Understanding the Check Engine Light

Many people panic when the dreaded "Check Engine" Light comes on in their car. They are convinced that an expensive repair will be needed and the car needs to be taken to the dealer or a mechanic right away.  This is not always the case.

Understanding what the Check Engine light is and what it means will help you save a lot of money if you own a car.

Check Engine Lights are related to emissions controls, period. When the light comes on, it usually means some emission-related item on your car is not working properly. This can mean the engine is not working properly as well (emissions controls are part and parcel of engine controls). But unlike the TEMP and OIL lights, it does not necessarily mean the car is going to melt down in a matter of minutes or seconds.  On the other hand, I am not suggesting you should drive around with the Check Engine light on for the rest of your life.

A brief history of the Check Engine light will help to understand how it works - and also how the light on your particular year of car works.

In the late 1970's, the EPA mandated that emission controls be warranted for 50,000 miles. Carmakers fought this on technical and financial grounds. The key component that was problematic was the oxygen sensor (or Lambda probe, named for the 16.67:1 air to fuel ratio that provides optimal combustion).

While exhaust systems could be made to last 50,000 miles (up to the catalytic converter, anyway) by aluminizing them or using stainless steel, the fragile Lambda probe would only last about 25,000 miles. The probe detects oxygen in the exhaust and uses rare earth elements in its construction.

The solution was to provide a light on the dashboard that would illuminate after 25,000 miles. For most cars of the late 1970's, this was triggered by a device attached to the odometer - a simple gear driven switch. It could be reset by a using unbent paper clip to preset a reset button, for another 25,000 miles. Many drivers did this, instead of replacing the oxygen sensor.  However, usually this meant the car ran poorly after the sensor went bad.

OBD I (On Board Diagnostics, version I) was introduced in the early 1990's. These systems used generally one oxygen sensor (or one per bank) ahead of the catalytic converter. A computer system monitored the various sensors and conditions and if an error was detected, it would log a code internally and then illuminate the "check engine" light. When the "check engine" light was illuminated, it could be read (on some cars) using a "stomp test" where jamming the acceleration pedal to the floor with the key in the ON position would cause the light to blink in a morse-code fashion.  Different makes and model cars had different techniques for reading codes - you have to consult a service manual or search online for your specific car.  Or a code reader could be used to read the digital code and then the error message looked up in an accompanying booklet.

OBD I cars were fairly robust and not prone to generating spurious codes. But it should be noted that just because a code mentions a particular car part, does not mean the corresponding repair is to replace that part. Even experienced car mechanics often fail to see this.

Oxygen sensor technology had improved by that time to the point where the sensors were deemed good for 50,000 miles. Most manufacturers recommend replacing them at a predetermined interval, although I have found that time, as well as mileage, is indicative.  Most will last longer than 50,000 miles, if the car is driven a lot in a short period of time.  On the other hand, a "garage queen" car may need a new one at 30,000 miles, if it is eight years old.

OBD II was introduced around the mid 1990's. This newer system features on-board emissions testing, as well as diagnostics, and as a result, in many States, emissions testing consists of plugging the car into an analyzer using the OBD II port (often under the dash) and see if there are any error codes.  If there are no codes, it passes.

Note that error codes may be stored, even if the check engine light is OFF. In many instances, the CE light will not be lit until an error code is present for a predetermined amount of time, or if a number of error codes are present. This was an attempt by the manufacturers to get OBD II to stop crying "Wolf" so much, and it works, to some extant.

OBD II cars constantly monitor emissions by using a second oxygen sensor (or sensors, if multiple banks are used) to measure oxygen content after the catalytic converter. Similarly, the evaporative emissions system is checked by monitoring pressure in the fuel tank to detect leaks in gas caps and fuel lines. It is a much more complex and error prone system than the older OBD I cars.

Most OBD II cars require a code reader in order to read out the error codes and diagnose problems. Usually a specialized code reader for a particular make will have a code book specific to that make and model car. Generic readers, often offered for free use at many auto parts stores (Autozone or the like) generate "P-codes", which are generic codes to all makes and models (and often less useful in diagnosing problems). Usually these codes are the letter P with four digits after them.

Oxygen sensor technology has improved even further with OBD II, with many lasting 100,000 miles or more, although again, they may fail before then, if the car is just plain old - even if it has low mileage.

Error codes on OBD I and OBD II cars may reset themselves over time (after a predetermined number of starts) if the underlying error is no longer present. So if a spurious error code is generated, you may get a Check Engine light and then a week later, it may go away. It happens.

SRS (airbag) lights, generally do NOT reset over time, as these are safety related items. Usually a separate code reader is required for these items. A typical problem with SRS systems occurs, for example, when seatbelt switches become momentarily disconnected, when the connectors are kicked by back seat passengers or are pulled when moving the seat from one extreme to another. Usually, cleaning the contact and wire-tying the connectors together snugly fixes the problem. Many dealers instead recommend expensive replacement of seat switches.  However, given the serious  safety concerns in dealing with airbag systems, I would not recommend trying to work on these yourself, unless you really know what you are doing.  Airbags can be set off by accident, and if you are in the wrong place, can literally take your head off.

Bear in mind that Check Engine error codes are generally emissions related and do not necessarily mean the engine is going to fail.  If your engine does fail, the Check Engine light may come on, but the reverse is not necessarily true.  The CE light is not as critical as your temperature light, and particularly your OIL light. If the temp light comes on, you have minutes to shut off the engine to prevent further damage. If the oil light comes on, seconds. Most cars can run for years with an illuminated CE light, depending on what error is generating it.

To avoid panic on the part of consumers, some manufacturers experimented with different wordings, such as "check engine soon" or "service engine soon" or the like.  When these lights first appeared, many consumers thought the car was going to blow up right away, and would have the car towed to the dealer in a panic.

The Check Engine light should not be confused with engine service indicators for oil changes and services, such as used by BMW for its oil change interval detector (which bases oil change intervals on number of starts, speeds driven, temperatures, loads, time, etc.) or the mileage based Inspection I and Inspection II protocols.

For the do-it-yourself mechanic you can often Google search an error code (ANDed with the model car name) and find websites, discussion groups, blogs, and even photos, describing the likely problem and illustrating do-it-yourself repair procedures.  You will generally need to have a code reader, however.

Again, just because an error code mentions an engine part does not mean that part is at fault. For example, "Oxygen sensor out of range" usually means that a leak has formed in an inexpensive ($15) rubber intake bellows, which can be easily replaced. It generally does not mean the more expensive oxygen sensor is broken.

Similarly, a "Catalytic Converter Efficiency Below Threshold" message may indicate a faulty oxygen sensor, which is a far cheaper part than the catalytic converter. OBD II codes do not indicate the part at fault, but the symptom of the underlying problem. They are a diagnostic tool, not a "read and replace" instruction device.

Think of it this way.  You go to the doctor and he notices you have an irregular heartbeat.  Does this mean you need a heart transplant?  Heck no.  There are dozens of causes of such conditions.  The symptom is only one aspect of the underlying diagnosis.  You don't just start replacing internal organs because they are misbehaving.  It could be something as simple as a drug interaction.  OBD II diagnostics work the same way.  They tell you the symptom, not the cause of the problem.  And sometimes the cause can be exceedingly trivial!

Many consumers have been angered by OBD II cars, which, like a Windows-based computer, often generate spurious error codes. A loosely attached gas cap, for example, will generate a "Check Engine" light, with the code translating to "Large Leak - Evaporative Emissions System" or the like. Usually tightening the gas cap fixes the problem. A thin film of plumbers silicone grease on the gasket also helps.  But this is little comfort to the owner who just spent $90 at a dealer to figure this out.

Occasionally, the OBD II computer just goes berserk and "throws codes", for example, if you stall the car. These codes are not an indication of any real failure, just the computer receiving a voltage spike or inconsistent readings and generating error messages.

Just as you would not start tearing apart a Windows computer every time it generates an error message, you should not immediately go looking for broken parts on your car because the Check Engine light is on (particularly with an OBD II car). Reset the code first and see what happens.

Usually, resetting the code (again, a code reader makes this easy for the consumer to do) will clear the problem, with no need for a trip to the dealer. If a code reappears immediately, it is likely a "hard fault" and a broken part is likely indicated (e.g., camshaft position sensor, throttle position switch, etc.).

If an error code reappears over time, then a condition may exist that needs repair. Writing down the codes and the date and monitoring them can be an effective diagnostic, just as checking your blood pressure is and effective diagnostic of your circulatory health.

In some instances, if a serious enough fault occurs, the car won't run at all. You'll have to have it towed to a mechanic (or fix it yourself). In other instances, the car may still run, but not as well as before. Many "fail safe" modes exist that allow the car to run, but not in an optimal fashion. An Oxygen sensor may malfunction, but the car will continue to run, albeit rich. The car may run rough and gas mileage will suffer. It may be necessary to have the car serviced to get it back into top running condition. Letting Check Engine light problems fester for years is never a good idea, even if the car continues to run, however.  A too-rich mixture in "limp home mode" will eventually clog the catalytic converter, for example.

Note that for emissions testing, the computer on an OBD II car needs 50 to 100 miles of sample data to "log" in order to determine whether the car is operating properly. Resetting the check engine light just prior to emissions testing will usually result in an immediate failure for lack of logging data. If you reset the CE light, drive the car at least 50 miles before testing, or it will fail on low data.

That's probably more than you wanted to know. In reality, the engine management computer is a fairly primitive device, compared to the computer on your desk. It is not too difficult to diagnose problems with the error codes associated with the CE light, particularly with the help available on the Internet.

If you are handy with tools and have patience and don't mind searching on the Internet, you can fix a lot of minor problems on your car (various sensors, gaskets, etc.) by reading check engine codes with a code reader and searching online for solutions. As with everything on the Internet, it pays to look a lot and get a consensus as to what is going on, as there is a lot of bad information out there.

If you are not handy with tools, then don't attempt such repairs. However understanding what the Check Engine light means and how it works will help you to understand the repair procedure better.

Good Luck!