Knowledgebase: General
Check Engine Light (MIL) On! What It Means & What To Do
Posted by Alex (Im) E. on 15 December 2012 10:50 PM

CHECK ENGINE LIGHT (Malfunction Indicator Light/Lamp or MIL for short)

Malfunction Indicator Light lamp Check Engine light (which is officially called "Malfunction Indicator Lamp" or MIL) — is supposed to alert the driver when something has gone wrong with the vehicles operations.

Depending on how the system is configured and the nature of the problem, the Malfunction Indicator lamp/light may come on and go off, remain on continuously or flash -- all of which can be very confusing because you have no way of knowing what the light means.

Here's how it works:

When the OBD II system detects a fault, it sets a "pending code" in cars computer.

The MIL light doesn't 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 OBD2 system will set a diagnostic trouble code (DTC) and turn on the Malfunction Indicator (MIL) lamp.


Ok, Now What?

If the Check Engine Light comes on — does it indicate a serious problem or not?

The only way to know is to connect a code reader or scan tool to the OBD-II diagnostic connector or your car (all vehicles have one by law) — and read out the code.

It's also important to note if any other warning lights are on, or if the engine is making unusual noises, running poorly, steaming, smoking or otherwise showing obvious signs of trouble.

  • If the OIL WARNING LAMP is on, shut the engine off immediately. Running the engine low on oil can cause serious damage to the bearings, rings and valvetrain.
  • If the TEMPERATURE WARNING LAMP is on, shut the engine off immediately. The engine is overheating possibly due to a coolant leak, low coolant level, bad thermostat, failed fan motor or fan clutch, broken belt or radiator obstruction.


Reading Codes

If you don't have a code reader or scan tool, some auto parts stores may do a code check for you at little or no cost (most dealers and repair shops do charge for this service). But the code alone won't tell you which part has failed and needs to be replaced.

A trouble code is only a starting place. It's not the final diagnosis. Someone still has to check out the various components in the affected circuit or system to determine what exactly is causing the problem. This often requires following a lengthy diagnostic chart to isolate the fault. Jumping to conclusions often results in a faulty diagnosis.

For example, let's say the Check Engine lamp is on and you find an OBD II 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.

Harder to diagnose are misfire codes. OBD-2 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 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.


Beware of False Codes

Something else to keep in mind about OBD2 fault codes is that some codes are false codes.

GM has had problems with certain 3.8L engines setting P1406 codes, which indicates a fault in the EGR valve.

Replacing the EGR valve doesn't fix the problem 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 "flash reprogram" the computer so it is less sensitive to this condition. Referring to vehicle manufacturer technical service bulletins (TSBs) can save a lot of time and frustration for these kinds of problems.


Generic & Enhanced OBD-II Codes

Something else that complicates diagnosis is that "standardized" OBD II codes really aren't.

There are actually two different types. "Generic" OBD II codes are the same in the sense that all vehicle manufacturers use the same code number to indicate the same type of problem. But each vehicle manufacturers also has their own special "enhanced" codes that cover problems not included in the basic OBD2 code list.

These include many problems not covered by the generic codes as well as problems that are outside the engine management system such as ABS codes, climate control codes, body codes, air bag codes, etc.

Generic OBD II codes all start with "P0" while the OEM enhanced codes all start with a "P1." Enhanced codes are often vehicle specific, and may not be readable with some code readers or scan tools.

In other words, it may require OEM scanner or a dealer scan tool to read the enhanced codes.


Turning the Check Engine Light OUT

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

If an intermittent fault does not reoccur after three consecutive trips, the Malfunction Indicator Light (MIL lamp) will go out but the code will remain in memory. If the fault does not reoccur for for 40 trips, the code will be erased.

The only safe way to clear fault codes is to use a scan tool or code reader.

On many pre-OBD II vehicles, all you had to do was disconnect the battery or pull the PCM fuse to clear the memory and turn the Check Engine light off. If the problem had not been fixed, the Check Engine light would eventually come back on. But this procedure should NOT be used on OBD II cars for the following reason:


WARNING: On many 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. 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 the use of a scan tool and a special relearning procedure to reset the PCM.

CAUTION: Are you sure you want to erase the codes? Codes contain important diagnostic information a technician may need later to troubleshoot the system. If the codes are cleared, it may take some time for the codes to reset - -which will delay diagnosing and repairing the fault. History codes, in particular, can shed light on why a certain fault may be occurring.


If a vehicle is being given an OBD II test, an illuminated Malfunction Indicator Light (MIL lamp) is grounds for failing the test. Clearing the codes just before the test may allow the vehicle to pass -- provided the codes do not reappear while the vehicle is being driven to the test facility.

Some codes, such as those for a bad catalytic converter or an EVAP leak are difficult to set and may take several day of driving before the right conditions are met to complete the monitor.

One way to make sure there are no pending codes that will turn on the Malfunction Indicator Light ( MIL lamp) later is to use your scan tool to look at the "Mode 06" data in the PCM. Most scan tools can display Mode 06 data.

The Mode 06 menu is usually found by choosing "generic" or "global" OBD II on the scan tool main menu rather than entering the vehicle year, make and model.

When you go to the Mode 06 menu, you can see the operating status of all the monitored components, whether there are any DTCs or not. If a component is not operating within normal limits, it will be flagged and may set a DTC later.

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