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The "Service Engine Soon" light came on yesterday and isn't going away on its own. The car drives fine, no weird noises, smells, or other symptoms. The manual is not very helpful, just directing me to take it to a professional for service. Before I do that, I'd like to try and eliminate any possible causes that are easy to check for myself.

The specific car I'm working with is a 2002 Nissan Maxima that passed smog two weeks ago, but I'm interested in general advice too.

I'm comfortable changing oil and filters but not much more. My tools are limited to a pair of jack stands, oil pan, and a wrench and socket set. I would prefer not to invest in new tools unless they are going to be useful again and again.

What are some of the possible causes for the "check engine" light that can be diagnosed at home? Where should I start?

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Welcome to the site. Hook up the vehicle to a scan tool. That will give you direction. Once you have those, please post them up here and I'm sure the users here will do their best to help you out. Great question btw – Zaid Jan 11 at 10:16
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I think you may have mistaken a service interval indicator for a check engine light. Modern cars will ask to be serviced at regular intervals, even if no fault exists. There indicators can usually be reset the but procedure is usually manufacturer specific. A check engine light is something else and generally indicates that a fault has been logged by the ECU. – Steve Matthews Jan 11 at 11:09
    
@SteveMatthews fair point, you should post that as an answer – Zaid Jan 11 at 11:38
    
Check out my answer re: diagnosing check engine lights. – Lynn Crumbling Jan 11 at 21:28
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Number one cause of a check engine light is a loose gas cap. Always check this first! It may take a few minutes or up to a day of driving before the light goes away if this is indeed the cause. – makinbacon Jan 11 at 23:16
up vote 22 down vote accepted

You've come to the right place. There's a lot that you can do on your own. Even if all you end up doing is gathering more information for the mechanic who ends up working on the car, they never mind a hint as to where the problems might be!

What are some of the possible causes for the "check engine" light that can be diagnosed at home?

All of them. They can be all over the map, ranging from things as simple as "the gas cap is loose" to "major engine parts have broken off."

Where should I start?

You should start with a tool that will help you determine what the singularly unhelpful check engine light is actually trying to tell you: a code reader.

A code reader will query the engine computer to see what's on its mind. The engine computer will respond with a series of codes (from one to an arbitrary number of them), each of which will stand for a short diagnostic message. Depending on the tool that you use, you might also get a paragraph description and background information to go along with the code. Some tools will even let you clear out a code to see if it comes back (much like rebooting your PC to see if the problem goes away).

If the tool doesn't give you much more than some letters and numbers, try typing the results into the search box in the upper right of this page. There is a 100% chance that you are not the first person to have a particular problem and some of those people have come here with the same question. For example, I've had a P0420 code on my wife's old Outback. That lead me to conclude that one of the O2 sensors in the exhaust system was going bad. A little cash and a fair amount of annoying wrench work and the problem was solved.

If searching isn't helpful enough, come back with a new question letting us know the make and model of your car, the codes that you've found and any other symptoms that you've observed. This is a place where car nerds will happily help you for the fun of it!

PS: I just ran across Lynn's answer to the question What CAN'T the check engine light tell you? Lynn gives provides some links to the Wikipedia list of OBD codes as well as some examples of where those codes might lead you in diagnosis.

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Worth noting, many parts stores will offer to read codes for free, saving you the cost of buying a code reader. They figured out a long time ago that people who read the code in their parking lot tend to buy parts from the store shortly thereafter ;-) – Cort Ammon Jan 11 at 5:01
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This. Very plain code readers are under US$50, and go on sale sometimes. More money will get you a nicer interface and a more extensive on-board database of codes (with the cheapest ones you have to look up manufacturer specific codes on the internet; not a big deal in the age of smart phones). – dmckee Jan 11 at 6:41
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You can get simple bluetooth based readers that works with your smartphone for <$10. Search ebay for ELM327 and you get a bunch that are small enough to leave there and then use smartphone for dashcam with engine stats. – user3553030 Jan 11 at 7:33
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I lack the rep to comment on Bob's answer, but to echo him, as a weekend mechanic the best $25 I've spent on my car was on a Bluetooth OBD2 adapter. It allows you to pull codes to narrow down the problem and clear them to see if they reoccur. If you know the error code some googling will usually present you with a list of likely culprits. amazon.com/BAFX-Products-34t5-Bluetooth-Android/dp/B005NLQAHS Should work on any car 1996 or newer. – thunder2709 Jan 11 at 17:27
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Some vehicles will have a dashboard readout of service codes as well. On some Chrysler vehicles, for example, this could be triggered by turning on and off the key 3 times. (just to the run position, not starting/stopping the car) This would scroll through recent engine codes in the mileage display. – GalacticCowboy Jan 11 at 19:42

I think you may have mistaken a service interval indicator for a check engine light. Modern cars will ask to be serviced at regular intervals, even if no fault exists. There indicators can usually be reset the but procedure is usually manufacturer specific. A check engine light is something else and generally indicates that a fault has been logged by the ECU.

share|improve this answer
    
I appreciate the suggestion and I had to double-check the manual. For the 2002 Nissan Maxima, the "Service Engine Soon" light is referred to as the "Malfunction Indicator Lamp", and indicates a potential emission control malfunction. So I believe it is analogous to a "Check Engine" light in this case. – Robert Jan 11 at 19:49
    
Wow, that's a vendor specific bit of confusion isn't it! I wonder if this is a translation issue. – Steve Matthews Jan 12 at 9:17

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