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How to Diagnose Your Check Engine Light

by Larry Carley copyright

The Check Engine light, or "Malfunction Indicator Lamp" (MIL) alerts you when your vehicle's OBD II system has detected a potential emissions problem. Depending on the nature of the problem, the Check Engine lamp may come on and go off, remain on continuously or flash. The only way to know what is causing the light to come on is to plug a scan tool into your vehicle's diagnostic connector so you can read out the fault code(s).

Don't panic if you see the Check Engine light come on because it seldom indicates a serious problem. If there are no other warning lights on and your engine seems to be running normally, you should be able to continue driving without harming anything. But you should investigate the problem at your earliest convenience.

Here's how the Check Engine Light works. When the OBD II system detects any fault that may cause an increase in emissions, it sets a "pending code" in the computer's memory. The Check Engine Light does not come on yet because the system needs to make sure the problem is real and not a temporary glitch. If the same problem occurs on a second trip (under the same driving conditions), the OBD II system will then set a diagnostic trouble code (DTC) and turn on the Check Engine Light.

What To Do If Your Check Engine Light Is On

If any other warning lights are also illuminated (such as engine temperature, oil pressure, charging voltage, etc.), or your engine is making noise or not running normally, STOP. The cause of the warning light should be investigated before driving any further as serious engine damage result depending on the problem.

If no other warning lights are on and only the Check Engine light is illuminated, you can continue driving. But you need to do a diagnosis to find out what is amiss.

Use a Scan Tool

The only way to know why your Check Engine light is on is to connect a scan tool or code reader to the 16-pin OBD II diagnostic connector under the instrument panel and read out the code. If you do not have a code reader or scan tool to do this yourself, you can take your car to an auto parts store for a free diagnosis. If you can't get a free diagnosis at an auto parts store, you will have to take your vehicle to a repair shop or new car dealer for a plug-in diagnosis. Be warned that this is usually expensive. Most charge $75 to $100 to perform this service. For the same money, you could buy a code reader or basic scan tool and do it yourself.

Reading Trouble Codes With Your Scan Tool

When you plug a code reader or scan tool into the diagnostic connector, the tool will display any trouble codes that are in the Powertrain Control Module (PCM) memory. There may be only one code, or there may be multiple codes. Basic code readers may only display a number, which you then have to look up in a reference book or online to find out what it means. Better scan tools display the trouble code number and a short definition of what the trouble code means.

Write down any trouble codes and definitions that are displayed for future reference.

What you do next will depend on the code(s) your found. A trouble code by itself does not tell you what part needs to be replaced. It only tells you the circuit or component where the fault occurred (oxygen sensor, for example), or the nature of the fault (misfire, for example).

See Trouble Code Diagnostic Help for Types of Codes for additional information on how to diagnose and repair common trouble codes.

Further diagnosis is usually necessary to isolate the fault and figure out what is causing the problem and which part (if any) needs to be replaced. This often requires following a lengthy diagnostic chart and step-by-step checks to rule out various possibilities. This kind of information can be found in the vehicle service literature or on service information websites such as AlldataDIY or Mitchell1DIY.

For example, let's say your Check Engine Light is on and you find a trouble code for one of the oxygen sensors (code P0130). The code might indicate a bad sensor, or it might indicate a loose connector or wiring problem. You should check the wiring first before replacing the sensor.

Harder to diagnose are misfire codes. OBD II can detect misfires in individual cylinders as well as random misfires. If it generates a misfire code for a single cylinder (say P0301 for the #1 cylinder), it only tells you the cylinder is misfiring, not why the cylinder is misfiring. The underlying cause could be a bad spark plug, a bad plug wire, a weak coil on a distributorless ignition system (DIS) or coil-on-plug (COP) system, a dirty or dead fuel injector, or a compression problem (bad valve, leaky head gasket, rounded cam lobe, etc.). As you can see, there are multiple possibilities so it takes some diagnostic expertise to isolate the fault before any parts can be replaced.

A "random misfire code" (P0300) is even harder to diagnose because there can be numerous causes. A random misfire usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty injectors, low fuel pressure, a weak ignition coil(2), bad plug wires or compression problems.

For a detailed look at all the operating parameters that can set trouble codes, Click Here to view a PDF file on GM 4.6L diagnostic parameters.

The best advice in situations like this is to take your car to a repair facility that has the proper tools and expertise to accurately diagnose the fault.

Watch Out for False Trouble Codes

Sometimes circumstances will set a code that indicates a fault has occurred, but actually there is no real problem. Some cars will set codes because the OBD II system is over-sensitive or there is a glitch in the factory software. For example, older GM cars with certain 3.8L engines will often set a P1406 code, which indicates a fault in the EGR valve. Replacing the EGR valve doesn't fix the problem on these cars because the OBD II system is overly sensitive to how quickly the EGR valve opens when it is commanded to do so by the PCM. The cure here is not to replace the EGR valve, but to have your car dealer or repair shop reprogram your engine computer. This is called "reflashing the PCM" and it involves installing updated software that fixes the problem. The process typically takes about a hour and costs $100 to $200.

Vehicle manufacturers frequently release Technical Service Bulletins (TSBs) that provide fixes for faults like these. This type of information is available on the vehicle manufacturer service information website, or through Alldata or Mitchell.

Turning The Check Engine Light Off

The OBD II Check Engine light will generally remain on as long as a fault persists.

If an intermittent fault does not reoccur after three consecutive trips, the MIL lamp will go out, but the trouble code will remain in memory. If the fault does not reoccur for for 40 to 50 trips, the code will be erased.

The only safe way to clear fault codes and turn of the Check Engine Light is to use a scan tool or code reader. Most of these tools have a button or menu choice that says "Clear Codes?" When you press the button or choose the option, it wipes the code from the PCM's memory. This will take you back to ground zero.

ADVICE: Write down any codes you have found BEFORE you erase them! Don't think you'll remember them because in a few days you probably won't.

If the Check Engine Light comes back on again (which is usually does if there is a hard fault in the system), you can check the codes again to see if they are the same ones as before. This would confirm the fact that you have an emissions problem, and that further diagnosis and repairs are probably necessary.

NOTE: Many emission faults that sets codes won't have any noticeable effect on the way your car starts, drives or behaves. So you may be tempted to just ignore them. That's up to you. But if you live in an area that requires emissions testing, your vehicle will NOT pass an emissions test if the Check Engine Light is on.

On older pre-OBD II vehicles (1995 and back), trouble codes can also be cleared from the PCM's memory by disconnecting the battery. Unhooking the battery ground cable for 10 seconds, then reconnecting it will "reset" the computer. But it will also wipe all of the other learned settings from the PCM's memory, too. That means your engine may not idle smoothly or feel quite right for some time until the PCM relearns what it needs to know. Same for the transmission controller. You will also lose the channel presets on the radio, and any other electronic settings (memory seats, mirrors, etc.). That's why a code reader or scan tool should be used to clear the codes only.

WARNING: On many 1996 and newer OBD II cars, pulling the PCM fuse or disconnecting the battery may NOT clear the codes, and may cause a loss of important information the PCM needs to function correctly. This is certainly true on 2004 and newer vehicles with Controller Area Network (CAN) electrical systems. DO NOT DISCONNECT THE BATTERY ON THESE VEHICLES! On some vehicles, loss of power to the PCM may cause it to forget

transmission settings, climate control functions and other essential data. This, in turn, may require an expensive trip to the new car dealer so they can use a factory scan tool to reset or reprogram the information that was lost.

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Other Warning Lights (TEMP, OIL, ALT/GEN, BRAKES, ABS, AIR BAGS, etc.)

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Understanding OBD II Driveability & Emissions Problems

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Controller Area Network (CAN) Diagnostics

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