Check Engine Light
Check Engine Light on due to OBDII fault

When the check engine light in your vehicle flashes, comes on and stays on, or comes on intermittently, you must make a decision about what to do. Should you keep driving? Pull off the road and call a tow truck? Or call your shop for suggestions about what to do?

Knowing about the system on your vehicle that controls the check engine light and how it works can help with the decision making process. Fear of the unknown can make life very uncomfortable, especially when it relates to our vehicles.

Check Engine System Introduction

In the 1980’s the On-Board Diagnostic (OBD) system was introduced to lower vehicle emissions and help auto repair technicians diagnose and repair computerized engine systems on our vehicles. The computerized systems would turn on a check engine light if a problem was detected.

In 1996 the government mandated new cars and light trucks have a more advanced OBDII system. This computer engine control system would monitor components that affect the emission performance of a vehicle to ensure it is running correctly so exhaust pipe emissions will be at a minimum.

If there is a problem with one of the monitored systems the check engine light (Service Engine Soon light) may come on to alert the driver that a problem has been detected.

Components or systems that can cause the light to come on:

  • temperature sensors, gas caps, engine misfire and running rough, speed sensors, air flow sensors
  • broken or loose hoses, lines or caps
  • lack of recommended maintenance of vehicle systems causing them to not operate as designed

Should you stop driving when the Check Engine Light comes on?

The light coming on is intended to inform the driver of a possible need for service but is not telling you to stop driving.

There are a number of reasons for the light to come on and a number of different ways the light will display depending on the type of fault that is detected.

  1. Light on and staying on – the vehicle seems to run okay, with no noticeable problems. It is okay to keep driving. The light may not be on the next time you drive, or it may stay on for several trips before it goes out. If it stays on you should schedule an appointment with your shop.
  2. Light flashing – a flashing check engine light indicates there is a fault that can harm one or more of the components on your vehicle. It is recommended to safely pull off the road and stop the vehicle. Many manufacturer’s would recommend having it towed to a shop. If it was not flashing too long you can shut the engine off, restart and try again. If it flashes again you should again pull over and call a tow truck.
  3. Light intermittently coming on – as long as it is not flashing and the vehicle is running okay you can keep driving. If the problem persists the light will stay on. If it is an intermittent condition the light at times may not be on. Schedule a visit to your shop to have it diagnosed.

Don’t worry if the light is not on when you take your vehicle in. When the light comes on the computer retains a “history code” that can be retrieved by the technician even days after the light has gone out.

Why does the shop tell me to drive the vehicle after they clear a code?

A technician who is working on your vehicle will clear the code from the system if he does not find a reason for the light being on, or if he finds and fixes the problem.

The vehicle will then have to be driven under certain conditions to allow the system to run all its readiness monitors. If the fault is corrected or the light does not return after the monitors run, the vehicle is fixed. If the check engine light returns the problem still exists.

Specific driving conditions or “drive cycles” must be met before all the monitors can run. This may take a day or more of driving.

You do not want to pay your auto repair shop to drive your vehicle when you can do it for free, especially if it will take a day or more of driving.

For more information about the on-board diagnostic system and to access frequently ask questions refer to this Environmental Protection Agency website.

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