We all dread the ominously vague Check Engine Light coming on to tell us something’s wrong with the car we’re driving. But really, the Check Engine Light is your friend. The Check Engine Light exists to help you, and Car Autance will explain how.
If your car’s a 1996 model or newer it almost definitely has a universal onboard diagnostic system known as OBD-II (or “OBD2” in colloquial typing). If you already knew that, you might be ready to go ahead and scroll straight down to the Codes Explained section.
If your car’s a ’95 or older, it may also have a check engine light and a more primitive diagnostic system, but I’m sorry to say this particular post won’t help you too much. Try Googling your specific car plus “trouble code diagnostics.”
For those of you still with me, I’ll briefly explain why the Check Engine Light is a blessing before we walk through what to do when one shows up. Two reasons, really: the CEL can alert you to minor issues before they become big ones. And when you do have a malfunction to diagnose, that light (or, rather, the aforementioned OBD system it’s connected to) narrows down your problem points. Quicker diagnosis means less downtime, less needlessly replaced parts, less pain in your ass.
Thanks to your OBD2 system, if your car’s running weirdly or the CEL has come on, you can plug a little computer (we’ll get to this shortly) into a port near your steering wheel (we’ll show you that, too) and it will spit back codes that can be translated to descriptions of specific problems.
If this little primer piques your interest and you want to go deeper on the history and science of OBD-II systems, check out this ancient OBD-II Home Page that appears to have been put together by a company shilling a now-discontinued scanner tool. That site is decidedly unsexy but it’s loaded with well-organized information.
The rest of this post will run down what to do when your CEL comes on, then link out to our ever-expanding library of how to interpret those codes I just mentioned.
So let’s imagine you were just driving along when suddenly the scary old-timey-engine-shaped light popped up to ruin your day, and now you’re pulled over Googling how to deal with it on your phone. Was I close?
Step One: Panic!
Hah, just kidding, you’re going to be fine. Take a deep breath and we’ll get through this together.
Feel, Smell, Look, and Listen
Has your car been behaving dramatically differently since around the same time CEL popped on? Are you feeling new vibrations? Hearing odd rumbles? Smelling gross smells? If not, you’re probably OK to proceed on, at least until you can get your codes read.
“But my car’s been rumbly and smelly for years,” you say? Well… relatable. But I have to err on the side of caution when I’m dispensing broad advice to a hypothetical audience. Trust your instincts.
If your CEL was accompanied by the car seeming off in a disconcerting way, I would seriously consider calling a trusted mechanic if you’ve got one, getting towed, or at least doing some more internet searching on your specific symptoms before doing any more driving,
Locating Your OBD2 Scan Plug
Before you can translate OBD2 codes into useful information you’ve got to find the dang plug. The exact location of this will vary a little from car to car, so may just want to open another tab and quickly image-search your “year make model + OBD port.”
If you can’t be bothered to do that or want to feel a sweet rush of satisfaction, hunt for it yourself! I can almost promise you it’ll be somewhere near where the driver’s feet or knees usually go. So check around the pedals and under the steering wheel for a female plug that’s trapezoid-shaped and maybe a little smaller than a Bic lighter.
Here’s what you’re looking for:
The OBD2 (or OBD-II) plug port might be oriented sideways or upside-down relative to this picture, but it will be shaped exactly like this. – Image: Andrew P. Collins
Getting Your Codes Read
Any pro mechanic will definitely be able to scan your OBD2 codes, and the benefit of bringing your car to a professional is that they’ll also be able to tell you pretty quickly what might be broken along with, more importantly, how serious the problem is and how much time and money they’d want to sort it out. They might charge you for the diagnosis, though.
Here you can see that my ’98 Montero’s OBD2 plug is oriented vertically. That dangling cord is my scan tool; your car won’t have that from the factory. – Image: Andrew P. Collins
Almost any auto parts store should be able to scan your codes for free. If you go this route you might also be able to get some insight on the car’s condition from the person working the counter, but generally speaking you want to take advice from those folks with a grain of salt. Not everybody employed by parts stores are automotive experts.
Your third option is to get your own scan tool. Some even interface with your phone.
Scan Tools
There are about a zillion OBD2 scan tools for sale on Amazon alone, and probably at least two on the shelf at your local car parts and hardware stores. Even the cheapest one will be able to translate your CEL into trouble codes, which you can then use our library below to interpret. I personally am a big fan of one called the ScanGauge II, which spits out realtime information on engine parameters in addition to reporting codes. I paid about $150 10 years ago and it doesn’t look like the price has come down since.
A ScanGauge II looks awesome in the cab of a ’90s SUV, though. I got mine more than a decade ago when I had a Land Rover Discovery; these days it’s hard-mounted in my Mitsubishi Montero. I keep it plugged in at all times so I can watch precise measurements on my engine’s coolant temperature and air intake temperature, which comes in handy when I go off-roading in the desert.
As you can see, I’ve got my ScanGauge II installed in a single DIN storage tray slot. Since it didn’t quite fill out the space, I cut and stained a block of wood, then put a Mitsubishi emblem on it, to fill out the space. – Image: Andrew P. Collins
My colleague Corey Foster is fond of his Autel MS309, which is much cheaper at just $23. He describes it as “idiot-proof,” as it provides a lot of information without requiring a second reference manual to operate. So if you’re just finding this site for the first time and want to keep your car modding simple, that might be a good starting point.
On the other end of that scale, Car Autance writer Peter Nelson has an APEX Pro plugged into his OBD2 port which can feed a lot of engine and vehicle speed info into a computer program for him to study later. Chris Rosales, also a writer here, uses an OBDEleven which lets him access Volkswagen-specific information through the OBD2 port in his GTI.
Clearing Codes
Most OBD scan tools have the ability to clear trouble codes, making the Check Engine Light go away. Problem solved? Well, actually… sometimes.
Trouble codes, and the Check Engine Light, are triggered by the car’s computer detecting a malfunction. But it can occasionally be set off by a brief hiccup especially if your car’s older. When I get a CEL but my car doesn’t feel like it’s malfunctioning, I’ll usually scan the code, write it down, look it up, clear it out (usually just a button on a scan tool), and keep driving to see if it comes back. Sometimes it doesn’t and I chalk it up to a momentary stumble.
That might not be ASE-approved protocol, but, it can be a good place to start troubleshooting. More more caveat: If you clear the code, and still want the car to get looked at by a pro, they might have a harder time getting the info they need from the car’s computer. So put some more miles on after code-clearing before you bring it in.
Codes Explained
However you figure out which codes your CEL is indicating, what you really want to understand is how to get your car back to optimal condition. Hence, our library of OBD2 trouble codes. There are thousands of codes in total, so we’ll probably be updating this list for eternity, but we’re starting with the most common and working down to the obscure so hopefully you’ll find some useful info here.
P0420 “Catalyst System Efficiency Below Threshold (Bank 1)”
P0171 “System too lean (bank 1)”
P0300 “Random misfire; misfires”
This library is still being updated. For any other code, we’ll have to send you back to your search engine for now. Good luck!
