The Verify Engine light, and that is officially termed the “Malfunction Indicator Lamp” (MIL) alerts you when your vehicle’s OBD II system has detected a prospective emissions trouble. Dependant upon the nature from the problem, the Verify Engine lamp may possibly come on and go off, remain on continuously or flash. Certainly, none of this gives you any clue whatsoever as to what might be happening.

Lots of people panic once they see the light, fearing their engine is experiencing some kind of important issue. But fear not, for the reason that in most situations, the situation is generally minor and it is nothing that involves your instant consideration.

Here’s how the Check Engine Light performs. Once the OBD II system detects any fault that could lead to an increase in emissions, it sets a “pending code” inside the computer’s memory. The Check out Engine Light does not come on yet since the system needs to be certain the trouble is actual and never a short-term glitch. When the identical dilemma occurs on the 2nd trip (under the identical driving disorders), the OBD II system will then set a diagnostic problems code (DTC) and turn to the Check out Engine Light.


If your Examine Engine comes on, what should really you do?

If no other warning lights are on (temp, oil pressure, charging, and so forth.), Along with your automobile is driving ordinarily (no uncommon sounds, smells, vibrations, reduction of power or other signs of problems), you dont need to do anything at all promptly. But you should determine why the light came on when it truly is easy to try and do so.

The only strategy to know why your Examine Engine light is on should be to connect a scan tool or code reader to the OBD II diagnostic connector under the instrument panel and study out the code. If you don’t have a code reader or scan instrument to try and do this oneself, you can take your car to an auto parts retailer to get a no cost diagnosis (Autozone & Advance Auto are currently offering this). When you cant get a free diagnosis at an auto parts retailer, you will need to consider your automobile to a repair shop or new car dealer to get a plug-in diagnosis. Be warned that this is usually expensive. Most charge $75 or more to perform this service. For the identical money, you could buy a code reader or basic scan tool and do it by yourself.


When you plug a code reader or scan instrument into the diagnostic connector, the instrument will display any trouble codes that are in the powertrain control module (PCM) memory. There may perhaps be only one code, or there may well 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 difficulty code means.

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

So what do you do now that you possess a code? It depends about the code. A trouble code by itself does not tell you what part desires to be replaced. It only indicates the circuit or component where the fault occurred (oxygen sensor, for example), or the nature of the fault (misfire, for example).

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

For example, let’s say your Examine Engine Light is on and you discover a difficulty code for one of the oxygen sensors (code P0130). The code may possibly indicate a bad sensor, or it may indicate a loose connector or wiring trouble. You really should test 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 lead to could be a bad spark plug, a bad plug wire, a weak coil on the distributorless ignition system (DIS) or coil-on-plug (COP) system, a dirty or dead fuel injector, or a compression challenge (bad valve, leaky head gasket, rounded cam lobe, and so on.). As it is possible to 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 simply because there can be numerous causes. A random misfire normally means the air/fuel mixture is running lean. But the result in may be anything at all from a hard-to-find vacuum leak to dirty injectors, low fuel strain, a weak ignition coil(2), bad plug wires or compression problems.

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

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


Something else to keep in mind about OBD II fault codes is that some codes are false codes. Some cars will set codes as the OBD II system is over-sensitive or there is a glitch while in the factory software. For example, older GM cars with certain 3.8L engines will often set a P1406 code, which indicates a fault inside the EGR valve. Replacing the EGR valve doesn’t fix the situation on these cars since the OBD II system is overly sensitive to how quickly the EGR valve opens when it’s commanded to accomplish so by the PCM. The cure here is not to replace the EGR valve, but to have the PCM “flash reprogrammed” by a GM dealer or repair shop who will download the updated software from GM into your car’s PCM.

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


The OBD II Check out 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 difficulties code will remain in memory. In the event the fault does not reoccur for for 40 to 50 trips, the code will be erased.

The only safe approach to clear fault codes and turn of your 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 simply because in a few days you probably won’t.

If the Test Engine Light comes back on again (which is ordinarily does if there is a hard fault while in the system), you may check out the codes again to see if they are precisely the same ones as before. This would confirm the fact that you have an emissions dilemma, 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 possibly be tempted to just ignore them. That’s up to you. But should you live in an area that needs emissions testing, your car will NOT pass an emissions test in case the Verify Engine Light is on.

On older pre-OBD II vehicles (1995 and back), problems 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 perhaps not idle smoothly or feel quite right for some time until the PCM relearns what it demands to learn. Similar for the transmission controller. You will also lose the channel presets on the radio, and any other electronic settings (memory seats, mirrors, and so on.). That’s why a code reader or scan instrument ought to be used to clear the codes only.

WARNING: On many 1996 and newer OBD II cars, pulling the PCM fuse or disconnecting the battery might NOT clear the codes, and may well bring about a loss of important information the PCM demands 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 your PCM may possibly lead to it to forget

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