert, but you might cause additional engine problems driving it this way. Plus, with your CEL on you will not be able to tell if a new problem developes.
Why your mechanic says your car is not worth fixing I do not understand. Either ask around friends for a different mechanic or check yelp reviews and find one. Expect to pay a diagnosis fee, usually around $125, some shops will waive the fee if you have the work performed there.


Because HE cannot diagnose the problem. This mechanic is essentially admitting he doesn’t know how to fix it. Taking a big hit on the value at trade it because he won’t admit that is unacceptable. Find a better mechanic.

Talk to friends, look on Yelp, Google and in your area. As @Purebred posts, expect to pay a diagnostic fee. This car has plenty of life left in it and it can be fixed.

1 Like

Thank you for both (PureBred and Mustangman) for your input. I appreciate all suggestions. I can provide some additional information. The mechanic and I have discussed additional diagnostics. The technical information recommends a MAP diagnostic (removing the sensor, cleaning, etc.)($125). If that is inconclusive, the next step he would suggest is a smoke test to try to find a vacuum leak (another $150 or more). Other possibilities include an evap cannister solenoid and a fuel tank pressure sensor, each minimum $300. But in any of these areas, we would expect to see constant performance problems, not just the one problem on starting the car after depressurizing (sort of), changing fuel volume and repressurizing the fuel system, as happens when adding fuel to the tank. In posting this, I was hoping that someone might have seen this unique and distinct issue. I have talked to two friends who are great garage mechanics. Neither has seen this and both are in agreement with both of you–likely evap system related. I just think my mechanic is hesitant to start down the road of $250+ worth of dianostics and $600 of solenoids and sensors without more confidence that the check engine light will not come back on within an hour of leaving the shop when the car is seemingly performing perfectly at 60mph on the interstate. (As happened the first time he turned it off.) In researching info on the code and this car on other sites, I am not finding anything other than the steps that I described. This is my first attempt at posting anything. The next thing I am going to do is fill my 2-gallon gas can (for my lawn mower) and add that to the tank manually and see if that sort of very low pressure, low volume addition creates the same starting issue. I am also thinking of going to a junk yard to see if I can view a fuel neck for a Chevy Sonic to see the construction of it and to assess if there is a likely place of obstruction or corrosion or other damage or problem. One friend suggested using a long funnel to add gas to bypass a sensor in the fuel neck (but my research does not say there is one), but anti-siphoning mechanisms in tanks might preclude that. Any suggestions for diagnostics or work arounds are welcome. Is there such a thing as a Chilton’s manual for these systems in cars now-a-days? I lived on Chilton’s manuals for my Chevies in the '70’s and '80’s.
I just thought this issue might make a good story for Mr. Tappet, with the unique nature of the issue and the added dynamic of my psychological denial of accepting this car upon loss of my beloved Sebring in such an unfair demise. :slight_smile:

IMHO, it’s worth fixing your car until the cost of the repair exceeds the cost of any alternative you’d consider, like buying a new car. That calculation gets complicated, weighing several unknown factors based on your budget and your current car loan, if any.

Some say it’s a matter of what the car is worth, but I don’t see it that way.

1 Like

Once it starts, those rough starts, how long until it smooths out?

Maybe something in the EVAP system that runs for a short time after startup is allowing too much air into the engine, leaning out the mixture. A valve that should be closed, or a pump running that shouldn’t be.

Whitey is absolutely right. What your car may be worth to someone else is meaningless. The calculation is between what it costs to fix this car vs. what it would cost to buy something else.

A mechanic who’s not willing to spend a customer’s money chasing a problem he can’t solve sounds pretty good to me. Yes, the OP can always try a different mechanic but he might not be as honest.

Honest is admirable but won’t get the OP’s car fixed.


The first thing you do is replace the evap purge valve

P0496 is very often caused by a faulty evap purge valve

Don’t bother cleaning the MAP sensor . . . I don’t know why the guy even recommended that. Nobody cleans MAP sensors

You really need a better mechanic, one who is capable of diagnosing lean conditions. Even if the condition isn’t present right now, unless you’ve cleared the fault code, there will be freeze frame data, which can often give a smart guy important clues

I think you can also forget the fuel tank pressure sensor

I REALLY get the feeling this guy is out of his depth and is grasping at straws

By the way, there are many things besides vacuum leaks which can cause P0171

Your car is only 3yrs old and definitely worth fixing


I concur w/@db4690 above, very good chance the cause of the hard to start after filling the tank is a faulty purge valve.

Here’s why. As gasoline fills the tank during re-fueling, the air & fuel vapors above the fuel level are pushed into the car’s charcoal canister, which holds onto the fuel, and allows the rest – the air — to vent to the outside. If this continued refueling after refueling, eventually the canister would get full of gasoline. No good. To prevent that, the computer commands the purge valve to open when you are driving the car down the road, which allows any fuel in the canister to be sucked into the engine and burned. The problem you are most likely having is that purge valve is open when it shouldn’t be; when you are starting the car. It’s also likely it is open during re-fueling. This would allow the fuel in the canister to be pushed into the engine as you refuel, which shouldn’t happen as it would tend to flood the engine.

The P0171 is sort of the opposite of the above, too lean rather than too rich, and could be caused by various things. It could well be something simple. But its pointless to try to solve the P0171 until the purge valve problem is repaired.

As far as what to do, the first task imo is to locate an independent shop that works on Chevys. You don’t need a dealership’s expertise for these problems. Ask your friends, relatives co-workers, church-goers who own Chevys who they use to get their cars repaired. Then choose one of those shops and make an appointment. Be sure to tell the shop which of their customers, one of your friends etc, recommended them to you. This gives the shop an incentive to do a good job for you.

As far as throwng in the towel and replacing the Sonic? I don’t think you are to that point yet. Get a good inde shop’s appraisal on what’s wrong first. With only 77K miles on a 5 year old car, and presuming it’s never been driven with low oil or seriously overheated, there’s an excellent chance this isn’t anything serious.

Why did the mechanic not try defeating the cannister output for a test? Block or clamp the output hose to the air intake shut. Loosen and then tighten the gas cap. Start the engine. If it starts easily and runs smoothly, you have your answer. I recently read elsewhere that there is an epidemic of cannister solenoid valve failures in cannisters with an integral solenoid so that the entire cannister assembly had to be replaced to fix the problem.

Interesting - I have a 2013 sonic (with 88K miles on it) which was doing the same thing. P0496. I took it to Firestone - they replaced the Fuel Injection canister solenoid, removed and replaced the fuel tank, replaced my spark plugs with a fuel system cleaning. I went to the gas station - and it died… again. However no check engine light. I took it back and they replaced (at their cost, other than labour) the evap canister purge solenoid. I filled it with gas and No problem starting - yay!
But the next day, the check engine light came on and it was a P0171 - too lean. Its now back at Firestone getting diagnosed.
Im wondering if this is a problem with Chevy Sonics.
Im not trying to pirate your thread… Im only giving my experience and what I had fixed, to hopefully help you. Firestone will figure it out and when they do, Ill post an update here.

Good luck with that . And no it is not an across the board problem with Sonic .

Oh thank goodness! Cause its a pain to get figured out!! lol

Update - Firestone said that my air intake hose (tube?) is cracked. They are going to eat the cost of the part, but I will need to pay $53 for labour. They said its a totally different system than the Evap system -
Regardless, hopefully this will do the trick. Im hoping my experience will help others. :slight_smile:

@LisaMcIntire, I know this is a month late, but the P0171 is probably not significant. It is likely caused by the P0496. If there is a leak in the purge valve, or it is not closing properly, then extra air is being drawn into the intake manifold causing the lean condition.

What is significant here is that it occurs when you fill the gas tank. Overfilling the gas tank (filling after the first click) could cause this problem, maybe, but I’m not sure how. If the purge valve is commanded to be open more right after filling for some reason, then I might suspect that the canister vent is not closing when the purge valve is opened.

BTW, have you recently driven through deep water? There is a filter on the vent valve that could be not letting enough air into the canister if it got wet.

Update - well, Firestone replaced the intake hose - and I brought it home and it drove fine yesterday… today the check engine light came back on - so tomorrow Ill take it back over and see what the code is NOW.

As a matter of interest - Ive filled the car up twice now since they did the evap canister purge solenoid I havent had a problem starting it - so that might be your issue, Lisa.

On another note - they had told me that the P0171 was a totally different system than the P0496 - hence me having to pay for labour for the intake hose - and they said they were doing it as a ‘favour’ for me. I thought that didnt make sense that they werent connected, however I dont know about cars.

Im hoping that any of this will help someone else… so they dont get the fuel tank replaced, the fuel injection canister solenoid replaced and spark plugs replaced along with a system cleaning, just to find out its a evap canister solenoid!

Your shop is correct saying that the evap system is separate from the air intake. However it is still possible – albeit a little unlikely — for an evap system problem to generate a “overly lean” diagnostic code. In any event the intake path has to be air-tight in any modern car, so that fix was required.

Overly lean codes are often caused by vacuum leaks. Could be a vacuum hose that has split or fallen off. Another common cause is a vacuum operated device (like the brake booster) has failed. Splt rubber in the air intake path is another common cause. There’s many possibilities, like trying to find where a balloon is leaking, so good shops have a systematic way to check for vacuum leaks. Sometimes they us a gadget called a “smoke machine” to inject visible smoke into the air intake path and look for where the smoke might be escaping. If so they’ve found an air leak.

The fuel trim test can be pretty helpful for finding clues for overly lean. You might want to ask you shop if they’ve done that yet.

Beyond that, the common advice here is to mostly avoid chain repair shops. Instead ask your friends, co-workers who they use, and from that list interview a few that work on Sonics, then choose a local independent auto repair shop of your own liking.

Fault P0171 is a very common problem for the Chevrolet Sonic.
There is a service bulletin about the leaking air intake hose that was replaced.
There are 750 reports by technicians for warped valve covers causing this fault, a very high number.
There are other emission control valves that can cause this fault, it will need to be diagnosed.

1 Like