Stubborn check engine light ideas
March 29, 2013 9:48 AM   Subscribe

A few months ago, my car's check engine light came on. They fixed it, it came back on again, they fixed it again, and now it's back on. I'm looking for possible explanations so I'm more prepared when I take it back yet again.

My car is a 2004 Mazda3 hatchback with only 30k miles, purchased new by me back in '04. It has been a reliable car and given me no trouble up until now. I live in a big city and don't drive much - it's mostly a grocery getter and weekend-only car.

My check engine light came on right around the time a groupon I had for an oil change was about to expire, so that's where I took it. They get pretty good reviews so I felt ok about it. The shop pulled the codes for me without doing the "full diagnostic", saving me the $90. Luckily (they said), the code came back as the thermostat being bad, which is a relatively quick and easy replacement. They asked me if I was having trouble with the heat taking a while to warm up, and while I didn't think I was, I don't drive much and we had been having a mild winter. I also got the oil changed and they found a transmission leak around the pan gasket, so they replaced that for me as well. They said I'm also due for whatever fuel injection system work is required at my mileage, but I'm broke so I have put it off for now.

Like a week or so later, the check engine light came back on. I took it back to them - they said it there was air in the coolant system and fixed it for free - I'm assuming they must have introduced the air when they were working on the car before because they certainly acted like it was their mistake. While I was there I got my rear brakes done, which is likely irrelevant but hey they got another $400 from me.

A week or two after that, as I'm turning left out of my alley the check engine light came back on AGAIN. Since then (it's been a month; I've been too busy to take it in) I've driven the car and things seem mostly fine. The heat is maybe slow to warm up but it's really hard to tell - not something I would probably even notice if they hadn't asked me about it originally. The car is definitely not overheating. It seems to drive mostly fine, though the idle can be rough sometimes and the transmission doesn't shift as smoothly as it used to, but that's been the case for a while.

Any ideas? What's the likelihood this is still related to the thermostat/cooling system? Should I insist on (and pay for) the full check engine light "diagnostic" - does that give us more information to work with than just the codes? If the thermostat has just been replaced, what other possible problems that would throw the same code should I be aware of?

Thanks! Like I said, I'm taking it back in but would love to be maybe a little more prepared. I don't have the actual code the computer gave them so I can't google very effectively, and I am not noticing any related symptoms other than the light coming on.
posted by misskaz to Home & Garden (33 answers total) 2 users marked this as a favorite
 
Go to Autozone and get the diagnostic codes pulled (they'll do this for free). Then a bit of Googling should point you in the right direction.
posted by kindall at 9:51 AM on March 29, 2013 [2 favorites]


I'd find another mechanic in the area or take it to a Mazda dealer. Seems like maybe you're getting hustled.
posted by kinetic at 9:52 AM on March 29, 2013 [2 favorites]


It could even be a touchy fuse for the light and they are fixing nothing.
posted by agregoli at 9:56 AM on March 29, 2013


I'm with kinetic, except rather than the place hustling you, I think maybe the place just isn't that good. As you noted, they already made one mistake on your first trip there. This is sounding suspiciously like they're two for two, and that's enough for me to look elsewhere.
posted by Etrigan at 9:58 AM on March 29, 2013 [1 favorite]


Go to Autozone and get the diagnostic codes pulled (they'll do this for free).

You can even do this yourself.
posted by capricorn at 10:00 AM on March 29, 2013 [3 favorites]


I would second getting the codes pulled at Autozone. Then, find an online forum for your make/model, and post your question there (or update your question here so we can better help you).

I would also suggest researching a well-rated mechanic who specializes in your make of car (or Japanese cars more generally). A busy (good) shop probably won't be found via any GroupOn promotion.
posted by nacho fries at 10:01 AM on March 29, 2013 [1 favorite]


I second people to get a new opinion. Look up some new places that have good Google ratings (or yelp.. etc.) I know next to nothing about cars, however I am a quick learner. If you have time try to research the thing they say is wrong before you pay out for it. (If you have a smartphone you should be able to do some searching on the spot if you can't make time for another appointment.)

Not sure if it matters, but the Car Talk Guys loved to say that the solution for the Check Engine light was a piece of black electrical tape.

I had mine come on for an hour after towing the car across two states, it was fine, just being a baby about something.
posted by Crystalinne at 10:01 AM on March 29, 2013 [2 favorites]


Yeah, Autozone, find a friend with an OBD-II reader, or spend the $90 on one yourself (as capricorn suggests).

That the light is coming back on suggests that what they're really doing is just resetting the computer, and whatever other stuff they're doing has nothing to do with what triggered the fault. Generally, the computer has to detect some number of faults before it turns on the "check engine" light, and those faults don't happen all the time, so if you just hit the reset button on the reader the light will go off for a few hundred or thousand miles.
posted by straw at 10:03 AM on March 29, 2013 [1 favorite]


Autozone is a good choice, but if there seems to be nothing wrong with the car, you can start by googling "[model name] check engine false positive" or something like that.

I did that with my old Toyota, and it was simple as removing and putting back a fuse to "reboot" and get the stupid light to turn off.
posted by drjimmy11 at 10:06 AM on March 29, 2013 [1 favorite]


straw raises a very good point. Because you don't drive many miles, and may not put your car through the necessary paces for the computer to reset (getting up to a certain speed and sustaining it), you won't know till some time has passed that the underlying issue wasn't addressed.

When I had a CE issue, my mechanic gave me specific instructions on what type of driving / number of miles / mph to put on the car, post-repair, before he'd consider the issue fixed. Did your guys give you that info?
posted by nacho fries at 10:07 AM on March 29, 2013


I wonder if you have a gas cap leak. My check engine light comes on when my gas cap isn't tightened enough. If there's something wrong with the threads or whatever, that might be causing it to come on. I don't know if/how this would show up in the codes though.
posted by rabbitrabbit at 10:08 AM on March 29, 2013 [2 favorites]


Knowledge is power, and you need more power in this relationship. I second what capricorn suggested -- get your own scan tool and pull the codes yourself. They can be had for less than $30 from Amazon (e.g., the Autel MaxiScan MS300 and MS309 seem well reviewed and are $30 or less). Get one that can both read and clear codes, and don't worry about clearing codes needlessly; any code that has an unresolved underlying problem will come back. Which you already know.

Also, this is a 2004 vehicle, so you need a reader that will pull CAN (controller area network) codes, not OBDII codes, as the OBD protocol was replaced by the CAN protocol beginning in 2004.
posted by mosk at 10:08 AM on March 29, 2013 [2 favorites]


You are getting hustled. Find a reliable mechanic, AKA not a chain oil-change place.

You may also want to actually watch the mechanic read the codes on your engine. Certainly if this is not allowed or encouraged, you're probably not at an honest shop. My mechanic will carry the scan tool over to the car and do it right in front of me.

It's one of those things where there's either a code, or there isn't, and solving whatever the code is (usually) makes the problem go away. This is not a situation where they should be digging around for stuff in your car that could be wrong. Anything they find that is not indicated by the codes they read in the diagnostic has nothing to do with your check engine light. You may want to pay them to fix it, or you may not, but it's not going to make your check engine light stop going off.
posted by Sara C. at 10:16 AM on March 29, 2013


Nthing that there is little to go on here and your first step is to pull the code yourself or have someone else do it for you.

However, it is worth noting that there probably isn't any definitive way for your car to tell if the thermostat itself it bad. The code itself is probably set when the coolant doesn't warm up as expected, which could mean there is a problem with the thermostat, the sensor that measures the coolant temperature, or some other sneaky thing that affects the measured coolant temperature.

Because you don't drive many miles, and may not put your car through the necessary paces for the computer to reset (getting up to a certain speed and sustaining it), you won't know till some time has passed that the underlying issue wasn't addressed.

This is not generally true. If a code is cleared, the fact that it comes back means something is setting the code again. There may be some special cases where operating time is required to clear up some problem, but I can't see how this is one of them.
posted by ssg at 10:16 AM on March 29, 2013 [1 favorite]


It could even be a touchy fuse for the light and they are fixing nothing.

This happened to me, and my awesome, honest mechanic told me this, straight up. Told me they checked the codes and found nothing. Told me my engine looks great. Then they dicked around in the fuses in my instrument panel to get the light to go off long enough for me to pass my smog test and sent me on my way.

Any mechanic who suspects this is the problem and uses the excuse to dig around in your car and find stuff to "fix" is dishonest.
posted by Sara C. at 10:18 AM on March 29, 2013


Had any wet weather in your area? Mine goes on and will stay on for a couple days (and then go off again) if I get water in my undercarriage, or if I get a car wash with the undercarriage wash feature on. Once I told them to stop using the undercarriage wash, I ended a lengthy battle with the check-engine light and saw a whole lot less of my mechanic.
posted by dlugoczaj at 10:43 AM on March 29, 2013


Our car had reoccurring check engine light problems for a few months despite the fact that it seemed to be running fine. Ultimately, after a few trips to the garage, our mechanic decided that it must be that the car computer was bad, because the codes that were showing up weren't making any sense. ("Valve was detected open" when mechanic did not find valve open, that sort of thing.) We took it to the dealer at that point, and all the codes had a common denominator of "going through one relay". They replaced that relay, not the whole computer, and the check engine light hasn't come back on and it has been at least a couple of months.
posted by molasses at 10:44 AM on March 29, 2013


It could even be a touchy fuse for the light and they are fixing nothing.

A weird fuse is not going to cause a light to light up. A fuse can only prevent a circuit from working, it can't cause it to energize when it doesn't want to.

These lights always come on for some reason. Could be the computer is crazy, could be a manufacturer-specific code that a generic code reader can't access, could be a transmission code that the code reader can't access, could be a real code going off for a bad reason, could be a loose gas cap. (Although many newer cars have a separate light for that now.)

If you get nothing out of an Autozone code check, go to the dealership and get the codes read. They have equipment that can read all the codes and find out what the problem is.
posted by gjc at 10:46 AM on March 29, 2013 [2 favorites]


I would also mention that even though the car runs fine, it can be doing bad things that build up over time. For example, if the car is running slightly rich, you'll never notice the difference. But after some thousands of miles, you are going to start getting things like clogged catalytic converters going bad. You've got to have an actual code to know what's going on.
posted by gjc at 10:49 AM on March 29, 2013 [1 favorite]


This happened to us for years. Take it to a different mechanic.
posted by walla at 10:52 AM on March 29, 2013


I'm kind of hoping Brockles shows up here, because it seems to me that this exactly is one of his pet peeves.

The mechanic can't tell you from the diagnostic code alone that your thermostat is bad. You'll get a code like "cooling system fault." In a modern car, that could be the thermostat, it could be the ECT sensor for the computer, you just can't tell without knowing more about the symptoms and maybe taking it for a test drive to check your theory. Air in your coolant? I'm no expert, but I haven't heard that one before.

Between a cheap hand-held code scanner you can get at a parts store and the Googles, and being the most familiar person on the planet with the symptoms you are seeing, you can probably figure out what the most likely problem with your car is before you take it in. I usually spring for a full diagnostic test, but then ask them a bunch of questions about WHY they think the problem is what they think it is. I require convincing when their diagnosis doesn't match my amateur one or doesn't seem to make any sense to me.

You'd be surprised how often I get a stammering, backpedaling "maintenance supervisor" who conveniently doesn't know anything but "that's what the computer said" and the actual mechanic is "unavailable" to explain. I don't go there again.

I finally found a shop where the actual mechanic would come out and help me understand the problem and proposed fix, rather than just telling me I would have to pay X money. I was very happy. Then I moved, so now I'm looking again.

What I'm saying is, you seem to be the kind of person who just believes what they say, in a shop where they take advantage of that.
posted by ctmf at 11:32 AM on March 29, 2013 [1 favorite]


they dicked around in the fuses in my instrument panel to get the light to go off long enough for me to pass my smog test

Just as an FYI, the CA smog diagnostic process detects if the CEL has been merely reset or disabled while an underlying issue exists. Many a driver has had their car fail smog with the CEL off. A good smog shop will ask upfront if you've had CEL issues recently, and ask if you've put enough miles on the car since the repair to reset the system properly. A shady shop will let you go through the test process, charge you, and fail you due to the ECU not being in a "ready" state.
posted by nacho fries at 12:06 PM on March 29, 2013


Forgot to mention: after you get the CEL-related issue repaired, your mechanic should invite you to return to the shop (after you've put miles/mph on your car) to have him confirm that the ECU has completed its checks. He just needs to do a quick check with the diagnostic tool (should be no additional cost to you).
posted by nacho fries at 12:43 PM on March 29, 2013


To be fair, sometimes it's not the obvious thing. And maybe they did tell you or thought they were telling you, but you misheard, that they couldn't be sure it was the thermostat, BUT that a thermostat is a common problem and is only a ten-dollar part, so it had a good chance of fixing it for cheap.

My car is pretty predictable - when I turn it on, it takes some time to warm up to dead center in the temperature gage and then stays there no matter what. If it started staying low or high or varying all over while I drove, I'd suspect the thermostat. If yours isn't doing anything unusual for your car, then I'd think the more likely suspect is the engine coolant temperature sensor for the engine computer. A failure of that sensor probably won't have any immediately obvious effects, other than a slight hit on your gas mileage. But it should be fixed, and even that's pretty cheap (but not as cheap as a thermostat.)

(Or it could be a new problem unrelated to the last one.)
posted by ctmf at 12:49 PM on March 29, 2013


ctmf makes a good point. When my CEL came on because of a bad thermostat, it was pretty obvious what it was because my temp gauge had also recently started showing that my car was running cooler than usual.
posted by rabbitrabbit at 12:55 PM on March 29, 2013


Get a new mechanic, it doesn't sound like you've got a good one. I've had good luck finding excellent mechanics using the Car Talk Mechanics Files.
posted by quince at 1:17 PM on March 29, 2013 [2 favorites]


I'd find another mechanic in the area or take it to a Mazda dealer. Seems like maybe you're getting hustled.

I tend to agree with this, too. Especially the "They said I'm also due for whatever fuel injection system work is required at my mileage..." part. I can't imagine what sort of "work" might be required on the injection system at such low mileage. Sounds like they want an opportunity to charge you $200 for pouring a bottle of injection cleaner in your tank.

Do you have the owner's manual for the car? It will have a table listing the scheduled maintenance milestones and what needs to be done.
posted by Thorzdad at 2:01 PM on March 29, 2013 [1 favorite]


They said I'm also due for whatever fuel injection system work is required at my mileage, but I'm broke so I have put it off for now.

30,000 miles with modern quality fuels? Snake oil. If it really bothers you, $10 bottle of injector cleaner is all you need.

What's the likelihood this is still related to the thermostat/cooling system?

With no other symptoms, I'd say almost zero. But it's also almost impossible to tell without knowing the codes or diagnosing the car. Being as there weren't any other symptoms before, I am wondering if you aren't getting lightly scammed - maybe even with the best of intentions/incompetence.

Like a week or so later, the check engine light came back on.

With the low mileage you seem to do, I have to wonder if that was the same fault triggering again after the ECU had got it's act back together after they cleared the faults. Possibly something completely unrelated to the work they did - I wouldn't take it there or get any more work done without (as people have said) seeing the codes myself and doing some googling.
posted by Brockles at 2:29 PM on March 29, 2013 [3 favorites]


I just want to thank everyone for chiming in. I don't know a lot about cars because they're not a huge part of my life, but I am mechanically inclined (taught myself to fix/replace just about every part on my bike) so I like actually learning about what could be going on here. Everyone's comments were helpful, but I marked as best answer the ones that taught me something I didn't know - that the codes aren't specific enough to say it was definitely the thermostat; and that after resetting the computer it may take some time with my low mileage for the CEL to come back on again - that was what my instinct was telling me after the second time it came back on, but it's good to have it confirmed. They likely never fixed the problem in the first place, especially given that my temperature gauge has been indicating normal behavior on that front, heating up to the exact mid point and staying there.

It's also good to know I don't have to worry about my fuel injector, thanks Brockles.

I'm disappointed because this wasn't a chain place - it's locally/family owned, and everyone there was super nice - explaining and showing me the things that needed fixing (like the transmission leak and the brakes) while also checking in to make sure they weren't talking down to me just because I'm a woman. But, you all have convinced me to get the codes myself and then take it elsewhere. I also suspect they busted one of the plastic covers on the underside of the engine area, cause it's now dragging. Not a critical part and I'll probably just remove it if I can get under the car, but still annoying. Thanks, Mefi!
posted by misskaz at 9:00 AM on March 30, 2013


everyone there was super nice

That was why I stressed the possibility of the best of intentions. It's quite easy to make a faulty conclusion from CEL codes - it doesn't necessarily mean you are being scammed maliciously, if you know what I mean.

I also suspect they busted one of the plastic covers on the underside of the engine area

That happens to the best of us. Those things are often really badly made and warp and crack at the slightest provocation. Don't mark them down for that necessarily.
posted by Brockles at 9:45 AM on March 30, 2013


Ugh, tell me about it - I broke one of the clips off the one covering the battery not too long ago. It just fell apart when I touched it, seemed like.
posted by misskaz at 12:36 PM on March 30, 2013


Couple of thoughts -

1) OBDII is an industry standard, some mfg have their own codes. I ended up popping $150 for a BMW specific code reader that narrowed the issue I was having. I still had to do troubleshooting from there. I have 2 cars with almost identical engines so I was able to swap some parts to figure out what the issue was. Taking it to the dealer or looking for such a tool for your car might be a place to start to get the manufacturer's specific codes.

2) This could be a faulty sensor in the cooling system. When they "fix" the issue the normal thing to do is to clear the ODBII error. The sensor may not glitch out right away thus causing your check engine light to come back on. (see ssg's post). This was happening to my in my case it would take 300 miles or so for it to come back.
posted by empty vessel at 7:54 PM on March 30, 2013


I'm late to this, but if it's any consolation, I had the exact same problem. I drive a 2000 VW Cabrio, which I bought used when it was a few years old. I am not much of a driver - right now it has 111,000 miles on it. The check engine light was on for several years. I had a couple people fix it, and the fix would last long enough for me to get a few blocks away from their shop before it would come on again. So I started putting my glasses case on the ledge in front of the light so it wouldn't distract me, and eventually the light went off and stayed off.
posted by tizzie at 8:50 AM on April 15, 2013


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