Why is my check engine light on?

Why is my check engine light on?

The mystery of the check engine light.

It’s amazing how much of a mystery the check engine light in a car can be, to many people.  With some people are not worried at all if it’s on, as long as the car runs. Other people have their car towed in to the shop the second it turns on.  Just like the people who think that the codes stored will tell the mechanic exactly what to fix.  Why the check engine light is on, seems to be a mystery to most people.  The simplest answer to why a car’s check engine light is on, is because something is wrong with your car that is causing it to produce higher than government allowable emissions.

The history check engine lights and OBD.

The clean air act of 1970 required certain emissions related equipment for all cars.  As time went of these emission requirements got stricter.  Requiring more equipment and better ways of fuel delivery on cars.  Which brought on the wide spread adoption of electric fuel injection in the 1980’s.

As fuel-injection evolved so did the software to run these cars. This brought on OBD software.  Which stands for on board diagnostics.  OBD is a way of stating the computer is monitoring different sensors in your car.  When one or more sensors register something the computer (PCM) thinks is out of range it turns on a light, and stores a code.  This code give a direction for the mechanic to start with when trying to solve the problem.  It is by no means the tell all end all answer.

When Fuel-Injection first can out every manufacturer had its own software, and scan tools.  By the mid 90’s if you wanted to know why your check engine light was on you had to have a high dollar scan tool or take your car to a dealership.  The government then got more involved.  It was determined that anyone should be able to affordably buy a scan tool that will read the basic government required codes.  Thus OBDII was born.

In 1996 all cars made in the US where required to have OBDII.  OBDII required a data link connector placed within a few feet of the steering wheel.  Every car was required to have to same connector.   It also added specific emissions monitoring and specific codes.  This allowed for generic scan tools to read generic OBDII codes.  It also made it easy for any one to find the Data link connector and to attach code reader or scan tool to that car.  Thus making it easy to find the starting point for diagnosing the problem.

What are the codes?

There are potentially over 9999 codes your PCM can store.  All codes start with one of 4 Letters.  You have 4 more digits to be used.  These codes give a direction to figure what’s wrong with your car when one of the cars warning lights comes on.  However you can have codes stored in the PCM that do not make the light turn on.

The are four basic types of codes.

  • Pxxxx are for powertrain codes (engine)
  • Bxxxx are for body codes (electrical things inside car)
  • Cxxxx are for chassis codes (steering and suspension)
  • Uxxxx are for class 2 network codes (various computers can’t talk to the main PCM)

Powertrain Codes

Powertrain codes are the most common codes that turn on your check engine light.  The government mandates all P0xxx codes  P1xxx codes are manufacture specific codes.  As you go further on the numbers Px1xx and Px2xx codes are for air and fuel problems.  The most common codes are Px3xx, which are ignition related problems.  The Px4xx codes are for emission control codes.  Px5xx are speed and idle regulation. Computer and output signals are Px6xx codes.  Both Px7xx and Px8xx are for transmission problems.  Px9xx is for control modules, input and output signals.

The last 2 digits are where the code gets more specific.  Such as a P0301 code is a misfire on number 1 cylinder.  They even go as far as things like a P0961 which is a transmission pressure control solenoid “A” control circuit problem.  As you can see they are not always specific.

What turns on the Light

There are plenty of things that can turn on the check engine light.  Every code stored will turn on the light.  The PCM is constantly checking your car for faults, but it doesn’t always turn on the light right away.  There are two ways the computer determines when to turn on the light.    Some codes will store the second a fault is found.  Others need at least one complete drive cycle to determine if it needs to turn on the light.  These drive cycles are used to make sure that it wasn’t just a fluke thing and that there really is something wrong.

Codes like misfires have a set number of times the computer sees a miss in a set time frame to turn on the light.  Other things like an EVAP leak requires that your gas tank be between 1/4 and 3/4 full, and not hold vacuum while driving for prolonged time at set speeds.  Every manufacture is a little bit different as to what turns on the light for a specific code on their car.  Every code has a set test parameters that pass or fail.  The same can be said for turning off the light.  Some codes must be reset with a scan tool.  Others just require the problem to go away for a few seconds.  It’s really amazing the amount of programming it takes for just one little light.

What to do when the check engine light turns on?

There are two things you can do when the check engine light comes on.  The first thing is ignore it.  This is not a good plan.  Just because your car is running good doesn’t mean you should ignore the light.  Many times that light coming on is more than just a simple problem causing high tailpipe emissions.  Left unfixed it could cause serious engine damage or costly repairs later on.  The better choice is to have it looked at by your trusted mechanic.  If you are good with research and have an OBDII code reader you can read the code yourself.  Finding out the code is just a starting point.  The internet and some basic diagnostic skills will be required.  No matter what you should get it looked at.


Do not be scared of the check engine light.  It’s there because of a problem with your car.  All the codes stored because of the light will be related to your car producing higher than government mandated emissions.  Although your car can store many more codes that doesn’t turn on the light.  None of these codes do anything more than tell you where to start looking to fix the problem with your car.  Think of them like a compass on a giant road map.  The next time your light comes on you will have some knowledge about what it means, what it tells you, and what your mechanic is going to do to fix.

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