Rick's Free Auto Repair Advice

Fix Check Engine Light On

Why is my check engine light on

I volunteer in many different automotive forums and I constantly see people say they have a check engine light on. They want to know why it’s on. But when a you have a check engine light on, there’s no way to guess. Because the system is capable of setting up to 1,800 different trouble codes. That doesn’t even include manufacturer-specific codes that can be used to tell if there’s a problem with a body control computer or chassis system. With 1,800 possibilities, there’s simply no way anybody can get any kind of answer when they ask, “Why did my Check Engine light come on?”

The ONLY way to start the diagnostics to find out why the check engine light came on is to plug in a code reading tool and poll the computer. Once a trouble code sets and lights the light, that code is stored in computer memory. The code reader or scan tool (there’s a difference between the two) polls the computer memory and pulls up the trouble code. Some units decipher the code and tell you what it means in plain English, while others just pop up the code.

Once you have the code, you can do two things: 1) Fix the problem, or 2) Clear the code. Now, I get asked a lot “How to I clear the code or turn off the light.” I’ve got news for you, you’re not fooling anybody if you think you can turn off the light and then take your car or truck in for an emissions inspection. Because the computer will tell the inspector that you’ve just cleared the codes. The inspector will tell you to come back and have the car checked again after the computer has reset its readiness monitors. (See drive cycles to understand what you have to do after clearing codes to be able to take and emissions test).

And, you CANNOT reset ABS or SRS (airbag) lights. You have to fix the problem.

Finally, be aware that the computer tracks how many times you’ve started the car with the Check Engine light on. So, if you’re under warranty or driving a leased vehicle and think that you can dodge the charges for engine damage by claiming that the “light just came on,” think again. They can tell when the light first came on.

How to check codes

1) Take it to an auto parts store that checks codes for free. Most major retail auto parts stores (like Autozone, Advance Auto, O’Reilly) will scan your computer for free. But beware, their parts guys are NOT technicians. So if the trouble code translates into “Oxygen sensor lean,” I guarantee you the parts guy will sell you a new oxygen sensor. Do you need one? Well, what if the exhaust stream really IS lean and the oxygen sensor is telling the truth? $80 down the drain. Remember, parts stores are in business to sell parts. Don’t rely on them to diagnose problems.

2) Buy your own code reader or scan tool
In the early days of OBDII, code readers and scan tools cost a fortune and only professional technicians could afford them. But today you can buy a code reader for $40. All right, I can hear you whining. But before you go into a full pity-party, remember that “in the old days” we had to buy special tools to work on cars. Remember tach/dwell meters for setting “points” in the distributor? Or that timing light you had to use to set timing? How about those distributor wrenches that you only used once every 3 years? So get over it—you’re going to have to spend money on a new tool.

What’s the difference between a code reader and a scan tool?

A code reader reads codes—period. It doesn’t tap into the vehicle’s computer to display “live data.” Knowing the code if helpful, but it’s just a start towards diagnosing the root cause. The majority of Do It Yourselfers who buy code readers end up guessing at the problem and replace a LOT of parts unnecessarily. So you saved yourself a bunch of money on the code reader only to throw far more money into parts you didn’t need.

A scan tool, on the other hand, displays the same data that the computer receives from it’s sensors. If we follow the oxygen sensor example from above, the first thing I’d check in live data is the value for “Short-term Fuel Trim.” When the computer sees a lean condition, it figures it calculated the wrong air/fuel mixture. To correct the problem, it adds more fuel. That additional fuel is referred to as Fuel Trim and the value runs from 0 to 25%. So, in this case, if I were to see a fuel trim of 25% along with an oxygen sensor “lean” code, I would know that I had a huge vacuum leak. In other words, too much air is getting into the engine, the computer is trying to compensate for it and it has max’ed out it’s ability to correct it, and the oxygen sensor is still seeing a lean condition. Remember I told you a parts guy would sell you an oxygen sensor? Well, in this case, I’d be looking for a broken vacuum hose or a bad intake manifold gasket. There, you just save almost enough money on this single repair to justify owning a scan tool.

What else can you read on a scan tool?

Engine RPM, engine coolant temperature, radiator fan operation, transmission operation, barometric pressure, MAF, MAP, VSS, TPS sensor values –getting the picture? You can literally “look” into the computer and get the data.

So buy a scan tool and read up on how to use it and interpret the values.

I’ve listed some of the more popular models of code readers and scan tools below. But read the fine print on the model you want to buy. MAKE SURE IT WORKS ON YOUR VEHICLE. Also, these inexpensive scan tools only pull “P” codes. They will not read ABS, Airbag, or other manufacturer-specific codes. If you want to read all those codes, you’ll need to pony up about $4,000 for a professional model with all the software.


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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