Q: Service engine soon error message

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I am looking at a car for sale. It was just safetied. I noticed the error message 'service engine soon'. The guy says it goes on sometimes and then does not appear. Is this a serious concern?

My car has 169000 miles.
My car has an automatic transmission.

If the check engine light is on (aka, "service engine soon"), that means that there is a malfunction in the engine, typically involving the engine controls such as ignition, fuel, air induction, emissions and so forth. There are thousands of trouble codes that can cause the light to set, ranging from the mundane (the gas cap is loose, increasing emissions) to the potentially catastrophic (the cylinders are misfiring which if not addressed promptly will destroy the catalytic converter). Most problems are "in between" but ALL problems must be addressed otherwise obviously the check engine light would be rendered meaningless as a guide to the operating status of the car. A good way to protect yourself in a used car transaction is to request a pre-purchase vehicle inspection. That inspection of the specific vehicle you want to purchase, carried out by a certified Mechanic, dispatched to your location, will give you very specific data regarding any current problems that the car has and in your case that would include the Mechanic using a code scanner to download the diagnostic trouble codes that have caused the check engine light to illuminate. Once the mechanic has downloaded those codes, he or she will be in a position to let you know of the potential significance of the codes.


Any time a vehicle’s check engine light illuminates, there is a malfunction that should be diagnosed and repaired. If the check engine light is flashing, that means there is a serious fault, such as cylinder misfiring, that requires immediate repair. Misfiring, in particular, has the potential to damage the catalytic converter which can result in additional costly repairs. If the check engine light is "steadily" illuminated (not flashing) the seriousness of the malfunction is hard to judge unless and until a Code Scanner is used by the Mechanic to download the specific trouble codes from your car’s computer (PCM or ECU) that have caused the light to illuminate. Sometimes the check engine light will illuminate if the gas cap is loose. If the rubber seal on the gas cap is intact and the cap is tight, then all other causes of the light illuminating would have to be evaluated in a check engine light diagnostic. During such a diagnostic, a certified mechanic from YourMechanic would use a code scanner to retrieve diagnostic trouble code(s) from your car’s PCM. Using those codes the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like) are tested to pinpoint the cause of the fault or the reason why the check engine light has illuminated. Once the faulty part or component is identified, it is explained to your satisfaction and the mechanic will let you know of the cost to repair. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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