How to Run a Car to Reset the Check Engine Light
Cars a generation ago were far easier to repair than those on the road today. The addition of complex emission-control systems and the incorporation of microprocessors to manage everything from fuel efficiency to the driver's seat position mean that repairing modern cars almost requires a degree in computer engineering.
Since the mid-1980s, vehicles have been equipped with a diagnostic computer that monitors the function of all those systems. Most of the time, it performs its function invisibly. When the computer senses a fault, however, it warns the driver by illuminating the Check Engine Light (CEL) on the instrument panel. Owner's manuals warn that when this light comes on, you should pull over immediately and stop to avoid damaging the engine.
The faults range from simple problems, like a missing gas filler cap, to problems that can cause major damage to an engine in moments. There are also documented cases of false codes and randomly-generated error codes. In pre-1996 vehicles a fault could be identified by "reading" the pattern of blinks when the ignition was first turned on, but that is no longer the case. On today's cars the owner must use a code reader to extract the trouble code stored in the diagnostic computer. Only then can the CEL be reset safely.
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Instructions
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Read the OBD (on-board diagnostic) trouble code from the diagnostic computer. OBD code readers are widely available and inexpensive, and many auto parts stores will check the code at no charge (they also stock the needed parts).
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Research the code to determine the severity of the fault. OBD-II codes are standardized four-digit numbers preceded by the letter "P." Some faults are obviously minor, such as a loose gas cap. Some are obviously false, such as a warning that the side airbag sensor has malfunctioned on a vehicle that has no side airbags. Other faults cause the vehicle to run inefficiently or exceed design limits for emissions, but can be ignored temporarily. Major faults require immediate attention.
The parts store staff may have advice, or owners can research the code on the Internet.
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Decide whether the fault is a false warning and ignoring it will not damage the vehicle. If so, disconnect the battery cable from the positive terminal---the one with the red plastic cover---and leave disconnected for half an hour. After reconnecting the battery cable the light will have gone off in most cases. Be aware that there is no guarantee that it will stay off.
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Repair major problems referenced by the trouble code. The light will go out by itself after the fault has been removed. It may take a two or more engine restarts to reset the light.
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Tips & Warnings
Enthusiast forums often have information about false error codes; search on the code number and vehicle model (such as "P0420 Honda CR-V") to find others who have had the same problem.
Driving with the Check Engine Light on will not make it go away. It is likely that doing so will make the problem worse.
References
Resources
- Photo Credit dead battery image by Katrina Miller from Fotolia.com
