I have a new car, a 2014 Mazda 6. Recently, the check engine light went on. At the time it went on, the car did not accelerate properly for about ten seconds (it accelerated, but slowly). After ten seconds, everything seemed fine, though the check engine light remained on for a while, then turned off. The scenario repeated itself a few days later, although there was no acceleration problem (the check engine light again went off after a while). I am going to the service department, but I wanted to ask a question here first.
Am I correct in assuming that the car’s computer retains both incidents and the specific codes that accompanied them?
Yes, issues that are currently effecting the vehicle will have codes stored as “active”. Issues that are intermittent will be stored in the history as “passive”. So even if the technician or yourself cannot duplicate the issue, there will still very likely be stored codes along with vehicle data such as engine temp, rpm, etc when the issue occurred.
To some degree, yes. The computer has a “freeze frame” feature that stores some of the data at the time a code was triggered, as described by Willcross. Sometimes this info is helpful in tracking down the cause of the problem. It does not necessarily store a freeze frame for every possible code, and if the same code was generated more than once it may only have one freeze frame for it.
A scan tool is a good thing to have. You can get simple ones that will just give you the numeric code, and the there are others that will display O2 sensor graphs, etc. Very nice to have if you have an older car.
The interpretation of the codes at such places is not reliable. Get the actual code number and look it up yourself. Having a Haynes manual for your model helps. Keep in mind that different errors can cause the same code so it just narrows things down a bit, it’s not 100% diagnostic.
In between the two extremes, you can get intermediate ones that will at least translate the codes into English for you. The one I have also has a customizable mode that will tell you at a glance if the car will pass the type of emissions test that relies on the computer fault codes, which many states do.
How useful this is I suppose depends on how inclined you are to do your own troubleshooting and repairs, or if you want to double check the info your mechanic is giving you. I’m not so inclined, but after an emissions repair when the computer is reset and it takes up to a week for all the monitors to come up to “ready” status, it was at least able to tell me when the car was ready for the test and that it would pass, without wasting more time and money. And when something weird like described in the OP happens, it’s the equivalent of having a much more sophisticated dashboard, where “Check Engine” translates into actual problem descriptions.
This is a “used to” in CA. Shops were charging $80 to plug the reader into the socket. They didn’t like the competition.
So now, in CA, they offer a range of readers - IIRC, $80 got you your basic OBDCII code, plus a booklet telling you what each meant.
$120 got the text meaning of the code as well as on-board text.
I knew I’d lose the booklet.
In my case is was the air inlet temp - a thermistor mounted on the bottom of te air cleaner housing. $10 at Napa.
The state of California Bureau of Automotive Repair made the decision that code reading and recommending repairs crossed the line from a parts house to being an ARD (Auto Repair Dealer) and requires a license.
Also it’s a 14 and covered under the warranty, why would you take it to Auto Zone?
Here is why you can’t just take a code and replace that part.
Here is the perfect example of, is the code the source of the problem or the result of another issue?
A code for a bad cat can be caused by
A bad cat (well duh!)
Bad front O2 sensor
Bad rear O2 sensor
And a few other things.
When you bring your car to me I will charge you to diagnose the problem, not just read the code. You are paying for my knowledge, experience, and skill.
Getting back to the OP. Modern cars have the ability to turn off a check engine light if the problem no longer exists. The code will be stored in memory for a period of time and if the problem does not reoccur it will go away totally.
The law requires between 2 and 4 running parameters be stored when the code sets, however many car makers store a bunch more codes that can be accessed with the factory diagnostic equipment. At Volvo we stored about 20 parameters, now at Mercedes it’s even more.
My suggestion is take the car to the dealer and describe the issue just like you did in the OP.
As Rick says in the part of his post I trimmed, a hand held OBD II will only point you in the general direction of what area/system might be at fault. If you start making guesses based on a couple of codes you can end up throwing a lot of money at the problem. It will not say “your catalytic converter is bad”, it might say something like “P0420 Catalyst system efficiency below threshold (bank 1)”, do you really know what is wrong? No. This is why you take it to the shop because they will be able to retrieve more detail and have the training to interpret the meaning of the information. And with most of these readers, as Rick also says, when the light is on, a code is stored and when the light goes back out, the code is gone.
Checking the gas cap is good advice and a common source of a lit engine light. But then again, if you plug in your OBD II code reader it is going to say something like “P0440 Evaporative emission control system malfunction.” Oh my god! what is wrong with the car? It would be nice if the code read “the gas cap is loose, go put it on right”, but it doesn’t. Because it could be a leak in some other area that is more serious.
Personally I love figuring out a new problem when it arises with my car. But the car is special to me and will never be sold. And it has its own special building full of almost all the equipment I need. It is not unheard of for me to tell my wife, “honey, the car is going in the shop, I’m taking a week off work, and I may need a $1000 or so for tools and parts.” She will just say “Whatever.” And I get away with it because I can.
Different car (2010 Honda Accord) - check engine light was blinking and the car was running rough. Went to the nearest dealer and they told me that if the light is on, see a dealer when you get a chance. If the light is blinking stop and be towed to the nearest dealer. My opinion, they should make that a different color light.